Deposition is the geological process where sediments, soil, and rock fragments are added to a landform or landmass, increasing its size or creating new features. This constructive process builds up the Earth’s surface by accumulating material previously worn away and transported by other forces. Unlike erosion, which removes material, deposition occurs when the transporting agent—such as wind, ice, or water—loses the kinetic energy required to carry its sediment load. The material settles out and collects, forming layers of sediment that can eventually lithify into sedimentary rock. The resulting landform, including the sorting and size of the deposited particles, is directly influenced by the specific agent and the environment’s energy.
The Role of Water (Fluvial and Coastal Landforms)
Water is a powerful agent of deposition, shaping landscapes through confined channel flow in rivers and the dynamic action of waves and currents along coastlines. Fluvial deposition, driven by rivers and streams, is linked to a decrease in water velocity, which reduces the stream’s capacity to transport its load.
Fluvial Deposition
Rivers carrying large amounts of sediment rapidly deposit coarser materials when they flow from a steep mountain valley onto a gentler, open plain. This sudden loss of gradient and speed forms large, fan-shaped accumulations of sediment known as alluvial fans.
During flood events, water overflowing the banks slows dramatically, dropping fine-grained silts and clays to form broad, flat floodplains. These deposits often build up low, linear ridges along the riverbanks called natural levees, which are thickest closest to the channel. When a river meets a standing body of water, like a lake or the ocean, its velocity drops to near zero, forcing the deposition of its remaining sediment load in a fan or bird-foot shape called a delta.
Coastal/Marine Deposition
The coastal environment is an energetic zone where deposition is dominated by the interaction of waves, tides, and currents. Sediment transport parallel to the shoreline, known as longshore drift, is the primary mechanism for building coastal features.
Beaches form from the accumulation of sand and gravel pushed ashore by wave action, constantly moved and sorted by the energy of the waves and currents. When longshore drift encounters a change in the coastline, such as a bay or river mouth, the sediment continues to deposit in the direction of the current, creating an elongated ridge called a spit. If a spit grows across a bay mouth, it may form a baymouth bar, often leaving a sheltered lagoon. Barrier islands, which are long, narrow strips of sand parallel to the mainland coast, are constructed by river discharge sediment reworked by wave action and longshore transport.
Sculpting by Wind (Aeolian Features)
Wind, or aeolian, deposition is prominent in arid and semi-arid environments lacking vegetation and having a plentiful supply of loose, dry sediment. The wind transports particles primarily through suspension (carrying fine dust and silt high into the atmosphere) and by saltation (the bouncing of sand grains along the surface). Deposition occurs when wind velocity decreases or when an obstacle causes the air to slow down, allowing the sediment to settle.
The most recognizable aeolian features are sand dunes, which are mounds of sand built up when the wind loses its carrying capacity. The dune’s shape results directly from the prevailing wind direction and sand supply. For instance, transverse dunes form long ridges perpendicular to consistent wind flow, while parabolic dunes, often stabilized by vegetation, have a U-shape with arms pointing upwind.
Loess deposits are another form of wind deposition, consisting of extremely fine, wind-blown silt. These particles originate from glacial grinding or desert surfaces and are carried over vast distances before accumulating in thick, unstratified blankets. Loess plains are important because the sediment forms highly fertile soils, such as those found in the Chinese Loess Plateau.
Shaping by Ice (Glacial Deposits)
Glaciers transport massive amounts of material of all sizes, from fine rock flour to large boulders, encased within the ice. The deposited material, known as glacial till or glacial drift, is unsorted and unstratified because the ice cannot hydraulically sort the sediment. Till is deposited directly by the melting ice, creating distinct landforms.
Moraines are the most common depositional features, forming ridges of till that mark where the glacier margin once stood. Terminal moraines represent the farthest extent of the ice, while lateral moraines form along the sides of the glacial valley. Medial moraines are created when two tributary glaciers merge, combining their lateral moraines into a single ridge running down the center of the ice flow.
Drumlins are another feature formed beneath the ice; these are elongated, asymmetrical hills composed of glacial till. Drumlins are shaped like an inverted spoon, with the steep side facing the direction of ice advance, indicating past glacial movement. Meltwater flowing away from the glacier carries finer, sorted sediment (sand, gravel, and silt) which is deposited to form broad, flat outwash plains beyond the moraine.
Gravity’s Contribution (Mass Wasting Deposits)
Gravity is a constant force that causes the downward movement of soil and rock, a process termed mass wasting. Mass wasting does not require a fluid medium for transport but relies on the failure of slope stability. This downward movement, which includes landslides, slumps, and slow creep, results in the accumulation of material at the base of slopes.
Talus slopes are a direct result of gravitational deposition, forming cone-shaped piles of angular rock debris that accumulate at the foot of cliffs and steep rock faces. The material in a talus cone is coarse and poorly sorted, having simply fallen or rolled down the slope. Colluvium is a broader term for the loose, unsorted material deposited by gravity and non-channelized flow on or at the base of hillslopes. This deposit is often gravelly and non-stratified, reflecting the short transport distance and the dominance of simple slope failure mechanisms.