Deposition is the constructive geological process where sediments, soil, and rock fragments are added to a landmass, building up features on the Earth’s surface. This process is the natural counterpart to erosion, which is the destructive force that wears down and removes material from the land. Whenever the transporting agents of wind, water, or ice lose their ability to carry the material they have moved, that material is laid down in a new location. This accumulation transforms the landscape, creating distinct landforms that can range from small hills to vast plains.
The Physics of Sediment Settling
The fundamental mechanism driving all deposition is a reduction in the energy of the transporting medium. Whether the carrier is a rushing river or a strong wind, it can only hold a certain amount of sediment in suspension or move it along the surface based on its velocity. When the speed of the carrier drops, its capacity to transport material decreases, and the force of gravity begins to dominate.
As the velocity slows, the heaviest and coarsest particles, such as pebbles and sand, are the first to be dropped out of the flow. This sorting mechanism often results in layered sediments, with larger grains settling before smaller, silt-sized particles. Friction and obstacles can also trigger deposition by locally slowing the transporting agent, causing the material to accumulate around the point of resistance.
Landforms Created by Flowing Water and Glaciers
Flowing water, or fluvial action, shapes the land through deposition primarily when a river encounters a significant drop in gradient or enters a standing body of water. Alluvial fans are created where a stream abruptly exits a mountain canyon onto a flatter valley floor, causing an immediate, fan-shaped spread of coarse sediment. The sudden loss of velocity forces the river to deposit its heavy load, building a gently sloping, conical landform.
Where a river meets a sea or lake, its speed drops to near zero, leading to the formation of a delta. The sediment load is dumped at the river mouth, causing the main channel to split into multiple branching paths called distributaries.
Floodplains are constructed over time when a river overflows its banks, depositing fine silt and clay, known as alluvium, across the low-lying adjacent areas. The coarsest sand drops immediately along the channel edge, gradually building up elevated natural levees that act as earthen embankments.
Glaciers, as massive, slow-moving rivers of ice, deposit material differently because they carry unsorted debris embedded within the ice itself. This chaotic mix of clay, sand, pebbles, and boulders is known as till, and it is released only when the ice melts. Moraines are the most common glacial depositional landforms, appearing as long, linear ridges of till.
A terminal moraine marks the farthest point of the glacier’s advance, acting as a bulldozed pile of debris at its snout. Lateral moraines form ridges of rock and sediment along the edges of a valley glacier where the ice meets the valley walls.
Another distinct feature is the drumlin, an elongated, streamlined hill composed of till that resembles an inverted spoon. Drumlins often occur in swarms and are oriented parallel to the former direction of ice flow, forming beneath the glacier as it molds and deposits sediment.
Landforms Created by Wind and Gravity
Wind, or aeolian action, is a highly selective agent of deposition, capable of carrying fine material over vast distances before it settles. Sand dunes form when wind encounters an obstacle, causing its speed to decrease and the sand grains to fall out of transport. Sand grains move up the gentle windward slope of the dune through a bouncing motion called saltation.
Once the grains reach the crest, they cascade down the steeper leeward side, forming a slip face that rests at the natural angle of repose for dry sand, typically around 30 to 34 degrees. Loess deposits are composed of extremely fine, silt-sized particles that have been carried high into the atmosphere by strong winds. These deposits often originate from glacial grinding that produced rock flour, and they can accumulate in thick, unstratified blankets up to 90 meters deep, forming fertile plains far from the source.
Gravity alone is responsible for the most immediate form of deposition, often referred to as mass wasting. Talus, or scree slopes, are distinct landforms created entirely by the accumulation of rock fragments at the base of a steep cliff or rock face. Individual pieces of rock break away from the cliff, usually through weathering processes like freeze-thaw cycles, and fall directly to the bottom.
These deposits form a conical or apron-like slope whose angle is determined by the friction and size of the angular rock fragments. This accumulation of fallen material is a continuous process that slowly builds up the base of mountainous slopes.