The Earth’s surface is constantly being reshaped by a two-part geological cycle: erosion wears down existing landforms, and deposition builds new ones. Deposition is the constructive process where materials settle out of their transporting medium and accumulate in a new location. This occurs when the kinetic energy of the carrier (water, wind, or ice) is no longer sufficient to keep the sediment suspended or in motion. The resulting accumulation of sediment, soil, and rock fragments forms layers that give rise to distinctive landforms across the globe.
Shaping the Land with Flowing Water
Flowing water, through rivers and ocean currents, is the most pervasive agent of deposition, creating diverse landscapes as its velocity drops. Rivers carry a massive load of weathered material; when they overflow their banks during a flood, the sudden loss of speed causes an immediate drop-off of the heaviest sediment. This process repeatedly deposits coarser sand and silt directly adjacent to the river channel, slowly building up raised embankments known as natural levees. These elongated ridges slope gently away, leaving finer clay particles to settle much further out across the floodplain.
The largest riverine landforms are created when a river meets a standing body of water, such as a lake or the ocean, and its velocity abruptly halts. This loss of energy causes the entire sediment load to be dumped, forming a fan-shaped structure called a delta. Deltas exhibit distinct sorting patterns: the coarsest sand and gravel drop closest to the river mouth, often forming foreset beds. The lightest clay and silt particles remain suspended longer, traveling further out to settle on the seafloor as bottomset beds, which form the base upon which the delta advances.
Coastal deposition is heavily influenced by the energy of waves and the movement of currents parallel to the shore, known as longshore drift. Waves approaching a coastline at an angle transport sand in a zigzag motion, continually moving sediment down the coast. Where wave energy is dispersed, such as in sheltered bays or behind headlands, the sediment accumulates to form sandy beaches.
The continuous movement of longshore drift is responsible for building elongated features like sandbars and barrier islands. Barrier islands are transient, parallel offshore deposits that protect the mainland, often forming as submerged offshore bars gradually rise or as spits are extended and breached by storms. The size of the deposited material directly correlates with wave energy; high-energy environments leave behind coarser gravel and pebbles, while calmer waters allow fine sands and silts to settle.
The Impact of Glaciers and Gravity
Glacial ice acts as a slow-moving conveyor belt, carrying enormous quantities of rock and sediment that are released upon melting. The material deposited directly by the ice is called till, which is characteristically unsorted and unstratified. This chaotic mixture, containing everything from fine clay to large boulders, starkly contrasts with the well-sorted sediments laid down by water. This debris accumulates to form various landforms, collectively known as moraines.
Moraines are ridges of till that mark the former margins of the ice mass. Lateral moraines form as parallel ridges of debris deposited along the sides of a valley glacier, typically sourced from rockfall onto the ice surface. A terminal moraine is a transverse ridge built at the maximum point of the glacier’s furthest advance.
Glaciers create streamlined hills called drumlins, which are composed of till molded beneath the moving ice sheet. These oval-shaped landforms are aligned parallel to the direction of ice flow, with a blunt, steeper side facing the oncoming ice (stoss end) and a tapered side pointing down-ice (lee end). Drumlin formation involves either the deposition of sediment from subglacial water or the erosion and streamlining of pre-existing material by the overriding ice mass.
Gravity is a direct depositional force, causing immediate accumulation through mass wasting events like rockfalls and landslides. When rock fragments break away from a steep cliff face, often due to weathering processes, they fall to the base of the slope. This material accumulates to form cone-shaped piles known as talus slopes or scree. The deposited fragments are generally angular and poorly sorted, reflecting the chaotic nature of gravitational collapse.
Wind-Formed Landscapes (Aeolian Deposition)
Wind (aeolian deposition) is highly effective in arid and semi-arid environments where vegetation is sparse and loose sediment is abundant. Unlike water and ice, wind transports only fine, well-sorted particles, primarily sand and silt. Deposition occurs when the wind encounters an obstacle, causing its speed to drop and its carrying capacity to diminish.
This reduction in wind velocity forces the carried sand to settle, initiating the growth of a sand dune. A crescent-shaped landform called a barchan dune forms in areas with a consistent, unidirectional wind and a limited sand supply. These dunes are asymmetrical, featuring a gentle windward slope where sand is pushed up and a much steeper leeward slope, known as the slip face, where sand slides down at its angle of repose.
Wind-blown silt, carried over vast distances from deserts or glacial outwash plains, accumulates to form thick, homogeneous deposits known as loess. Loess deposits are significant because they create some of the world’s most fertile agricultural soils. The fine silt particles ensure excellent water retention and aeration, while the material’s mineral-rich composition naturally supplies plant nutrients.