Australia

Official name : Commonwealth of Australia

Area: 7,686,300 square kilometers (2,966,200 square miles)

Highest point on mainland: Mount Kosciusko (2,229 meters/7,314 feet)

Highest point in Australian territory: Mawson Peak (2,745 meters/9,000 feet), an active volcano on Heard Island near Antarctica

Lowest point on land: Lake Eyre (16 meters/52 feet below sea level)

Hemispheres: Southern and Eastern

Time zone: 10:00 p.m. in New South Wales, Australian Capital Territory, Victoria, Tasmania and Queensland = noon GMT; 9:00 p.m. in South Australia and Northern Territory = noon GMT; 8:00 p.m. in Western Australia = noon GMT

Longest distances: 4,000 kilometers (2,485 miles) from east to west; 3,837 kilometers (2,374 miles) from north to south

Land boundaries: None

Coastline: 36,735 kilometers (22,831 miles)

Territorial sea limits: 4.8 kilometers (3 miles)

1 LOCATION AND SIZE

The nation of Australia, which also happens to be the world's smallest continent, is situated in the Southern Hemisphere southeast of Asia, between the Pacific and Indian Oceans. Australia covers an area of 7,686,300 square kilometers (2,966,200 square miles). It is slightly smaller than the contiguous United States (not including Alaska and Hawaii). Australia is divided into six states and two territories.

Many Australian place-names reflect the country's history as a British colony, as well as the influence of Dutch and French explorers who visited the region during the seventeenth, eighteenth, and nineteenth centuries. In the late twentieth century, some Aboriginal place-names replaced the British colonial names.

The following table lists the area of each of the six Australian states in both metric and English units:

2 TERRITORIES AND DEPENDENCIES

Mainland Australia has two territories: Northern Territory and Australian Capital Territory. The following table lists the area of each region in metric and English units:

Since 1936, Australia has claimed an additional 6.1 million square kilometers (2.4 million square miles) on the continent of Antarctica as Australian Antarctic Territory—about 40 percent of the total land area. Three scientific bases are in operation there: Mawson (established in February of 1954), Davis (established in January of 1957), and Casey (established in February of 1969).

Furthermore, Australia claims authority over several nearby inhabited islands including Christmas Island, which is located in the Indian Ocean 2,623 kilometers (1,630 miles) northwest of Perth. Christmas Island covers an area of about 135 square kilometers (52 square miles), and in 1996 it had an estimated population of 813; 61 percent of the island's residents were Chinese and 25 percent were Malay. Not far from Christmas Island, the Cocos (Keeling) Islands consist of twenty-seven islets with a total land area of 14 square kilometers (5 square miles), two of which are inhabited. In 1996, the estimated population of these two islands was 609. Another possession, Norfolk Island, is northeast of Sydney and covers an area of 36 square kilometers (14 square miles). British explorer James Cook discovered Norfolk Island in 1774; the British government later sent prisoners here during the late eighteenth and early nineteenth centuries. In 1856, descendants of the British sailors who had carried out a mutiny on the ship, HMS Bounty, in 1789, joined the prisoners and settled on Norfolk Island. As of 1996, the estimated permanent population was 2,209.

Australia also claims authority over a number of uninhabited islands. The Coral Sea Islands were declared a territory of Australia in 1969; they have no permanent inhabitants, but researchers temporarily take up residence at a meteorology station on one of the islands. The mountainous Heard Island, which is about 4,000 kilometers (2,500 miles) southwest of Perth, covers an area of 910 square kilometers (350 square miles) and has a dormant volcano known as Big Ben (at an elevation of 2,740 meters/8,990 feet). Shag Island is just north of Heard Island; only 42 kilometers (26 miles) to the west are the small McDonald Islands. About 1,600 kilometers (1,000 miles) southeast of Tasmania, the rocky Macquarie Island measures 34 kilometers (21 miles) in length and about 3 to 5 kilometers (2 to 3 miles) in width. Macquarie Island is uninhabited except for a base maintained at its northern end since February 1948; at its southern end, it houses the biggest penguin rookery (a breeding ground) in the world.

3 CLIMATE

The climate of Australia is warm and dry. The following table summarizes seasonal temperatures and precipitation levels in the capital city of Sydney:

4 TOPOGRAPHIC REGIONS

Australia has one of the flattest terrains of any country in the world. Erosion over thousands of years has rounded and flattened the mountains of Australia, so that only 6 percent of the land is over 610 meters (2,000 feet) above sea level. The country may be divided into regions according to topography (description of the surface of the land).

The Eastern Highlands (also called the Eastern Uplands) encompass the eastern portion of the country, stretching from the Cape York Peninsula in northern Queensland south through New South Wales and Victoria. Average elevation in this region is about 152 meters (500 feet). The country's highest peak, Mount Kosciusko—at 2,229 meters (7,314 feet)—is found in the southeast corner of the mainland between Melbourne and Canberra.

The Western Plateau is a large desert region, covering approximately the western two-thirds of the country. The Western Plateau rests on an ancient rock shield or foundation, and the average elevation throughout is 305 meters (1,000 feet) above sea level. The Western Plateau has one mountain range (Hamersley) at its western edge, and three mountain ranges (Macdonnell, Musgrave, and Petermann) that stretch to its eastern edge. From these ranges southward, the Western Plateau is generally a flat tableland, with dramatic outcroppings of granite or sandstone. Four deserts are situated on the Western Plateau. The dry central part of the Western Plateau is popularly referred to as the "Outback." The Darling Range, also known as the Darling Scarp, is found along the plateau's southwest coast.

5 OCEANS AND SEAS

Several bodies of water surround Australia. Along the northern coast lie the Timor Sea (northwest of Darwin) and the Arafura Sea (directly north of Darwin between Australia and the neighboring nations of Indonesia and Papua New Guinea). The Coral Sea lies east of the Cape York Peninsula along the northeast coast. Stretching directly east is the Pacific Ocean. The Tasman Sea lies along the southeast shore of mainland Australia northeast of Tasmania Island. (Tasmania and the Tasman Sea are both named for the Dutch explorer Abel Tasman, who arrived in Tasmania in 1642.) Finally, the Indian Ocean surrounds the southern and western coasts of mainland Australia.

Seacoast and Undersea Features

The Grea Barrier Reef, the world's longest coral reef, extends for 2,010 kilometers (1,250 miles) just off the northeast coast of Queensland. It encompasses 207,000 square kilometers (79,902 square miles), and it supports a marine ecosystem that includes islands as well as coral reefs. Lake Alexandrina, a coastal inlet that is sometimes referred to as a coastal lake, is situated near Meningie to the southeast of Adelaide and to the east of the Great Australian Bight.

Sea Inlets and Straits

The coastline of Australia features a number of gulfs where the land curves around the sea. The Gulf of Carpentaria forms a deep U -shape on the northeast coast between Arnhem Land and Cape York Peninsula. In 1623 Djan Carstensz, a Dutch explorer, named the gulf in honor of Pieter de Carpentier, who was then the governor-general of the Dutch East Indies (present-day Indonesia). Another Dutch East Indies governor-general, Anthony van Diemen, gave his name in 1644 to Van Diemen Gulf, which lies just west of the Gulf of Carpentaria between Darwin and Melville Island. To the south of Van Diemen Gulf is Joseph Bonaparte Gulf, named in honor of eighteenth-century French emperor Napoleon Bonaparte's older brother by a French explorer in 1803.

To the south, the Great Australian Bight is formed by a large semicircular curve in the southern coast. ("Bight" describes a bend in a coastline or the bay that is formed by a curving coastline.) Along its eastern edge near Port Lincoln is Spencer Gulf, a finger-shaped gulf which points northward about 320 kilometers (198 miles) into South Australia. Bass Strait lies between Tasmania and the mainland. In 1798, explorers George Bass and Matthew Flinders sailed through the strait, demonstrating for the first time that Tasmania was an island.

Islands and Archipelagos

The state of Tasmania (sometimes called Tasmania Island) is a large island located 241 kilometers (150 miles) off the southeastern coast of the mainland. Tasmania has the same geology as the Eastern Highlands, with rugged terrain and a large central plateau. Elevations reach 1,524 meters (5,000 feet) on Tasmania. Between Tasmania and the mainland in the Bass Strait lie King Island and Flinders Island.

Two of Australia's largest islands lie off the northern coast of Northern Territory. To the west of Darwin is the largest, Melville Island, measuring 5,786 square kilometers (2,333 square miles). To the east in the Gulf of Carpentaria is Groote Eylandt (Dutch for "Great Island"), which covers 2,285 square kilometers (882 square miles), and Mornington Island. North of Broome in Western Australia lie the three uninhabited Ashmore Islands, as well as Cartier Island, which was annexed as part of the Northern Territory in 1938. Kangaroo Island, off the southern coast near Adelaide in South Australia, measures 4,416 square kilometers (1,718 square miles). Fraser Island, a part of Queensland that covers 1,643 square kilometers (634 square miles), is the largest all-sand island in the world.

To the northwest, the Bonaparte Archipelago features numerous small, rocky islands and a deeply indented coastline.

Coastal Features

Many peninsulas extend along the coast. In the northeast, the Cape York Peninsula points north toward Papua New Guinea. Across the Gulf of Carpentaria, Arnhem Land represents the edge of the Western Plateau and features rugged highlands and broad valleys. To the northwest, the Eighty Mile Beach, a stretch of sandy beachfront, marks the coastal edge of the Great Sandy Desert. Just off the high cliffs that mark the shore southwest of Melbourne, limestone pillars known as the Twelve Apostles emerge from the sea.

6 INLAND LAKES

There are no notable lakes in Australia.

7 RIVERS AND WATERFALLS

The most important and longest continuous river system in Australia, referred to as the Murray-Darling River System, flows through parts of four states: Queensland, New South Wales, Victoria, and South Australia. This river system provides the water for 80 percent of the irrigated land in the country. With an annual runoff volume of 22.7 billion cubic meters (801.6 billion cubic feet) of water, the Murray-Darling River System is Australia's largest. Compared to the world's largest river system, the Amazon River in South America, however, the Murray-Darling River system carries less than one percent of the water volume that is transported by the Amazon

The Murray-Darling River System drains an area of 1.1 million square kilometers (410,318 square miles), or about 14 percent of the total land area of the country. Measured from its source in Queensland to its mouth at Lake Alexandrina south of Adelaide, Murray-Darling measures 3,370 kilometers (2,022 miles), or about one-half the length of the world's longest river, the Nile in Egypt. The Murray River, the Darling River, and their tributaries are among the few river systems in Australia that have year-round water flow.

The Murray River measures 2,520 kilometers (1,512 miles), flowing west and southwest, eventually emptying into Lake Alexandrina, a coastal lake south of Adelaide that opens into the Indian Ocean. The Murrumbidgee River, one of the Murray's tributaries, measures 1,575 kilometers (950 miles). Other tributaries include the Lacklan and Goulburn Rivers.

DID YOU KNOW?

A river system is made up of a principal river and its tributaries (the rivers that flow into it). A river system begins with the drainage of rainfall and ends in a large body of water, usually an ocean. After a rainstorm, rainwater—called runoff—drains downhill until it eventually accumulates at a low point and begins to flow. As the water flows from higher to lower elevations, two or more small rivers join together to form a larger river. This larger river—usually the one that gives its name to the river system—continues to flow. Sometimes several other smaller rivers, called tributaries, join with the main river as it flows toward a larger body of water such as a lake or ocean.

The point at which a river flows into the ocean is called its mouth. A river system begins at a place called the source or headwaters. The source is the point farthest away from the mouth where water begins to flow. Ports—cities that support shipping activity—often develop at a river's mouth. Ports have docks and roads to allow goods to be transported by ships and other vehicles into and out of the country.

