The Earth’s surface is constantly being reshaped by the movement of materials. While erosion involves the wearing away and transport of rock, soil, and sediment, deposition is the complementary process where those materials are laid down or settled in a new location. This occurs when the agent carrying the sediment—be it water, wind, or ice—loses the energy needed to keep the material in motion. The resulting accumulation of transported material creates a vast array of distinctive geological structures.
River-Based Depositional Landforms
Rivers and streams create landforms through fluvial deposition when their flow velocity decreases. Heavier sediments, such as gravel and sand, drop out first as the current slows, while fine particles like silt and clay remain suspended until velocities are very low.
A delta forms where a river enters a larger, standing body of water, such as an ocean or lake. The sudden decrease in velocity causes the river to drop its entire sediment load, building a fan-shaped expanse of new land that pushes outward. This deposition often forces the main channel to break up into a network of smaller streams called distributaries.
Alluvial fans are cone-shaped deposits that form when a stream exits a steep valley and flows onto a much flatter, open plain. The abrupt change in slope causes the water to spread out and drastically slow down, quickly dumping its coarser sediment load, including gravel and boulders, near the mountain front. Floodplains are broad, flat areas adjacent to a river channel built up by fine sediment known as alluvium during periods of high water. When a river overflows, the water velocity drops sharply, allowing silt and clay to settle out and gradually raise the land’s elevation.
Wind-Based Depositional Landforms
Wind, particularly in arid or semi-arid regions, sculpts the landscape through aeolian deposition by carrying and dropping fine-grained sediment. When the wind encounters an obstacle or its speed decreases, it loses the capacity to carry its load and drops the sediment.
Sand dunes are the most recognizable landforms created by wind deposition, consisting of mounds of loose sand piled up when the wind slows over an obstruction. A dune typically features a gentle slope facing the prevailing wind and a steeper slip face on the leeward side where the sand cascades down. The shape and migration of these dunes are controlled by the consistency of the wind direction and the available sand supply.
Loess deposits consist of thick, widespread blankets of fine, wind-blown silt and clay. These sediments often originate from glacial outwash plains or deserts and can be carried thousands of miles before settling. Loess is highly porous and, due to the angularity of its grains, can form vertical cliffs, creating some of the most fertile agricultural soils in the world.
Ice-Based Depositional Landforms
Glaciers, acting as massive, slow-moving conveyor belts of ice, transport and deposit enormous volumes of rock and sediment. The material deposited directly by the ice is called till or glacial drift, which is characteristically unsorted and unstratified. This deposition occurs as the ice melts or retreats, leaving behind distinctive features that mark the extent and movement of the glacier.
Moraines are ridges of till that accumulate along the edges or at the terminus of a glacier. There are several types of moraines:
- Lateral moraines form parallel to the flow along the valley walls.
- Medial moraines result when two valley glaciers merge.
- Terminal moraines are prominent ridges marking the furthest point of the glacier’s advance.
Drumlins are smooth, elongated hills made of till, shaped like an inverted spoon. These features are streamlined by the overriding ice, with their long axis running parallel to the direction of the former ice flow. Beyond the glacier’s edge, vast, flat areas known as outwash plains are formed by meltwater streams carrying and depositing sediment. This water sorts the material, depositing stratified layers of gravel, sand, and silt in a broad apron.
Coastal and Marine Depositional Landforms
The energy of waves, tides, and ocean currents drives deposition along coastlines, creating dynamic features where the accumulation of sediment exceeds its removal. A primary mechanism for this transport is longshore drift, where waves approach the shore at an angle, causing sediment to move in a zigzag pattern along the coast.
Beaches are the most common coastal depositional landforms, consisting of an accumulation of sediment—sand, shingle, or even cobbles—along the shoreline. The composition and profile of a beach constantly adjust in response to the local wave energy, with gentle, constructive waves building up the shore. A spit is a long, narrow ridge of sand or shingle that extends from the mainland into open water. It forms when longshore drift carries sediment past a bend in the coastline, and the material is deposited as the current loses energy.
Barrier islands are long, narrow islands composed of sand that lie parallel to the mainland coast, separated by a lagoon or bay. These islands are built and maintained by the deposition of sediment and the continuous action of waves and longshore drift. They serve as a natural buffer, absorbing the energy of storms and protecting the mainland shoreline from erosion.