Deposition is a geological process where sediments, soil, and rocks are added to a landform or landmass. This process constantly reshapes Earth’s surface, working in conjunction with erosion and weathering. It contributes to the formation of new features over time.
The Process of Deposition
Deposition occurs when a transporting agent, such as wind, water, or ice, loses the energy required to carry its sediment load. As energy diminishes, particles settle out, building up layers of material. This happens when the forces moving the sediment are no longer sufficient to overcome gravity and friction. Gravity also plays a role, particularly in mass wasting events, aiding in the movement and eventual deposition of rock and soil. The type, size, and composition of deposited sediments can offer insights into the environment where they accumulated.
River and Coastal Depositional Landforms
Water is a powerful agent of deposition, creating diverse landforms in both riverine and coastal environments. Rivers, or fluvial systems, deposit sediments as their flow slows down, leading to the formation of floodplains. These flat areas adjacent to a river channel are built by sediment deposited during flood events, enriching the soil with fine silt and clay. Deltas are another prominent riverine landform, forming at the mouth of a river where it enters a larger, slower-moving body of water like a lake or ocean. The reduction in water velocity causes the river to drop its sediment load, creating a fan-shaped landmass.
Alluvial fans are cone-shaped deposits of sediment that form where a stream exits a mountain canyon onto a flatter plain. As the water spreads out and loses energy, it deposits a wide range of sediment sizes, typically with coarser material near the mountain front and finer sands and silts further out. Natural levees are embankments that develop along river banks, particularly after repeated flooding. When a river overflows its banks, the water slows, depositing coarser sediments immediately along the channel edge, forming raised ridges.
Coastal areas also exhibit numerous depositional landforms shaped by the sea. Beaches, for instance, are accumulations of sand, shingle, or pebbles along coastlines, formed by wave action and currents. Spits are narrow ridges of sand or gravel that extend from the shoreline into a body of water, often forming where the coastline changes direction or where a river meets the sea. Longshore drift, the movement of sediment parallel to the shore, contributes significantly to their growth. Barrier islands are long, narrow islands that run parallel to the mainland coast, formed by the accumulation of sand. They provide a natural buffer against storms and wave action.
Wind-Formed Depositional Landforms
Wind, especially in arid or semi-arid regions, transports and deposits sediment to create distinct landforms. Sand dunes are hills or ridges of sand sculpted by wind. These formations arise when wind loses energy, causing sand grains to settle and accumulate. The process often begins when sand is trapped by an obstacle or vegetation, leading to gradual dune growth. Different types of dunes, such as crescent, linear, or star shapes, can develop depending on wind direction, sand supply, and vegetation.
Loess deposits are another wind-formed landform. These are extensive, fertile plains composed of fine, yellowish-brown, wind-blown silt. Loess forms from silt-sized particles picked up from glacial outwash plains or desert margins and transported long distances by wind. These deposits are homogeneous and porous, contributing to their agricultural productivity. Loess blankets can cover vast areas, such as the Chinese Loess Plateau or parts of the US Midwest.
Glacial Depositional Landforms
Glaciers, as massive bodies of ice, transport and deposit vast quantities of sediment, known as till, shaping landscapes. Moraines are ridges or mounds of unsorted glacial till deposited at the edges or beneath a glacier. Various types exist, including terminal moraines, which mark the maximum advance of a glacier, and lateral moraines, which form along the sides of a glacier. Ground moraines are irregular blankets of till deposited beneath a glacier as it melts.
Drumlins
Drumlins are elongated, teardrop-shaped hills composed of till, often found in groups. They typically have a steep side facing the direction from which the glacier advanced and a gentler slope on the opposite side.
Eskers
Eskers are long, winding ridges of stratified sand and gravel. These formations are deposited by meltwater streams that flow within or beneath a glacier, leaving behind their sediment load as the ice melts.
Kames
Kames are irregularly shaped hills or mounds of stratified sand and gravel. They form when meltwater streams deposit sediment in depressions on the glacier surface or in crevasses.
Outwash plains, also known as sandurs, are broad, flat plains of sand and gravel. These plains are created by meltwater streams flowing away from a glacier, which carry and deposit sediment in front of the ice margin. The meltwater sorts the material, with larger particles deposited closer to the glacier and finer sediments carried further away. Outwash plains often feature braided stream complexes due to the high sediment content in the water.
Other Depositional Landforms
Beyond the primary agents of water, wind, and ice, other processes contribute to depositional landforms, often on a more localized scale. Talus slopes, also known as scree, are accumulations of broken rock fragments at the base of cliffs. These slopes form as a result of physical weathering, such as freeze-thaw cycles, which break down rock. Gravity then causes the loosened rock fragments to fall and collect at the foot of the cliff.
Biological activity can also contribute to depositional structures, such as the accumulation of shells or organic material. Volcanic activity, through the deposition of ash (tephra) and other ejecta, can also create new landforms.