The Darling River, flowing from the junction of the Culgoa and Barwon Rivers in New South Wales, measures 1,390 kilometers (834 miles). The headwaters of the Darling River originate in the MacIntyre River, which forms part of the border between Queensland and New South Wales. The MacIntyre River eventually flows into the Barwon River, generally agreed to be the main source of the Darling River. The Barwon-MacIntyre section, sometimes called the Upper Darling River, measures 1,140 kilometers (700 miles).

8 DESERTS

About 35 percent of the land area of Australia is categorized as desert because it receives so little rainfall. The Great Victoria Desert (Western Australia and South Australia) is the largest individual desert, covering about 4.5 percent of Australia's total land area at approximately 348,750 square kilometers (134,618 square miles).

Other deserts, in descending order from largest to smallest, are: the Great Sandy Desert (Western Australia), representing 3.5 percent of Australia's total land area, covering 267,250 square kilometers (130,160 square miles); the Tanami (or Tanamy) Desert (Western Australia and Northern Territory), representing 2.4 percent of Australia's total land area, covering 184,500 square kilometers (71,220 square miles) just north of the MacDonnell Ranges; the Simpson Desert (Northern Territory, Queensland, and South Australia), representing 2.3 percent of Australia's total land area, covering 176,500 square kilometers (68,130 square miles); the Gibson Desert (Western Australia), representing about 2 percent of Australia's total land area, covering approximately 156,000 square kilometers (60,200 square miles)

in the center of the state along its western border; the Little Sandy Desert (Western Australia), representing about 1.5 percent of Australia's total land area, covering 111,500 square kilometers (43,040 square miles); the Strzelecki Desert (South Australia, Queensland, New South Wales), representing 1 percent of Australia's total land area, covering 80,250 square kilometers (30,980 square miles); the Sturt Stony Desert (South Australia, Queensland, New South Wales), representing less than 1 percent of Australia's total land area, covering 29,750 square kilometers (11,484 square miles); the Tirari Desert (South Australia), representing less than 1 percent of Australia's total land area, covering 15,250 square kilometers (5,888 square miles); and the Pedirka Desert (South Australia), representing less than 1 percent of Australia's total land area, covering 1,250 square kilometers (482 square miles).

9 FLAT AND ROLLING TERRAIN

Rimming the southern edge of the Western Plateau is the Nullarbor Plain, a flat lowland region of limestone along the Great Australian Bight. (Nullarbor comes from the Latin, meaning "no trees.")

The Central Plains, also called the Central Eastern Lowlands or the Interior Lowlands, rest on large horizontal deposits of sedimentary rock, and run from the Gulf of Carpentaria in the north to western Victoria. Lake Eyre, the nation's lowest point, lies in this region.

There are rolling hills on the west coast near Perth. Other hilly areas lie near Adelaide in South Australia, and in the Eastern Highlands.

10 MOUNTAINS AND VOLCANOES

Australia is one of the flattest continents on Earth. The summit (highest point) of the highest mountain, Mount Kosciusko (2,229 meters/7,314 feet) in the southeast, can be reached by car. Mount Kosciusko, along with its surrounding plateaus and extinct volcanoes, is in the larger range known generally as the Australian Alps; the specific system that includes Mount Kosciusko is known as the Snowy Mountains.

Geographers use the term Great Divide to describe the mountains that run the length of the country in the east. These mountains are also referred to as the Great Dividing Range. The coastline in this area features deep gorges and high, sheer rock cliffs. Moving north, the highlands gradually decrease in altitude. Along the northeastern coast, the Great Divide also includes the Eastern Highlands, where the elevation is just over 900 meters (3,000 feet).

The Western Plateau features several mountain ranges. At the far western edge lies the highest of these, the Hamersley Range, which includes a peak that exceeds 1,219 meters (4,000 feet). Extending to the eastern edge of the Western Plateau are the Macdonnell Range, the Musgrave Range, and the Petermann Range.

All three ranges run from east to west and are characterized by deep gorges. The Macdonnell and Musgrave Ranges have peaks that rise to almost 1,500 meters (4,900 feet). The Darling Range, named for Sir Ralph Darling, a former governor of New South Wales, lies in the extreme southwest corner of the country. Its highest peak is Mount Cooke (582 meters/ 1,920 feet).

11 CANYONS AND CAVES

A network of caves punctuate the Nullarbor Plain. Among the best known are the Abrakurrie Cave and the Koonalda Caves, huge caves which are situated about 76 meters (250 feet) below ground.

Some of the most spectacular caverns are underwater along the coast. These attract scuba divers from around the world.

12 PLATEAUS AND MONOLITHS

Forming the northern edge of the large Western Plateau, on the northwestern border of the state of Western Australia, lies the Kimberley Plateau, with elevations reaching over 900 meters (3,000 feet).

The western portion of the Western Plateau is generally a flat tableland, with dramatic outcroppings of granite or sandstone. The most well known of these is Uluru, the Aboriginal name for the location formerly known as Ayers Rock. Uluru is the world's largest monolith—a large cylindrical stone outcropping—and is over 335 meters (1,100 feet) high.

In the southwest near the Darling Range, limestone pillars about the size of a person protrude from the surface of a flat, barren plain.

13 MAN-MADE FEATURES

Dams have been built to create water storage reservoirs in every state and territory.

DID YOU KNOW?

The Outback is a popular term that refers to the interior of the country, especially the dry center of the Western Plateau and the northern plains. Australians use the term "the bush" to refer to rural areas, especially wilderness.

Life in the Outback may be compared loosely to the rough cowboy lifestyle of the historic American West. "Outback" was first used to describe remote areas far away from civilization. Now, however, "Outback" refers to a broader picture—a place where men and women struggle to live and work in a challenging environment; "the bush" simply describes the geographical places located far from cities and towns.

14 FURTHER READING

Books

Australia. Des Plaines, IL: Heinemann Library, 1999.

Berendes, Mary. Australia. Chanhassen, MN: Child's World, 1999.

Darian-Smith, Kate. Exploration into Australia. Parsippany, NJ: New Discovery Books, 1996.

Dolce, Laura. Australia. Philadelphia: Chelsea House Publishers, 1999.

Israel, Fred L. Australia: The Unique Continent. Philadelphia: Chelsea House Publishers, 2000.

Lowe, David. Australia. Austin, TX: Raintree Steck-Vaughn, 1997.

McCollum, Sean. Australia. Minneapolis, MN: Carolrhoda Books, 1999.

North, Peter. Welcome to Australia. Milwaukee, WI: Gareth Stevens, 1999.

Williams, Brian, and Brenda Williams. World Book Looks at Australia. Chicago: World Book, 1998.

Periodicals

"Australia's Southern Seas." National Geographic, March 1987, p. 286–319.

Brian, Sarah Jane. "What's Up Down Under." Contact Kids, September 2000, p. 20.

Bryson, Bill. "Australian Outback." National Geographic Traveler, October 1999, p. 86ff.

Gore, Rick. "People Like Us." National Geographic, July 2000, p. 90.

Web Sites

Australia's National Mapping Agency. http://www.auslig.gov.au (accessed March 12, 2003).

Australia Speleological Federation. http://www.caves.org.au/ (accessed March 12, 2003).

Bureau of Meteorology. "Climate Facts." http://www.bom.gov.au/ (accessed March 12, 2003).

SEDIMENT AND SEDIMENTATION

CONCEPT

The materials that make up Earth are each products of complex cycles and interactions, as a study of sediment and sedimentation shows. Sediment is unconsolidated material deposited at or near Earth's surface from a number of sources, most notably preexisting rock. There are three kinds of sediment: chemical, organic, and rock, or clastic sediment. Weathering removes this material from its source, while erosion and mass wasting push it along to a place where it is deposited. After deposition, the material may become a permanent part of its environment, or it may continue to undergo a series of cycles in which it experiences ongoing transformation.

HOW IT WORKS

Transporting Sediment

There are three types of sediment: rocks, or clastic sediment; mineral deposits, or chemical sediment; and organic sediment, composed primarily of organic material. (In this context, the term organic refers to formerly living things and parts or products of living things; however, as discussed in Minerals, the term actually has a much broader meaning.) There are also three general processes involved in the transport of sediment from higher altitudes to lower ones, where they eventually are deposited: weathering, mass wasting, and erosion.

The lines between these three processes are not always clearly drawn, but, in general, the following guidelines apply. When various processes act on the material, causing it to be dislodged from a larger sample (for example, separating a rock from a boulder), this is an example of weathering. Assuming that a rock has been broken apart by weathering, it may be moved farther by mass-wasting processes, such as creep or fall, for which gravity is the driving factor. If the pieces of rock are swept away by a river, high winds, or a glacier (all of which are flowing media), this is an example of erosion.

WEATHERING.

Weathering is divided further into three different types: physical, chemical, and biological. Physical or mechanical weathering takes place as a result of such factors as gravity, friction, temperature, and moisture. Gravity may cause a rock to roll down a hillside, breaking to pieces at the bottom; friction from particles of matter borne by the wind may wear down a rock surface; and changes in temperature and moisture can cause expansion and contraction of materials.

Whereas physical weathering attacks the rock as a whole, chemical weathering involves the breakdown of the minerals or organic materials that make up the rock. Chemical breakdown may lead to the dissolution of the materials in the rock, which then are washed away by water or wind or both, or it may be merely a matter of breaking the materials down into simpler compounds.

Biological weathering is not so much a third type of weathering as it is a manifestation of chemical and physical breakdown caused by living organisms. Suppose, for instance, that a plant grows within a crack in a rock. Over time, the plant will influence physical weathering through its moisture and the steady force of its growth pushing at the walls of the fissure in which it is rooted. At the same time, its specific chemical properties likely will induce decomposition of the rock.

MASS WASTING.

Mass wasting, sometimes known as mass movement, comprises a number of types of movement of earth material, all of them driven by gravity. Creep is the slow downward drift of regolith (unconsolidated material produced by weathering), while slump occurs when a mass of regolith slides over, or creates, a concave surface—that is, one shaped like the inside of a bowl. Slump sometimes is classified as a variety of slide, in which material moves downhill in a fairly coherent mass along a flat or planar surface. Such movements, sometimes called rock slides, debris slides, or landslides, are among the most destructive types of mass wasting.

When a less uniform, or more chaotic, mass of material moves rapidly down a slope, it is called flow. Flow is divided into categories, depending on the specific amounts of water: granular flows (0-20% water) and slurry flows (20-40% water), the fastest varieties of which are debris avalanche and mudflow, respectively. Mudflows can be more than 60 mi. (100 km) per hour, while debris avalanches may achieve speeds of 250 mi. (400 km) per hour.

Even these high-speed varieties of mass wasting entail movement along slopes that are considerably less than 90°, whereas a final variety of mass wasting, that is, fall, takes place at angles almost perpendicular to the ground. Typically the bottom of a slope or cliff contains accumulated talus, or fallen rock material. Nor is fall limited to rock fall: debris fall, which is closely related, includes soil, vegetation, and regolith as well as rocks. (For more on these subjects, see Mass Wasting.)

Erosion

Erosion typically is caused either by gravity (in which case it is generally known as mass wasting, discussed earlier) or by flowing media, such as water, wind, and even ice in glaciers. It removes sediments in one of three ways: by the direct impact of the agent (i.e., the flowing media that is discussed in the following sections); by abrasion, another physical process; or by corrosion, a chemical process.

In the case of direct impact, the wind, water, or ice removes sediment, which may or may not be loose when it is hit. On the other hand, abrasion involves the impact of solid earth materials carried by the flowing agent rather than the impact of the flowing agent itself. For example, sand borne by the wind, as discussed later, or pebbles carried by water may cause abrasion. Corrosion is chemical and is primarily a factor only in water-driven, as opposed to wind-driven or ice-driven, erosion. Streams slowly dissolve rock, removing minerals that are carried downstream by the water.

WATER.

