The Earth’s surface is in a constant state of change, shaped by erosion and deposition. Erosion breaks down and transports material using agents like wind, water, and ice. Deposition is the constructive phase of this geological cycle, occurring when sediments, soil, and rocks settle out of the transporting medium. This happens when the transporting agent loses enough kinetic energy to carry the sediment load. The continuous addition of material through deposition creates the varied topography seen across the globe.
Shaping by Water: Fluvial and Coastal Deposits
Water is the most widespread agent of deposition, creating distinct features in river and coastal environments. Fluvial deposition occurs when a river’s velocity slows, reducing its ability to carry its sediment load. In a meandering river, the lowest velocity is on the inside bend, forming point bars of sand and gravel. During a flood, water spreads out and rapidly loses energy, depositing coarser sediment near the channel to form natural levees. Finer silt and clay settle across the broad floodplain.
The river delta is the most dramatic fluvial landform, created when a river enters a standing body of water like a lake or ocean. The sudden drop in velocity causes the river to drop its entire sediment load. This rapid accumulation chokes the main channel, forcing the water to divide into smaller streams called distributaries. The resulting landform consists of three layers: coarse topset beds near the surface, seaward-sloping foreset beds, and fine bottomset beds extending into the basin.
Along coastlines, deposition is governed by the energy of waves and currents. Longshore drift moves sediment parallel to the coast. Where the coastline changes direction or wave energy decreases, this sediment forms features like spits, which are elongated ridges of sand projecting into the water. If a spit grows across a bay mouth, it forms a baymouth bar, separating the bay from the open ocean.
Waves also deposit sediment to create beaches, which cycle between erosion and deposition based on wave energy. Offshore, sand accumulation forms barrier islands. These are long, narrow accumulations of sand that run parallel to the mainland, separated from the shore by a lagoon or tidal flat. Barrier islands serve as a natural buffer, absorbing storm surge energy and protecting the inner coastline.
Shaping by Wind: Aeolian Landforms
In arid and semi-arid regions, or along sandy coastlines, wind is the primary agent of transport and deposition. These processes, known as aeolian, create distinctive desert landscapes. Wind transports sediment through three mechanisms. Very fine particles, such as silt and clay, are carried long distances in suspension. Larger sand grains move by saltation, a bouncing motion that accounts for the majority of sand movement. The largest grains are pushed along the surface by the impacts of saltating particles in a process called surface creep.
Deposition occurs when the wind speed drops below the threshold required to keep the grains moving, often behind an obstacle. The most recognizable aeolian features are sand dunes, which are mounds or ridges of sand that form and migrate downwind. The shape of a dune—such as a crescent-shaped barchan or a linear seif dune—is a direct result of the prevailing wind direction and the availability of sand.
Beyond sand, wind deposits thick layers of homogenous, unstratified silt known as loess, often far from its original source. Loess deposits are highly fertile and form rich agricultural soils in regions like the American Midwest and China.
Shaping by Ice: Glacial Deposits
Glaciers carry sediment of all sizes, from fine clay to massive boulders. When the ice melts or retreats, this material is dropped, creating a unique suite of depositional landforms. Material deposited directly by the ice is called till or glacial drift. Till is unsorted and unstratified, meaning rocks of all sizes are mixed together.
Moraines are the most prominent landforms composed of till, forming ridges at the margins of a glacier. A terminal moraine marks the farthest point of advance, while lateral moraines form along the sides of a valley glacier. The uneven blanket of till left after an ice sheet melts is called ground moraine. Another till feature is the drumlin, an elongated, streamlined hill shaped like an inverted spoon, with its steep side facing the direction of ice advance.
Meltwater also creates stratified drift. Water flowing from a melting glacier sorts the sediment by size, unlike the ice itself. Sand and gravel are deposited over a wide, flat expanse in front of the terminal moraine, forming an outwash plain. Streams flowing in tunnels within the glacier deposit sand and gravel, which remain as long, winding ridges called eskers after the ice disappears. Meltwater can also deposit sediment in depressions in the ice, forming steep-sided, conical hills known as kames.
The Long-Term Impact: Creating Sedimentary Rock
The continuous accumulation of deposited materials is the first step in the geological process that transforms loose sediment into solid rock. Over millions of years, as layers build up, the material at the bottom is buried deeper under pressure. This weight squeezes water out of the pore spaces between the grains, a process known as compaction.
Following compaction, dissolved minerals in the groundwater, such as quartz or calcite, precipitate into the remaining pore spaces. This process, called cementation, acts as a natural glue, binding the individual sediment grains together. Compaction and cementation are the primary mechanisms of lithification, which converts unconsolidated sediment into sedimentary rock. Sand deposits become sandstone, mud and clay become shale, and chemically precipitated minerals form limestone. These layered sedimentary rocks form the geological record, providing a preserved history of the Earth’s surface environments shaped by deposition.