Of the fluid substances driving erosion, liquid water is perhaps the one most readily associated in most people's minds with erosion. In addition to the erosive power of waves on the seashore, there is the force exerted by running water in creeks, streams, and rivers. As a river moves, pushing along sediment eroded from the streambeds or riverbeds, it carves out deep chasms in the bedrock beneath.

Moving bodies of water continually reshape the land, carrying soil and debris down slopes, from the source of the river to its mouth, or delta. A delta is a region formed when a river enters a larger body of water, at which point the reduction in velocity on the part of the river current leads to the widespread deposition of sediment. It is so named because its triangular shape resembles that of the Greek letter delta, Δ .

Water at the bottom of a large body, such as a pond or lake, also exerts erosive power; then there is the influence of falling rain. Assuming that the ground is not protected by vegetation, raindrops can loosen particles of soil, sending them scattering in all directions. A rain that is heavy enough may dislodge whole layers of top-soil and send them rushing away in a swiftly moving current. The land left behind may be rutted and scarred, much of its best soil lost for good.

ICE.

Ice, of course, is simply another form of water, but since it is solid, its physical properties are quite different. It is a solid rather than a fluid, such as liquid water or air (the physical sciences treat gases and liquids collectively as "fluids"), yet owing to the enormous volume of ice in glaciers, these great masses are capable of flowing. Glaciers do not flow in the same way as a fluid does; instead, they are moved by gravity, and like giant bulldozers made of ice, they plow through rock, soil, and plants.

Under certain conditions a glacier may have a layer of melted water surrounding it, which greatly enhances its mobility. Even without such lubricant, however, these immense rivers of ice move steadily forward, gouging out pieces of bedrock from mountain slopes, fashioning deep valleys, removing sediment from some regions and adding it to others. In unglaciated areas, or places that have never experienced any glacial activity, sediment is formed by the weathering and decomposition of rock. On the other hand, formerly glaciated areas are distinguished by layers of till, or glacial sediment, from 200 to 1,200 ft. (61-366 m) thick.

WIND.

The processes of wind erosion sometimes are called eolian processes, after Aeolus, the Greek god of the winds. Eolian erosion is in some ways less forceful than the erosive influence of water. Water, after all, can lift heavier and larger particles than can the winds. Wind, however, has a much greater frictional component in certain situations. This is particularly true when the wind carries sand, every grain of which is like a cutting tool.

Wind erosion, in fact, is most pronounced in precisely those places where sand abounds, in deserts and other areas that lack effective ground cover in the form of solidly rooted, prevalent vegetation. In some desert regions the bases of rocks or cliffs have been sandblasted, leaving a mushroom-shaped formation owing to the fact that the wind could not lift the fine grains of sand very high.

Sediment Load

Eroded particles become part of what is called the sediment load transported by the fluid medium. Sediment load falls into three categories: dissolved load, suspended load, and bed load. The amount of each type of load that a fluid medium is capable of carrying depends on the density of the fluid medium itself: in other words, wind can carry the least of each and ice the most.

The wind does not carry any dissolved load, since solid particles (unlike gases) cannot be dissolved in air. Ice or water, on the other hand, is able to dissolve materials, which become invisible within them. Typically, about 90% of the dissolved load in a river is accounted for by five different ions, or atoms that carry a net electric charge: the anions (negative ions) chloride, sulfate, and bicarbonate and the cations (positive ions) of sodium and calcium.

Suspended load is sediment that is suspended, or floating, in the erosive medium. In this instance, wind is just as capable as water or ice of suspending particles of the sediment load, which are likely to color the medium that carries them. Hence, water or wind carrying suspended particles is usually murky. The thicker the medium, the larger the particles it is capable of suspending. In other words, ice can suspend extremely large pieces of sediment, whereas water can suspend much more modest ones. Wind can suspend only tiny particles.

Then there is bed load, large sediment that never becomes suspended but rather is almost always in contact with the substrate or bottom, whether "the bottom" is a streambed or the ground itself. Instead of being lifted up by the medium, bed load is nudged along, rolling, skipping, and sliding as it makes its way over the substrate. Once again, the density of the medium itself has a direct relationship to the size of the bed load it is capable of carrying. Wind rarely transports bed load thicker than fine sand, and water usually moves only pebbles, though under flood conditions it can transport boulders. As with suspended load, glaciers can transport virtually any size of bed load.

Sediment Sizes and Shapes

Geologists and sedimentologists use certain terms to indicate sizes of the individual particles in sediment. Many of these terms are familiar to us from daily life, but whereas people typically use them in a rather vague way, within the realm of sedimentology they have very specific meanings. Listed below are the various sizes of rock, each with measurements or measurement ranges for the rock's diameter:

• Clay: Smaller than 0.00015 in. (0.004 mm)
• Silt: 0.00015 in. (0.004 mm) to 0.0025 in. (0.0625 mm)
• Sand: 0.0025 in. (0.0625 mm) to 0.08 in. (2 mm)
• Pebble: 0.08 in. (2 mm) to 2.5 in. (64 mm)
• Cobble: 2.5 in. (64 mm) to 10 in. (256 mm)
• Boulder: Larger than 10 in. (256 mm).

This listing is known as the Udden-Went-worth scale, which was developed in 1898 by J.A. Udden (1859-1932), an American sedimentary petrologist (a scientist who studies rocks). In 1922 the British sedimentary petrologist C. K. Wentworth) expanded Udden's scale, adapting the definitions of various particle sizes to fit more closely with the actual usage and experience of researchers in the field. The scale uses modifiers to pinpoint the relative sizes of particles. In ascending order of size, these sizes are very fine, fine, medium, coarse, and very coarse.

REAL-LIFE APPLICATIONS

Sediments and Dust Bowls

Sediment makes possible the formation of soil, which of course is essential for growing crops. Therefore it is a serious matter indeed when wind and other forces of erosion remove sediment, creating dust-bowl conditions. The term "Dust Bowl," with capital letters, refers to the situation that struck the United States Great Plains states during the 1930s, devastating farms and leaving thousands of families without home or livelihood. (See Erosion for much more about the Dust Bowl.)

During the late 1990s, some environmentalists became concerned that farming practices in the western United States were eroding sediment, putting in place the possibility of a return to the conditions that created the Dust Bowl. However, in August 1999, the respected journal Science reported studies showing that sediment in farmlands was not eroding at anything like the rate that had been feared. Soil scientist Stanley Trimble at the University of California, Los Angeles, studied Coon Creek, Wisconsin, and its tributaries, a watershed for which 140 years' worth of erosion data were available. As Trimble discovered, the rate of sediment erosion in the area had dramatically decreased since the 1930s, and was now at 6% of the rate during the Dust Bowl years.

Some studies from the 1970s onward had indicated that farming techniques, designed to improve the crop output from the soil, had created a situation in which sediment was being washed away at alarming rates. However, if such sediment removal were actually taking place, there would have to be some evidence—if nothing more, the sediment that had been washed away would have had to go somewhere. Instead, as Trimble reported," We found that much of the sediment in Coon Creek doesn't move very far, and that it moves in complex ways." The sediment, as he went on to explain, was moving within the Coon Creek basin, but the amount that actually made it to the Mississippi River (which could be counted as true erosion, since it was removing sediment from the area) had stayed essentially the same for the past 140 years.

Deposition and Depositional Environments

Eventually everything in motion—including sediment—comes to rest somewhere. A piece of sediment traveling on a stream of water may stop hundreds of times, but there comes a point when it comes to a complete stop. This process of coming to rest is known as deposition, which may be of two types, mechanical or chemical. The first of these affects clastic and organic sediment, while the second applies (fittingly enough) to chemical sediment.

In mechanical deposition, particles are deposited in order of their relative size, the largest pieces of bed load coming to a stop first. These large pieces are followed by medium-size pieces and so on until both bed load and suspended load have been deposited. If the sediment has come to a full stop, as, for instance, in a stagnant pool of water, even the finest clay suspended in the water eventually will be deposited as well.

Unlike mechanical deposition, chemical deposition is not the result of a decrease in the velocity of the flow; rather, it comes about as a result of chemical precipitation, when a solid particle crystallizes from a fluid medium. This often happens in a saltwater environment, where waters may become overloaded with salt and other minerals. In such a situation, the water is unable to maintain the minerals in a dissolved state (i.e., in solution) and precipitates part of its content in the form of solids.

DEPOSITIONAL ENVIRONMENTS.

The matter of sediment deposition in water is particularly important where reservoirs are concerned, since in that case the water is to be used for drinking, cooking, bathing, and other purposes by humans. One of the biggest problems for the maintenance of clean reservoirs is the transport of sediment from agricultural areas, in which the soil is likely to contain pesticides and other chemicals, including the phosphorus found in fertilizer. A number of factors, including precipitation, topography, and land use, affect the rate at which sediment is deposited in reservoirs.

The area in which sediment is deposited is known as its depositional environment, of which there are three basic varieties: terrestrial, marginal marine, and marine. These are, respectively, environments on land (and in landlocked waterways, such as creeks or lakes), along coasts, and in the open ocean. A depositional environment may be a large-scale one, known as a regional environment, or it may be a smaller subenvironment, of which there may be hundreds within a given regional environment.

SEDIMENTARY STRUCTURES.

There are many characteristic physical formations, called sedimentary structures, that sediment forms after it has reached a particular depositional environment. These formations include bedding planes and beds, channels, cross-beds, ripples, and mud cracks. A bed is a layer, or stratum, of sediment, and bedding planes are surfaces that separate beds. The bedding plane indicates an interruption in the regular order of deposition. (These are concepts that also apply to the field of stratigraphy. For more on that subject, see the essay Stratigraphy.)

Channels are simply depressions in a bed that reflect the larger elongated depression made by a river as it flows along its course. Cross-beds are portions of sediment that are at an angle to the beds above and below them, as a result of the action of wind and water currents—for example, in a flowing stream. As for ripples, they are small sandbar-like protuberances that form perpendicular to the direction of water flow. At the beach, if you wade out into the water and look down at your feet, you are likely to see ripples perpendicular to the direction of the waves. Finally, mud cracks are the sedimentary structures that remain when water trapped in a muddy pool evaporates. The clay, formerly at the bottom of the pool, begins to lose its moisture, and as it does, it cracks.

The Impact of Sediment

It is estimated that the world's rivers carry as much as 24 million tons (21,772,800 metric tons) of sediment to the oceans each year. There is also the sediment carried by wind, glaciers, and gravity. Where is it all going? The answer depends on the type of sediment. Clastic and organic sediment may wind up in a depositional environment and experience compaction and cementation in the process of becoming sedimentary rock. (For more on sedimentary rock, see Rocks.)

On the other hand, clastic and organic particles may be buried, but before becoming lithified (turned to rock), they once again may be exposed to wind and other forces of nature, in which case they go through the entire cycle again: weathering, erosion, transport, deposition, and burial. This cycle may repeat many times before the sediment finally winds up in a permanent depositional environment. In the latter case, particles of clastic and organic sediment ultimately may become part of the soil, which is discussed elsewhere in this book (See Soil).

A chemical sediment also may become part of the soil, or it may take part in one or more biogeochemical cycles (also discussed elsewhere; see Biogeochemical Cycles). These chemicals may wind up as water in underground reservoirs, as ice at Earth's poles, as gases in the atmosphere, as elements or compounds in living organisms, or as parts of rocks. Indeed, all three types of sediment—clastic, chemical, and organic—are part of what is known as the rock cycle, whereby rocks experience endlessly repeating phases of destruction and renewal. (See Rocks for more details.)

Sedimentary Mineral Deposits

Among the most interesting aspects of sediment are the mineral deposits it contains—deposits that may, in the case of placer gold, be of significant value. A placer deposit is a concentration of heavy minerals left behind by the effect of gravity on moving particles, and since gold is the densest of all metals other than uranium (which is even more rare), it is among the most notable of placer deposits.

Of course, the fact that gold is valuable has done little to hurt, and a great deal to help, human fascination with placer gold deposits. Placer gold played a major role from the beginning of the famous California Gold Rush (1848-49), which commenced with discovery of a placer deposit by prospector James Marshall on January 24, 1848, along the American River near the town of Coloma. This discovery not only triggered a vast gold rush, as prospectors came from all over the United States in search of gold, but it also proved a major factor in the settlement of the West. Most of the miners who went to the West failed to make a fortune, of course, but instead they found something much better than gold: a gorgeous, fertile land like few places in the United States—California, a place that today holds every bit as much allure for many Americans as it did in 1848.

Despite the attention it naturally attracts, gold is far from the only placer mineral. Other placer minerals, all with a high specific gravity (density in comparison to that of water), include platinum, magnetite, chromite, native copper, zircon, and various gemstones. Nor are placer minerals found only in streams and other flowing bodies of water; wave action and shore currents can leave behind what are called beach placers. Among the notable beach placers in the world are gold deposits near Nome, Alaska, as well as zircon in Brazil and Australia, and marine gravel near Namaqualand, South Africa, which contains diamond particles.

An entirely different process can result in the formation of evaporites, minerals that include carbonates, gypsum, halites, and magnesium and potassium salts. (These specific mineral types are discussed in Minerals.) Formed when the evaporation of water leaves behind ionic, or electrically charged, chemical compounds, evaporites sometimes undergo physical processes similar to those of clastic sediment. They may even have graded bedding, meaning that the heavier materials fall to the bottom. In addition to their usefulness in industry and commerce (e.g., the use of gypsum in sheetrock for building), physical and chemical aspects of evaporites also provide scientists with considerable information regarding the past climate of an area.

WHERE TO LEARN MORE

Cherrington, Mark. Degradation of the Land. New York: Chelsea House, 1991.

"Erosion—Fast or Slow or None?" (Web site). <http://www.crcwater.org/issues9/19990828erosion.html>.

Middleton, Nick. Atlas of Environmental Issues. Illus. Steve Weston and John Downes. New York: Facts on File, 1989.

Schneiderman, Jill S. The Earth Around Us: Maintaining a Livable Planet. New York: W. H. Freeman, 2000.

Snedden, Robert. Rocks and Soil. Illus. Chris Fairclough. Austin, TX: Raintree Steck-Vaughn, 1999.

Soil Erosion and Sedimentation in the Great Lakes Region (Web site). <http://www.great-lakes.net/envt/pollution/erosion.html>.

Sediment/Soil Quality Related Links (Web site). <http://response.restoration.noaa.gov/cpr/sediment/sedlinks.html>.

State of the Land—Sedimentation and Water Quality (Web site). <http://www.nhq.nrcs.usda.gov/land/env/wq4.html>.

USDA-ARS-National Sedimentation Laboratory. United States Department of Agriculture, Agriculture Research Laboratory National Sedimentation Labora tory (Web site). <http://www.sedlab.olemiss.edu/>.

USGS Sediment Database. United States Geological Sur vey (Web site). <http://webserver.cr.usgs.gov/sediment/>.

Wyler, Rose. Science Fun with Mud and Dirt. Illus. Pat Ronson Stewart. New York: Julian Messner, 1986.

KEY TERMS

BED LOAD:

Sediment that is capable of being transported by an erosive medium (wind, water, or air) but only under conditions in which it remains in nearly constant contact with the substrate or bottom (e.g., a streambed or the ground). Bedload, along with dissolved load and suspended load, is one of three types of sediment load.

COMPOUND:

A substance made up of atoms of more than one element chemically bonded to one another.

CONSOLIDATION:

A process whereby materials become compacted, or experience an increase in density. This takes place through a number of processes, including recrystallization and cementation.

DEPOSITION:

The process wherebysediment is laid down on the Earth's surface.

DIAGENESIS:

A term referring to all the changes experienced by a sediment sample under conditions of low temperature and low pressure following deposition.

DISSOLVED LOAD:

Sediment load that is absorbed completely by the erosive medium (either water or ice) that carries it. Dissolved load is one of three types of sediment load, the others being suspended load and bed load.

EROSION:

The movement of soil and rock due to forces produced by water, wind, glaciers, gravity, and other influences. In most cases, a fluid medium, such as air or water, is involved.

FLUID:

In the physical sciences, the term fluid refers to any substance that flows and therefore has no definite shape—that is, both liquids and gases. Occasionally, substances that appear to be solid (for example, ice in glaciers), in fact, are flowing slowly; therefore, within the earth sciences, ice often is treated as another fluid medium.

ION:

An atom or group of atoms that has lost or gained one or more electrons and thus has a net electric charge. Positively charged ions are called cations, and negatively charged ones are called anions.

MASS WASTING:

The transfer of earth material down slopes by processes that include creep, slump, slide, flow, and fall. Also known as mass movement.

MINERAL:

A naturally occurring, typically inorganic substance with a specific chemical composition and a crystalline structure.

ORGANIC:

At one time, chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide.

PRECIPITATION:

In the context of chemistry, precipitation refers to the formation of a solid from a liquid.

REGOLITH:

A general term describing a layer of weathered material that rests atopbedrock.

ROCK:

An aggregate of minerals or organic matter, which may be consolidated or unconsolidated.

SEDIMENT:

Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock. There are three types of sediment: rocks, or clastic sediment; mineral deposits, or chemical sediment; and organic sediment, composed primarily of organic material.

SEDIMENTARY ROCK:

One of the three major types of rock, along with igneous and metamorphic rock. Sedimentary rock usually is formed by the deposition, compaction, and cementation of rock that has experienced weathering. It also may be formed as a result of chemical precipitation.

SEDIMENTATION:

The process of erosion, transport, and deposition undergone by sediment.

SEDIMENT LOAD:

A term for the particles transported by a flowing medium of erosion (wind, water, or ice). The types of sediment load are dissolved load, suspended load, and bed load.

SEDIMENTOLOGY:

The study and interpretation of sediments, including sedimentary processes and formations.

SUSPENDED LOAD:

Sediment that is suspended, or floating, in the erosive medium (wind, water, or ice). Suspended load is one of three types of sediment load, along with dissolved load and bed load.

TILL:

A general term for the sediments left by glaciers that lack any intervening layer of melted ice.

UNCONSOLIDATED ROCK:

Rock that appears in the form of loose particles, such as sand.

WEATHERING:

The breakdown of rocks and minerals at or near the surface of Earth due to physical, chemical, or biological processes.

Sediment and Sedimentation

Weathering

Erosion and transport

Agents of erosion and transport Gravity

Water

Wind

Glacial ice

Sediment erosion

Sediment size

Sediment load

Rounding and sorting of sediment

Deposition Mechanical deposition

Chemical deposition

Depositional environments

Sedimentary structures

The fate of sediments

Environmental impacts of sedimentation

Resources

Sediment consists of loose earth material such as sand that accumulates on the land surface, in river and lake beds, and on the ocean floor. Sediments form by weathering of rock. They then erode from the site of weathering and are transported by wind, water, ice, and mass wasting, all operating under the influence of gravity. Eventually sediment settles out and accumulates after transport; this process is known as deposition. Sedimentation is a general term for the processes of erosion, transport, and deposition. Sedimentology is the study of sediments and sedimentation.

There are three basic types of sediment: rock fragments, or clastic sediments; mineral deposits, or chemical sediments; and rock fragments and organic matter, or organic sediments. Dissolved minerals form by weathering rocks exposed at Earth’s surface. Organic matter is derived from the decaying remains of plants and animals.

Weathering

Clastic and chemical sediments form during weathering of bedrock or pre-existing sediment by both physical and chemical processes. Organic sediments are also produced by a combination of physical and chemical weathering. Physical (or mechanical) weathering—the disintegration of Earth materials— is generally caused by abrasion or fracturing, such as the striking of one pebble against another in a river or stream bed, or the cracking of a rock by expanding ice. Physical weathering produces clastic and organic sediment.

Chemical weathering, or the decay and dissolution of Earth materials, is caused by a variety of processes. However, it results primarily from interactions between water and rock. Chemical weathering may alter the mineral content of a rock by either adding or removing certain chemical components. Some mineral by-products of chemical weathering are dissolved by water and transported below ground or to an ocean or lake in solution. Later, these dissolved minerals may precipitate out, forming deposits on the roof of a cave (as stalactites ), or the ocean floor. Chemical weathering produces clastic, chemical, and organic sediments.

Erosion and transport

Erosion and transport of sediments from the site of weathering are caused by one or more of the following agents: gravity, wind, water, or ice. When gravity

acts alone to move a body of sediment or rock, the process is known as mass wasting. When the forces of wind, water, or ice act to erode sediment, they always do so under the influence of gravity.

Agents of erosion and transport Gravity

Large amounts of sediment, with individual particles ranging in size from mud to boulders, can move down slope due to gravity, a process called mass wasting. Rock falls, landslides, and mudflows are common types of mass wasting. Rock falls occur when rocks in a cliff face are loosened by weathering, break loose, and roll and bounce downslope. Landslides consist of downslope movement of a mass of rock or soil, in many cases because the soil or rock becomes saturated with water (for example, during heavy rainstorms or rainy seasons ). Debris flows occur when a hillside becomes nearly saturated by heavy rainfall, a landslide occurs, and the slide mass is transformed into a mass of liquefied mud and boulders quickly moves downslope.

Water

Water is the most effective agent of transport, even in the desert. Water can erode as channelized flow as occurs in streams, sheet flow that occurs over open soil or rock, or as a consequence of raindrop impact. The less vegetation that is present, the more water erodes— as droplets, in sheets, or as channelized flow.

Wind

Wind is a significant agent of erosion where little or no vegetation is present. For this reason, deserts are well known for their wind erosion. Even in deserts, however, infrequent but powerful rain storms are the most important agent of erosion. This is because relatively few areas of the world have strong prevailing winds with little vegetation, and because wind can rarely move particles larger than sand or silt.

Glacial ice

Glaciers are very effective at eroding and transporting material of all sizes, and continental glaciers can move boulders as large as a house hundreds of miles.

At times in the geologic past, continent-sized glaciers covered large areas of Earth at middle to high latitudes. Today, continental glaciers occur only on Antarctica and Greenland. Smaller alpine glaciers exist at high altitudes on some mountains.

Sediment erosion

Generally, erosive agents remove sediments from the site of weathering in one of three ways: impact of the agent, abrasion (both types of mechanical erosion, or corrasion ), or corrosion (chemical erosion ). The impact of wind, water, and ice erodes sediments. For example, flowing water exerts a force on sediments particles that allows them to be lifted and transported by water, wind, or flowing ice. The eroded sediments may already be loose, or they may be torn away from the rock surface by the force of the agent. If the flow is strong enough, clay, silt, sand, and even gravel, can be eroded in this way.

Abrasion is the second mechanism of sediment erosion. Abrasion is simply the removal of one Earth material by the impact of another. Rockhounds smooth stones by tumbling them in a container with hard sand or silt particles known as abrasives. Sandpaper is another everyday abrasive. In nature, when water (or wind or ice) flows over a rocky surface (for example, a stream bed), sedimentary particles that are being transported by the flow strike the surface and occasionally knock particles loose. Keep in mind that while the bedrock surface is abraded and pieces are knocked loose, the particles in transport are also abraded, becoming rounder and smoother with time.

Corrosion, or chemical erosion, the third erosional mechanism, is the dissolution of rock or sediment by the agent of transport. Wind is not capable of corrosion, and corrosion by ice is a much slower process than by liquid water. Corrosion in streams slowly dissolves the bedrock or sediments, producing mineral solutions (minerals dissolved in water) and aiding in the production of clastic sediments by weakening rock matrix.

Sediment size

Sediment particles come in all shapes and sizes. Sediment sizes are classified by separating them into a number of groups, based on metric measurements, and naming them using common terms and size modifiers. The terms, in order of decreasing size, are boulder (> 256 mm), cobble (256-64 mm), pebble (64-2 mm), sand (2-1/16 mm), silt (1/16-1/256 mm), and clay (1/256 mm). The modifiers, in decreasing size, are very coarse, coarse, medium, fine, and very fine. For example, sand is sediment that ranges in size from 2 millimeters to 1/16 mm. Very coarse sand ranges from 2 mm to 1 mm; coarse from 1 mm to 1/2 mm; medium from 1/2 mm to 1/4 mm; fine from 1/4 mm to 1/8 mm; and very fine from 1/8 mm to 1/16 mm. The entire classification is not as consistent as the terminology for sand—and not every group includes size modifiers (Figure 1).

Sediment load

When particles are eroded and transported by wind, water, or ice, they become part of the sediment load. There are three categories of load that may be transported by an erosional agent: dissolved load, suspended load, and bedload. Wind is not capable of dissolving minerals, and so it does not transport any dissolved load. The dissolved load in water and ice is not visible; to be deposited, it must be chemically precipitated.

Sediment can be suspended in wind, water, or ice. Suspended sediment makes stream water cloudy or turbid after a rainstorm and makes wind storms dusty. Suspended sediment is sediment that is not continuously in contact with the underlying surface (a stream bed or the desert floor) and is suspended within the medium of transport. Generally, the smallest particles of sediment are likely to be suspended; occasionally sand is suspended by powerful winds and pebbles are suspended by floodwater. However, because ice is a solid, virtually any size sediment can be part of the suspended sediment load of a glacier. Debris flows, which are dense mixtures of solids and water, can also transport large boulders.

Bedload consists of the larger sediment that is only sporadically transported. Bedload remains in almost continuous contact with the bottom, and moves by rolling, skipping, or sliding along the bottom. Pebbles on a river bed or beach are examples of bedload. Wind, water, and ice can all transport bedload, however, the size of sediment in the bedload varies greatly among these three transport agents.

Because of the low density of air, wind only rarely moves bedload coarser than fine sand. Some streams transport pebbles and coarser sediment only during floods, while other streams may transport, on a daily basis, all but boulders with ease.

Floods greatly increase the power of streams and their ability to transport large boulders. Flooding also may cause large sections of a riverbank to be washed into the water and become part of its load. Bank erosion during flood events by a combination of abrasion, hydraulic impact, and mass wasting is often a significant source of a stream’s load. Ice in glaciers, because it is a solid, can transport virtually any size material if the ice is sufficiently thick.

For a particular agent of transport, its ability to move coarse sediments as either bedload or suspended load is dependant on its velocity. The higher the velocity, the coarser the load that can be transported.

Rounding and sorting of sediment

Transport of sediments causes them to become rounder as their irregular edges are removed both by abrasion and corrosion. Beach sand, for example, becomes highly rounded due to its endless rolling and bouncing in the surf.

Sorting, or separation of clasts into similar sizes, also occurs during sediment transport. Sorting occurs because the size of grains that a medium of transport can move is limited by the medium’s velocity and density. For example, in a stream on a particular day, water flow may only be strong enough to transport grains that are finer than medium-grained sand. So all clasts on the surface of the stream bed that are equal to or larger than medium sand will be left behind. The sediment, therefore, becomes sorted. Beach sand is very well sorted because coarser grains are only rarely transported up the beach face by the approaching waves, and finer material is suspended and carried away by the surf.

Ice is the poorest sorter of sediment. Glaciers can transport almost any size sediment easily, and when ice flow slows down or stops, the sediment is not deposited, due to the density of the ice. As a result, sediments deposited directly by ice when it melts are usually very poorly sorted. Significant sorting only occurs in glacial sediments that are subsequently transported by melt-water from the glacier. Wind, on the other hand, is the best sorter of sediment, because it can usually only transport sediment that ranges in size from sand to clay. Occasional variation in wind speed during transport serves to further sort out these sediment sizes.

Deposition Mechanical deposition

When the velocity of the transport medium is insufficient to move a sediment particle, the particle is deposited. When velocity decreases in wind or water, larger sediments are deposited first. Sediments that were part of the suspended load will drop out and

become part of the bed load. If velocity continues to drop, nearly all bedload movement will cease, and only clay and the finest silt will be left suspended. In still water, even the clay will be deposited, over the next day or so, based on size—from largest clay particles to the smallest.

During its journey from outcrop to ocean, a typical sediment grain may be temporarily deposited, transported, and erorded thousands of times. When compacted fine-grained clay deposits are subjected to stream erosion, they are nearly as difficult to erode as pebbles and boulders (Figure 2). Because the clay particles are electrostatically attracted to one another, they resist erosion as well as much coarser grains. This is significant, for example, when comparing the erod-ibility of stream bank materials—clay soils in a river bank are fairly resistant to erosion, whereas sandy soils are not.

Eventually, transported sediment reaches a resting place where it remains long enough to be buried by other sediments. This is known as the depositional environment of the sediment.

Chemical deposition

Unlike clastic and organic sediment, chemical sediment can not simply be deposited by a decrease in water velocity. Chemical sediment must crystallize from the solution, that is, it must be precipitated. A common way for precipitation to occur is by evaporation. As water evaporates from the surface, if it is not replaced by water from another source (rainfall or a stream) any dissolved minerals in the water will become more concentrated until they begin to precipitate out of the water and accumulate on the bottom. This often occurs in the desert in salt pans or lakes. It may also occur along the sea coast in salt marshes.

Another mechanism that triggers mineral precipitation is a change in water temperature. When ocean water with different temperatures mixes, the end result may be sea water in which the concentration of dissolved minerals is higher than can be held in solution at that water temperature, and minerals will precipitate. For most minerals, their tendency to precipitate increases with decreasing water temperature. However, for some minerals, calcite (calcium carbonate) for example, the reverse is true.

Minerals may also be forced to precipitate by the biological activity of certain organisms. For example, when algae remove carbon dioxide from water, this decreases the acidity of the water, promoting the precipitation of calcite. Some marine organisms use this reaction, or similar chemical reactions, to promote mineral precipitation and use the minerals to form their skeletons. Clams, snails, hard corals, sea urchins, and a large variety of other marine organisms form their exoskeletons by manipulating water chemistry in this way.

Depositional environments

Landscapes form and constantly change as a consequence of weathering and sedimentation. The area where sediment accumulates and is later buried by other sediments is known as its depositional environment. There are many large-scale, or regional, environments of deposition, as well as hundreds of smaller subenvironments within these regions. For example, rivers are regional depositional environments. Some span distances of hundreds of miles and contain a large number of sub-environments, such as channels, backswamps, floodplains, abandoned channels, and sand bars. These depositional sub-environments can also be thought of as depositional landforms, that is, landforms produced by deposition rather than erosion.

Depositional environments are often separated into three general types, or settings: terrestrial (land), marginal marine (coastal), and marine (open ocean). Examples of each of these three regional depositional settings are as follows: terrestrial-alluvial fans, glacial valleys, lakes; marginal marin-beaches, deltas, estuaries, tidal mud and sand flats; marine-coral reefs, abyssal plains, continental slope.

Sedimentary structures

During deposition of sediments physical structures form that are indicative of the conditions that created them. These are known as sedimentary structures. They may provide information about water depth, current speed, environmental setting (for example, marine versus fresh water) or a variety of other factors. Among the more common of these are: bedding planes, beds, channels, cross-beds, ripples, and mud cracks.

Bedding planes are the surfaces separating layers of sediment, or beds, in an outcrop of sediment or rock. The beds represent episodes of sedimentation, while the bedding planes usually represent interruptions in sedimentation, either erosion or simply a lack of deposition. Beds and bedding planes are the most common sedimentary structures.

Rivers flow in elongated depressions called channels. When river deposits are preserved in the sediment record (for example as part of a delta system), channels also are preserved. These channels appear in rock outcrops as narrow to broad, v- or u-shaped, depressions at the base of otherwise flat beds.

Submerged bars along a coast or in a river form when water currents or waves transport large volumes of sand or gravel along the bottom. Similarly, wind currents form dunes from sand on a beach or a desert.

While these depositional surface features, or bedforms, build up in size, they also migrate in the direction of water or wind flow. This is known as bar or dunemigration. Suspended load or bedload material moves up the shallowly inclined, upwind or upcurrent (stoss) side and falls over the crest of the bedform to the steep, downwind or downcurrent (lee) side. A vertical cut through the bedform perpendicular to its long axis (from the stoss to the lee side) show a sequence of inclined beds of sediment, called cross-beds, that are the preserved leeward faces of the bedform. In an outcrop, these cross-beds can often be seen stacked one atop another; some may be oriented in opposing directions, indicating a change in current or wind direction.

When a current or wave passes over sand or silt in shallow water, it forms ripples on the bottom. Ripples are small scale versions of dunes or bars. Rows of ripples form perpendicular to the flow direction of the water. When formed by a current, ripples are asymmetrical in cross-section and move downstream by erosion of sediment from the stoss side of the ripple, and deposition on the lee side. Wave-formed ripples on the ocean floor have a more symmetrical profile, because waves move sediments back and forth, not just in one direction. In an outcrop, ripples appear as very small cross-beds, known as cross-laminations, or simply as undulating bedding planes.

When water is trapped in a muddy pool that slowly dries, the slow sedimentation of the clay particles forms a mud layer on the bottom of the pool. As the last of the water evaporates, the moist clay begins to dry up and crack, producing mud cracks as well as variably shaped mud chips known as mud crack polygons. Interpreting the character of any of the sedimentary structures discussed above (for example, ripples) would primarily provide information concerning the nature of the medium of transport. Mud cracks, preserved on the surface of a bed, give some idea of the nature of the depositional environment, specifically that it experienced alternating periods of wet and dry.

The fate of sediments

All clastic and organic sediments suffer one of two fates. Either they accumulate in a depositional environment, become buried, and are lithified (turned to rock by compaction and cementation) to produce sedimentary rock, or they are re-exposed by erosion after burial, but before lithification, and go through one or more new cycles of weathering-erosion-transport-deposition-burial.

Chemical sediments, while still in solution, can instead follow a number of different paths, known as

KEY TERMS

Bedload —The portion of sediment that is transported by rolling, skipping, and hopping along the stream bed at any given time because it is too heavy to be lifted by flowing stream water. It stands in contrast to suspended load.

Bedrock —The unweathered or partially weathered solid rock layer, which is exposed at Earth’s surface or covered by a thin mantle of soil or sediment.

Clay —The finest of sediment particles, less than 1/256 of a millimeter in diameter.

Delta —A landform that develops where a stream deposits sediment at the edge of a standing body of water (lake or sea).

Floodplain —The flat, low-lying area adjacent to a river or stream that becomes covered with water during flooding; flood waters deposit sand, silt and clay on this surface.

Geochemical cycle —A number of interrelated environments or settings through which a chemical can move as a result of changes in state or incorporation into different compounds.

Grain size —The size of a particle of sediment, ranging from clay to boulders; smaller size sediment is called fine grained, larger sediment is coarse grained.

Mass wasting —Movement of large masses of sediment primarily in response to the force of gravity.

Outcrop —A natural exposure of rock at Earth’s surface.

Pebbles —Coarse particles of sediment larger than sand (2 mm) and smaller than boulders (256 mm).

Sand —Sediment particles smaller than pebbles and larger than silt, ranging in size from 1/16 of a millimeter to 2 millimeters.

Sediment —Soil and rock particles that wash off land surfaces and flow with water and gravity toward the sea. On the sea floor, sediment can build up into thick layers. When it compresses under its weight, sedimentary rock is formed.

Sedimentation —The process by which sediment is removed from one place, and transported to another, where it accumulates.

Silt —Soil particles derived mainly from sedimentary materials that range between 0.0002 to 0.05 mm in size.

geochemical cycles. These pathways include ending up as: chemical sedimentary rocks, cement in clastic rocks, parts of living organisms, gases in the atmosphere, ice at the poles, or water in underground reservoirs. Dissolved minerals may remain in these settings for millions of years or quickly move on to another stage in the cycle.

Whether clastic, chemical, or organic, all sediments are part of what is called the rock cycle, an endless series of interrelated processes and products that includes all Earth materials.

Environmental impacts of sedimentation

Erosion, weathering, and sedimentation constantly work together to reshape Earth’s surface. These are natural processes that sometimes require humans to adapt and adjust to changes in the environment. However, human activity can increase sedimentation rates, leading to significant increases in the frequency and severity of certain natural disasters. For example, disturbance by construction and related land development is sometimes a contributing factor in the mudflows and landslides that occur in parts of California. The resulting damage can be costly both in terms of money and lives.

It is estimated that the world’s rivers carry as much as 24 million tons of sediment to the ocean each year. About two-thirds of this may be directly related to human activity, which greatly accelerates the natural rate of erosion. This causes rapid loss of fertile topsoil, which leads to decreased crop productivity.

Increased sedimentation also causes increased size and frequency of flooding. As stream channels are filled in, the capacity of the channel decreases. As a result, streams flood more rapidly during a rainstorm, as well as more often, and they drain less quickly after flooding. Likewise, sedimentation can become a major problem on dammed rivers. Sediment accumulates in the lake created by the dam rather than moving farther downstream and accumulating in a delta. Over time, trapped sediment reduces the size of the lake and the useful life of the dam. In areas that are forested, lakes formed by dams are not as susceptible to this problem. Sedimentation is not as great due to interception of rainfall by the trees and underbrush.

Vegetative cover also prevents soil from washing into streams by holding the soil in place. Without vegetation, erosion rates can increase significantly. Human activity that disturbs the natural landscape and increases sediment loads to streams also disturbs aquatic ecosystems.

Many state and local governments are now developing regulations concerning erosion and sedimentation resulting from private and commercial development. Only by implementing such measures can we hope to curb these and other destructive side effects, thereby preserving the environment as well as our quality of life.

See also Deposit.

Resources

BOOKS

Blatt, H., R. Tracy, and B. Owens. Petrology: Igneous, Sedimentary, and Metamorphic. New York: Freeman, 2005.

Boggs, S., Jr. Principles of Sedimentology and Stratigraphy. 4th ed. Upper Saddle River, New Jersey: Prentice Hall, 2005.

Gyr, A. and K. Hoyer. Sediment Transport: A Geophysical Phenomenon. Berlin: Springer, 2006.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall, 2004.

Clay Harris

Sediment and sedimentation

Sediments are loose Earth materials such as sand that accumulate on the land surface, in river and lake beds, and on the ocean floor. Sediments form by weathering of rock. They then erode from the site of weathering and are transported by wind , water , ice , and mass wasting , all operating under the influence of gravity. Eventually sediment settles out and accumulates after transport; this process is known as deposition. Sedimentation is a general term for the processes of erosion , transport, and deposition. Sedimentology is the study of sediments and sedimentation.

There are three basic types of sediment: rock fragments, or clastic sediments; mineral deposits, or chemical sediments; and rock fragments and organic matter , or organic sediments. Dissolved minerals form by weathering rocks exposed at Earth's surface. Organic matter is derived from the decaying remains of plants and animals.

Weathering

Clastic and chemical sediments form during weathering of bedrock or pre-existing sediment by both physical and chemical processes. Organic sediments are also produced by a combination of physical and chemical weathering. Physical (or mechanical) weathering—the disintegration of Earth materials—is generally caused by abrasion or fracturing, such as the striking of one pebble against another in a river or stream bed, or the cracking of a rock by expanding ice. Physical weathering produces clastic and organic sediment.

Chemical weathering, or the decay and dissolution of Earth materials, is caused by a variety of processes. However, it results primarily from various interactions between water and rock material. Chemical weathering may alter the mineral content of a rock by either adding or removing certain chemical components. Some mineral by-products of chemical weathering are dissolved by water and transported below ground or to an ocean or lake in solution . Later, these dissolved minerals may precipitate out, forming deposits on the roof of a cave (as stalactites), or the ocean floor. Chemical weathering produces clastic, chemical, and organic sediments.

Erosion and transport

Erosion and transport of sediments from the site of weathering are caused by one or more of the following agents: gravity, wind, water, or ice. When gravity acts alone to move a body of sediment or rock, this is known as mass wasting. When the forces of wind, water, or ice act to erode sediment, they always do so under the influence of gravity.

Agents of erosion and transport

Gravity

Large volumes of sediment, ranging in size from mud to boulders, can move downslope due to gravity, a process called mass wasting. Rock falls, landslides, and mudflows are common types of mass wasting. If you have ever seen large boulders on a roadway you have seen the results of a rock fall. Rock falls occur when rocks in a cliff face are loosened by weathering, break loose, and roll and bounce downslope. Landslides consist of rapid downslope movement of a mass of rock or soil , and require that little or no water be present. Mud flows occur when a hillside composed of fine grained material becomes nearly saturated by heavy rainfall. The water helps lubricate the sediment, and a lobe of mud quickly moves downslope. Other types of mass wasting include slump, creep, and subsidence .

Water

Water is the most effective agent of transport, even in the desert . When you think of water erosion, you probably think of erosion mainly by stream water, which is channelized. However, water also erodes when it flows over a lawn or down the street, in what is known as sheet flow. Even when water simply falls from the sky and hits the ground in droplets, it erodes the surface. The less vegetation that is present, the more water erodes—as droplets, in sheets, or as channelized flow.

Wind

You may think of wind as a very important agent of erosion, but it is really only significant where little or no vegetation is present. For this reason, deserts are well known for their wind erosion. However, as mentioned above, even in the desert, infrequent, but powerful rain storms are still the most important agent of erosion. This is because relatively few areas of the world have strong prevailing winds with little vegetation, and because wind can rarely move particles larger than sand or small pebbles.

Glacial ice

Ice in glaciers is very effective at eroding and transporting material of all sizes. Glaciers can move boulders as large as a house hundreds of miles.

If you look around, glaciers are not a very common sight these days. However, at times in the geologic past, continent-sized glaciers covered vast areas of the Earth at middle to high latitudes. Today, continental glaciers occur only on Antarctica and Greenland. In addition, many smaller glaciers exist at high altitudes on some mountains . These are called alpine glaciers.

Sediment erosion

Generally, erosive agents remove sediments from the site of weathering in one of three ways: impact of the agent, abrasion (both types of mechanical erosion, or corrasion), or corrosion (chemical erosion). The mere impact of wind, water, and ice erodes sediments; for example, flowing water exerts a force on sediments causing them to be swept away. The eroded sediments may already be loose, or they may be torn away from the rock surface by the force of the water. If the flow is strong enough, clay, silt, sand, and even gravel, can be eroded in this way.

Abrasion is the second mechanism of sediment erosion. Abrasion is simply the removal of one Earth material by the impact of another. Rock hounds smooth stones by "tumbling" them in a container with very hard sand or silt particles known as abrasives . When you use sand paper to smooth a wood surface, you are using the abrasive qualities of the sand embedded in the paper to erode the wood. In nature, when water (or wind or ice) flows over a rocky surface (for example, a stream bed), sedimentary particles that are being transported by the flow strike the surface, and occasionally knock particles loose. Keep in mind that while the bedrock surface is abraded and pieces are knocked loose, the particles in transport are also abraded, becoming rounder and smoother with time.

Corrosion, or chemical erosion, the third erosional mechanism, is the dissolution of rock or sediment by the agent of transport. Wind is not capable of corrosion, and corrosion by ice is a much slower process than by liquid water. Corrosion in streams slowly dissolves the bedrock or sediments, producing mineral solutions (minerals dissolved in water) and aiding in the production of clastic sediments by weakening rock matrix.

Sediment size

Sediments come in all shapes and sizes. Sediment sizes are classified by separating them into a number of groups, based on metric measurements, and naming them using common terms and size modifiers. The terms, in order of decreasing size, are boulder (> 256 mm), cobble (256-64 mm), pebble (64-2 mm), sand (2-1/16 mm), silt (1/16-1/256 mm), and clay (< 1/256 mm). The modifiers in decreasing size order, are very coarse, coarse, medium, fine, and very fine. For example, sand is sediment that ranges in size from 2 millimeters to 1/16 mm. Very coarse sand ranges from 2 mm to 1 mm; coarse from 1 mm to 1/2 mm; medium from 1/2 mm to 1/4 mm; fine from 1/4 mm to 1/8 mm; and very fine from 1/8 mm to 1/16 mm. Unfortunately, the entire classification is not as consistent as the terminology for sand—not every group includes size modifiers.

Sediment load

When particles are eroded and transported by wind, water, or ice, they become part of the transport medium's sediment load. There are three categories of load that may be transported by an erosional agent: dissolved load, suspended load, and bedload. Wind is not capable of dissolving minerals, and so it does not transport any dissolved load. The dissolved load in water and ice is not visible; to be deposited, it must be chemically precipitated.

Sediment can be suspended in wind, water, or ice. Suspended sediment is what makes stream water look dirty after a rainstorm and what makes a wind storm dusty. Suspended sediment is sediment that is not continuously in contact with the underlying surface (a stream bed or the desert floor) and so is suspended within the medium of transport. Generally, the smallest particles of sediment are likely to be suspended; occasionally sand is suspended by powerful winds and pebbles are suspended by flood waters. However, because ice is a solid, virtually any size sediment can be part of the suspended sediment load of a glacier.

Bedload consists of the larger sediment that is only sporadically transported. Bedload remains in almost continuous contact with the bottom, and moves by rolling, skipping, or sliding along the bottom. Pebbles on a river bed or beach are examples of bedload. Wind, water, and ice can all transport bedload, however, the size of sediment in the bedload varies greatly among these three transport agents.

Because of the low density of air, wind only rarely moves bedload coarser than fine sand. Some streams transport pebbles and coarser sediment only during floods, while other streams may transport, on a daily basis, all but boulders with ease.

Flood water greatly increase the power of streams. For example, many streams can move boulders during flooding . Flooding also may cause large sections of a river bank to be washed into the water and become part of its load. Bank erosion during flood events by a combination of abrasion, hydraulic impact, and mass wasting is often a significant source of a stream's load. Ice in glaciers, because it is a solid, can transport virtually any size material, if the ice is sufficiently thick, and the slope is steep.

For a particular agent of transport, its ability to move coarse sediments as either bedload or suspended load is dependant on its velocity . The higher the velocity, the coarser the load.

Rounding and sorting of sediment

Transport of sediments causes them to become rounder as their irregular edges are removed both by abrasion and corrosion. Beach sand becomes highly rounded due to its endless rolling and bouncing in the surf. Of the agents of transport, wind is most effective at mechanically rounding (abrading) clastic sediments, or clasts. Its low density does not provide much of a "cush ion" between the grains as they strike one another.

Sorting, or separation of clasts into similar sizes, also happens during sediment transport. Sorting occurs because the size of grains that a medium of transport can move is limited by the medium's velocity and density. For example, in a stream on a particular day, water flow may only be strong enough to transport grains that are finer than medium-grained sand. So all clasts on the surface of the stream bed that are equal to or larger than medium sand will be left behind. The sediment, therefore, becomes sorted. The easiest place to recognize this phenomenon is at the beach. Beach sand is very well sorted because coarser grains are only rarely transported up the beach face by the approaching waves, and finer material is suspended and carried away by the surf.

Ice is the poorest sorter of sediment. Glaciers can transport almost any size sediment easily, and when ice flow slows down or stops, the sediment is not deposited, due to the density of the ice. As a result, sediments deposited directly by ice when it melts are usually very poorly sorted. Significant sorting only occurs in glacial sediments that are subsequently transported by meltwater from the glacier. Wind, on the other hand, is the best sorter of sediment, because it can usually only transport sediment that ranges in size from sand to clay. Occasional variation in wind speed during transport serves to further sort out these sediment sizes.

Deposition

Mechanical deposition

When the velocity (force) of the transport medium is insufficient to move a clastic (or organic) sediment particle it is deposited. As you might expect, when velocity decreases in wind or water, larger sediments are deposited first. Sediments that were part of the suspended load will drop out and become part of the bed load. If velocity continues to drop, nearly all bedload movement will cease, and only clay and the finest silt will be left suspended. In still water, even the clay will be deposited, over the next day or so, based on size—from largest clay particles to the smallest.

During its trip from outcrop to ocean, a typical sediment grain may be deposited, temporarily, thousands of times. However, when the transport medium's velocity increases again, these deposits will again be eroded and transported. Surprisingly, when compacted fine-grained clay deposits are subjected to stream erosion, they are nearly as difficult to erode as pebbles and boulders. Because the tiny clay particles are electrostatically attracted to one another, they resist erosion as well as much coarser grains. This is significant, for example, when comparing the erodibility of stream bank materials—clay soils in a river bank are fairly resistant to erosion, whereas sandy soils are not.

Eventually the sediment will reach a final resting place where it remains long enough to be buried by other sediments. This is known as the sediment's depositional environment.

Chemical deposition

Unlike clastic and organic sediment, chemical sediment can not simply be deposited by a decrease in water velocity. Chemical sediment must crystallize from the solution, that is, it must be precipitated. A common way for precipitation to occur is by evaporation . As water evaporates from the surface, if it is not replaced by water from another source (rainfall or a stream) any dissolved minerals in the water will become more concentrated until they begin to precipitate out of the water and accumulate on the bottom. This often occurs in the desert in what are known as salt pans or lakes. It may also occur along the sea coast in a salt marsh.

Another mechanism that triggers mineral precipitation is a change in water temperature . When ocean waters with different temperatures mix, the end result may be sea water in which the concentration of dissolved minerals is higher than can be held in solution at that water temperature, and minerals will precipitate. For most minerals, their tendency to precipitate increases with decreasing water temperature. However, for some minerals, calcite (calcium carbonate ) for example, the reverse is true.

Minerals may also be forced to precipitate by the biological activity of certain organisms. For example, when algae remove carbon dioxide from water, this decreases the acidity of the water, promoting the precipitation of calcite. Some marine organisms use this reaction, or similar chemical reactions , to promote mineral precipitation and use the minerals to form their skeletons. Clams, snails , hard corals, sea urchins , and a large variety of other marine organisms form their exoskeletons by manipulating water chemistry in this way.

Depositional environments

Landscapes form and constantly change due to weathering and sedimentation. The area where a sediment accumulates and is later buried by other sediments is known as its depositional environment. There are many large-scale, or regional, environments of deposition, as well as hundreds of smaller subenvironments within these regions. For example, rivers are regional depositional environments. Some span distances of hundreds of miles and contain a large number of subenvironments, such as channels, backswamps, floodplains, abandoned channels, and sand bars. These depositional subenvironments can also be thought of as depositional landforms, that is, landforms produced by deposition rather than erosion.

Depositional environments are often separated into three general types, or settings: terrestrial (on land), marginal marine (coastal), and marine (open ocean). Examples of each of these three regional depositional settings are as follows: terrestrial-alluvial fans, glacial valleys, lakes; marginal marin-beaches, deltas, estuaries, tidal mud and sand flats; marine-coral reefs, abyssal plains, continental slope.

Sedimentary structures

During deposition of sediments physical structures form that are indicative of the conditions that created them. These are known as sedimentary structures. They may provide information about water depth, current speed, environmental setting (for example, marine versus fresh water) or a variety of other factors. Among the more common of these are: bedding planes, beds, channels, cross-beds, ripples, and mud cracks.

Bedding planes are the surfaces separating layers of sediment, or beds, in an outcrop of sediment or rock. The beds represent episodes of sedimentation, while the bedding planes usually represent interruptions in sedimentation, either erosion or simply a lack of deposition. Beds and bedding planes are the most common sedimentary structures.

Rivers flow in elongated depressions called channels. When river deposits are preserved in the sediment record (for example as part of a delta system), channels also are preserved. These channels appear in rock outcrops as narrow to broad, v- or u-shaped, "bellies" or depressions at the base of otherwise flat beds. Preserved channels are sometimes called cut-outs, because they "cut-out" part of the underlying bed.

Submerged bars along a coast or in a river form when water currents or waves transport large volumes of sand or gravel along the bottom. Similarly, wind currents form dunes from sand on a beach or a desert. While these depositional surface features, or bedforms, build up in size, they also migrate in the direction of water or wind flow. This is known as bar or dune migration . Suspended load or bedload material moves up the shallowly inclined, upwind or upcurrent (stoss) side and falls over the crest of the bedform to the steep, downwind or downcurrent (lee) side. If you cut through the bedform perpendicular to its long axis (from the stoss to the lee side) what you would observe are inclined beds of sediment, called cross-beds, that are the preserved leeward faces of the bedform. In an outcrop, these cross-beds can often be seen stacked one atop another; some may be oriented in opposing directions, indicating a change in current or wind direction.

When a current or wave passes over sand or silt in shallow water, it forms ripples on the bottom. Ripples are actually just smaller scale versions of dunes or bars. Rows of ripples form perpendicular to the flow direction of the water. When formed by a current, these ripples are asymmetrical in cross-section and move downstream by erosion of sediment from the stoss side of the ripple, and deposition on the lee side. Wave-formed ripples on the ocean floor have a more symmetrical profile, because waves move sediments back and forth, not just in one direction. In an outcrop, ripples appear as very small cross-beds, known as cross-laminations, or simply as undulating bedding planes.

When water is trapped in a muddy pool that slowly dries up, the slow sedimentation of the clay particles forms a mud layer on the bottom of the pool. As the last of the water evaporates, the moist clay begins to dry up and crack, producing mud cracks as well as variably shaped mud chips known as mud crack polygons . Interpreting the character of any of the sedimentary structures discussed above (for example, ripples) would primarily provide information concerning the nature of the medium of transport. Mud cracks, preserved on the surface of a bed, give some idea of the nature of the depositional environment, specifically that it experienced alternating periods of wet and dry.

The fate of sediments

All clastic and organic sediments suffer one of two fates. Either they accumulate in a depositional environment, then get buried and lithified (turned to rock by compaction and cementation) to produce sedimentary rock , or they are re-exposed by erosion after burial, but before lithification, and go through one or more new cycles of weathering-erosion-transport-deposition-burial.

Chemical sediments, while still in solution, can instead follow a number of different paths, known as geochemical cycles. These pathways include ending up as: chemical sedimentary rocks, cement in clastic rocks, parts of living organisms, gases in the atmosphere, ice at the poles, or water in underground reservoirs. Dissolved minerals may remain in these settings for millions of years or quickly move on to another stage in the cycle.

Whether clastic, chemical, or organic, all sediments are part of what is called the rock cycle, an endless series of interrelated processes and products that includes all Earth materials.

Environmental impacts of sedimentation

Erosion, weathering, and sedimentation constantly work together to reshape the Earth's surface. These are natural processes that sometimes require us to adapt and adjust to changes in our environment. However, too many people and too much disturbance of the land surface can drastically increase sedimentation rates, leading to significant increases in the frequency and severity of certain natural disasters. For example, disturbance by construction and related land development is sometimes a contributing factor in the mudflows and landslides that occur in certain areas of California. The resulting damage can be costly both in terms of money and lives.

It is reported that the world's rivers carry as much as 24 million tons of sediment to the ocean each year. About two-thirds of this may be directly related to human activity, which greatly accelerates the natural rate of erosion. This causes rapid loss of fertile topsoil, which leads to decreased crop productivity.

Increased sedimentation also causes increased size and frequency of flooding. As stream channels are filled in, the capacity of the channel decreases. As a result, streams flood more rapidly during a rainstorm, as well as more often, and they drain less quickly after flooding. Likewise, sedimentation can become a major problem on dammed rivers. Sediment accumulates in the lake created by the dam rather than moving farther downstream and accumulating in a delta. Over time, trapped sediment reduces the size of the lake and the useful life of the dam. In areas that are forested, lakes formed by dams are not as susceptible to this problem. Sedimentation is not as great due to interception of rainfall by the trees and underbrush.

Vegetative cover also prevents soil from washing into streams by holding the soil in place. Without vegetation, erosion rates can increase significantly. Human activity that disturbs the natural landscape and increases sediment loads to streams also disturbs aquatic ecosystems.

Many state and local governments are now developing regulations concerning erosion and sedimentation resulting from private and commercial development. Only by implementing such measures can we hope to curb these and other destructive side effects, thereby preserving the environment as well as our quality of life.

See also Deposit.

Resources

books

Dixon, Dougal, and Raymond Bernor. The Practical Geologist. New York: Simon and Schuster, 1992.

Hancock P.L. and Skinner B.J., eds. The Oxford Companion to the Earth. Oxford: Oxford University Press, 2000.

Leopold, Luna. A View of the River. Cambridge: Harvard University Press, 1994.

Middleton, Gerard V., and Celestina V. Cotti Ferrero. Encyclopedia of Sediments & Sedimentary Rocks. Boston: Kluwer Academic Publishers, 2003.

Siever, Raymond. Sand. Scientific American Library Series. New York: W.H. Freeman, 1988.

Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. 4th ed. John Wiley & Sons, 2000.

Westbroek, Peter. Life as a Geological Force: Dynamics of the Earth. New York: W. W. Norton, 1991.

Clay Harris

KEY TERMS

Bedload

—The portion of sediment that is transported by rolling, skipping, and hopping along the stream bed at any given time because it is too heavy to be lifted by flowing stream water. It stands in contrast to suspended load.

Bedrock

—The unweathered or partially weathered solid rock layer, which is exposed at the Earth's surface or covered by a thin mantle of soil or sediment.

Clay

—The finest of sediment particles, less than 1/256 of a millimeter in diameter.

Delta

—A landform that develops where a stream deposits sediment at the edge of a standing body of water (lake or sea).

Floodplain

—The flat, low-lying area adjacent to a river or stream that becomes covered with water during flooding; flood waters deposit sand, silt and clay on this surface.

Geochemical cycle

—A number of interrelated environments or settings through which a chemical can move as a result of changes in state or incorporation into different compounds.

Grain size

—The size of a particle of sediment, ranging from clay to boulders; smaller size sediment is called fine grained, larger sediment is coarse grained.

Mass wasting

—Movement of large masses of sediment primarily in response to the force of gravity.

Outcrop

—A natural exposure of rock at the Earth's surface.

Pebbles

—Coarse particles of sediment larger than sand (2 mm) and smaller than boulders (256 mm).

Sand

—Sediment particles smaller than pebbles and larger than silt, ranging in size from 1/16 of a millimeter to 2 millimeters.

Sediment

—Soil and rock particles that wash off land surfaces and flow with water and gravity toward the sea. On the sea floor, sediment can build up into thick layers. When it compresses under its weight, sedimentary rock is formed.

Sedimentation

—The process by which sediment is removed from one place, and transported to another, where it accumulates.

Silt

—Soil particles derived mainly from sedimentary materials that range between 0.0002–0.05 mm in size.

Sedimentation

Sediments are loose Earth materials such as sand that accumulate on the land surface, in river and lakebeds, and on the ocean floor. Sediments form by weathering of rock . They then erode from the site of weathering and are transported by wind, water, ice , and mass wasting , all operating under the influence of gravity . Eventually sediment settles out and accumulates after transport; this process is known as deposition. Sedimentation is a general term for the processes of erosion , transport, and deposition. Sedimentology is the study of sediments and sedimentation.

There are three basic types of sediment: rock fragments, or clastic sediments; mineral deposits, or chemical sediments; and rock fragments and organic matter, or organic sediments. Dissolved minerals form by weathering rocks exposed at the earth's surface. Organic matter is derived from the decaying remains of plants and animals.

Clastic and chemical sediments form during weathering of bedrock or pre-existing sediment by both physical and chemical processes. Organic sediments are also produced by a combination of physical and chemical weathering. Physical (or mechanical) weathering—the disintegration of Earth materials—is generally caused by abrasion or fracturing, such as the striking of one pebble against another in a river or stream bed, or the cracking of a rock by expanding ice. Physical weathering produces clastic and organic sediment.

Chemical weathering, or the decay and dissolution of Earth materials, is caused by a variety of processes. However, it results primarily from various interactions between water and rock material. Chemical weathering may alter the mineral content of a rock by either adding or removing certain chemical components. Some mineral by-products of chemical weathering are dissolved by water and transported below ground or to an ocean or lake in solution. Later, these dissolved minerals may precipitate out, forming deposits on the roof of a cave (as stalactites ), or the ocean floor. Chemical weathering produces clastic, chemical, and organic sediments.

Erosion and transport of sediments from the site of weathering are caused by one or more of the following agents: gravity, wind, water, or ice. When gravity acts alone to move a body of sediment or rock, this is known as mass wasting. When the forces of wind, water, or ice act to erode sediment, they always do so under the influence of gravity.

Large volumes of sediment, ranging in size from mud to boulders, can move downslope due to gravity, a process called mass wasting. Rock falls, landslides, and mudflows are common types of mass wasting. If you have ever seen large boulders on a roadway you have seen the results of a rock fall. Rock falls occur when rocks in a cliff face are loosened by weathering, break loose, and roll and bounce downslope. Landslides consist of rapid downslope movement of a mass of rock or soil , and require that little or no water be present. Mudflows occur when a hillside composed of fine-grained material becomes nearly saturated by heavy rainfall. The water helps lubricate the sediment, and a lobe of mud quickly moves downslope. Other types of mass wasting include slump, creep , and subsidence.

Water is the most effective agent of transport, even in the desert . When you think of water erosion, you probably think of erosion mainly by stream water, which is channelized. However, water also erodes when it flows over a lawn or down the street, in what is known as sheet flow. Even when water simply falls from the sky and hits the ground in droplets, it erodes the surface. The less vegetation that is present, the more water erodes - as droplets, in sheets, or as channelized flow.

Wind is an important agent of erosion only where little or no vegetation is present. For this reason, deserts are well known for their wind erosion. However, as mentioned above, even in the desert, infrequent, but powerful rainstorms are still the most important agent of erosion. This is because relatively few areas of the world have strong prevailing winds with little vegetation, and because wind can rarely move particles larger than sand or small pebbles.

Ice in glaciers is very effective at eroding and transporting material of all sizes. Glaciers can move boulders as large as a house hundreds of miles.

Generally, erosive agents remove sediments from the site of weathering in one of three ways: impact of the agent, abrasion (both types of mechanical erosion, or corrasion), or corrosion (chemical erosion). The mere impact of wind, water, and ice erodes sediments; for example, flowing water exerts a force on sediments causing them to be swept away. The eroded sediments may already be loose, or they may be torn away from the rock surface by the force of the water. If the flow is strong enough, clay , silt, sand, and even gravel, can be eroded in this way.

Sediments come in all shapes and sizes. Sediment sizes are classified by separating them into a number of groups, based on metric measurements, and naming them using common terms and size modifiers. The terms, in order of decreasing size, are boulder (>256 mm), cobble (256–64 mm), pebble (64–2 mm), sand (2-1/16 mm), silt (1/16–1/256 mm), and clay (<1/256 mm). The modifiers in decreasing size order, are very coarse, coarse, medium, fine, and very fine. For example, sand is sediment that ranges in size from 2 millimeters to 1/16 mm. Very coarse sand ranges from 2 mm to 1 mm; coarse from 1 mm to 1/2 mm; medium from 1/2 mm to 1/4 mm; fine from 1/4 mm to 1/8 mm; and very fine from 1/8 mm to 1/16 mm. Unfortunately, the entire classification is not as consistent as the terminology for sand - not every group includes size modifiers.

When particles are eroded and transported by wind, water, or ice, they become part of the transport medium's sediment load. There are three categories of load that may be transported by an erosion agent: dissolved load, suspended load , and bedload. Wind is not capable of dissolving minerals, and so it does not transport any dissolved load. The dissolved load in water and ice is not visible; to be deposited, it must be chemically precipitated.

Sediment can be suspended in wind, water, or ice. Suspended sediment is what makes stream water look dirty after a rainstorm and what makes a windstorm dusty. Suspended sediment is sediment that is not continuously in contact with the underlying surface (a stream bed or the desert floor) and so is suspended within the medium of transport. Generally, the smallest particles of sediment are likely to be suspended; occasionally sand is suspended by powerful winds and pebbles are suspended by floodwaters. However, because ice is a solid, virtually any size sediment can be part of the suspended sediment load of a glacier.

Bedload consists of the larger sediment that is only sporadically transported. Bedload remains in almost continuous contact with the bottom, and moves by rolling, skipping, or sliding along the bottom. Pebbles on a riverbed or beach are examples of bedload. Wind, water, and ice can all transport bedload, however, the size of sediment in the bedload varies greatly among these three transport agents.

Because of the low density of air, wind only rarely moves bedload coarser than fine sand. Some streams transport pebbles and coarser sediment only during floods , while other streams may, on a daily basis, transport all but boulders with ease.

Floodwater greatly increases the power of streams. For example, many streams can move boulders during flooding. Flooding also may cause large sections of a riverbank to be washed into the water and become part of its load. Bank erosion during flood events by a combination of abrasion, hydraulic impact, and mass wasting is often a significant source of a stream's load. Ice in glaciers, because it is a solid, can transport virtually any size material if the ice is sufficiently thick and the slope is steep.

For a particular agent of transport, its ability to move coarse sediments as either bedload or suspended load is dependant on its velocity. The higher the velocity, the coarser the load.

Transport of sediments causes them to become rounder as their irregular edges are removed by both abrasion and corrosion. Beach sand becomes highly rounded due to its endless rolling and bouncing in the surf. Of the agents of transport, wind is most effective at mechanically rounding (abrading) clastic sediments, or clasts. Its low density does not provide much of a "cushion" between the grains as they strike one another.

Sorting, or separation of clasts into similar sizes, also happens during sediment transport. Sorting occurs because the size of the grains that a medium of transport can move is limited by the medium's velocity and density. For example, in a stream on a particular day, water flow may only be strong enough to transport grains that are finer than medium-grained sand. So all clasts on the surface of the streambed that are equal to or larger than medium sand will be left behind. The sediment, therefore, becomes sorted. The easiest place to recognize this phenomenon is at the beach. Beach sand is very well sorted because coarser grains are only rarely transported up the beach face by the approaching waves, and finer material is suspended and carried away by the surf.

Ice is the poorest sorter of sediment. Glaciers can transport almost any size sediment easily, and when ice flow slows down or stops the sediment is not deposited due to the density of the ice. As a result, sediments deposited directly by ice when it melts are usually very poorly sorted. Significant sorting only occurs in glacial sediments that are subsequently transported by meltwater from the glacier. Wind, on the other hand, is the best sorter of sediment because it can usually only transport sediment that ranges in size from sand to clay. Occasional variation in wind speed during transport serves to further sort out these sediment sizes.

When the velocity (force) of the transport medium is insufficient to move a clastic (or organic) sediment particle it is deposited. When velocity decreases in wind or water, larger sediments are deposited first. Sediments that were part of the suspended load will drop out and become part of the bedload. If velocity continues to drop, nearly all bedload movement will cease, and only clay and the finest silt will be left suspended. In still water, even the clay will be deposited, over the next day or so, based on size—from largest clay particles to the smallest.

During its trip from outcrop to ocean, a typical sediment grain may be deposited, temporarily, thousands of times. However, when the transport medium's velocity increases again, these deposits will again be eroded and transported. Surprisingly, when compacted fine-grained clay deposits are subjected to stream erosion, they are nearly as difficult to erode as pebbles and boulders. Because the tiny clay particles are electrostatically attracted to one another, they resist erosion as well as much coarser grains. This is significant, for example, when comparing the erodibility of stream bank materials—clay soils in a river bank are fairly resistant to erosion, whereas sandy soils are not.

Eventually the sediment will reach a final resting place where it remains long enough to be buried by other sediments. This is known as the sediment's depositional environment.

Unlike clastic and organic sediment, chemical sediment cannot simply be deposited by a decrease in water velocity. Chemical sediment must crystallize from the solution; that is, it must be precipitated. A common way for precipitation to occur is by evaporation . As water evaporates from the surface, if it is not replaced by water from another source (rainfall or a stream) any dissolved minerals in the water will become more concentrated until they begin to precipitate out of the water and accumulate on the bottom. This often occurs in the desert in what are known as saltpans or lakes . It may also occur along the sea coast in a salt marsh.

Another mechanism that triggers mineral precipitation is a change in water temperature . When ocean waters with different temperatures mix, the end result may be seawater in which the concentration of dissolved minerals is higher than can be held in solution at that water temperature, and minerals will precipitate. For most minerals, their tendency to precipitate increases with decreasing water temperature. However, for some minerals, calcite (calcium carbonate) for example, the reverse is true.

Minerals may also be forced to precipitate by the biological activity of certain organisms. For example, when algae remove carbon dioxide from water the acidity of the water decreases, promoting the precipitation of calcite. Some marine organisms use this reaction, or similar chemical reactions, to promote mineral precipitation and use the minerals to form their skeletons. Clams, snails, hard corals, sea urchins, and a large variety of other marine organisms form their exoskeletons by manipulating water chemistry in this way.

Landscapes form and constantly change due to weathering and sedimentation. The area where sediment accumulates and is later buried by other sediments is known as its depositional environment. There are many large-scale, or regional, environments of deposition, as well as hundreds of smaller subenvironments within these regions. For example, rivers are regional depositional environments . Some span distances of hundreds of miles and contain a large number of subenvironments, such as channels, backswamps, floodplains , abandoned channels, and sand bars. These depositional subenvironments can also be thought of as depositional landforms , that is, land-forms produced by deposition rather than erosion.

Erosion, weathering, and sedimentation constantly work together to reshape the earth's surface. These are natural processes that sometimes require us to adapt and adjust to changes in our environment. However, too many people and too much disturbance of the land surface can drastically increase sedimentation rates, leading to significant increases in the frequency and severity of certain natural disasters. For example, disturbance by construction and related land development is sometimes a contributing factor in the mudflows and landslides that occur in certain areas of California. The resulting damage can be costly both in terms of money and lives.

The world's rivers carry as much as 24 million tons of sediment to the ocean each year. About two-thirds of this may be directly related to human activity, which greatly accelerates the natural rate of erosion. This causes rapid loss of fertile topsoil, which leads to decreased crop productivity.

Increased sedimentation also causes increased size and frequency of flooding. As stream channels are filled in, the capacity of the channel decreases. As a result, streams flood more rapidly during a rainstorm, as well as more often, and they drain less quickly after flooding. Likewise, sedimentation can become a major problem on dammed rivers. Sediment accumulates in the lake created by the dam rather than moving farther downstream and accumulating in a delta . Over time, trapped sediment reduces the size of the lake and the useful life of the dam. In areas that are forested, lakes formed by dams are not as susceptible to this problem. Sedimentation is not as great due to interception of rainfall by the trees and underbrush.

Vegetative cover also prevents soil from washing into streams by holding the soil in place. Without vegetation, erosion rates can increase significantly. Human activity that disturbs the natural landscape and increases sediment loads to streams also disturbs aquatic ecosystems. Many state and local governments are now developing regulations concerning erosion and sedimentation resulting from private and commercial development.