Deposition is the geological process where sediments, soil, and rock particles settle out of the transporting medium that carried them, ultimately building up layers on the Earth’s surface. This settling occurs when the transporting agent—whether water, wind, ice, or gravity—loses sufficient kinetic energy to keep the material in motion. It represents the final stage in the cycle of weathering, transport, and sedimentation. The resulting accumulation of material creates distinctive landforms and layers, which geologists study to understand past environmental conditions and Earth’s history.
Deposition in Water Systems
Water is the most significant agent for sediment transport and subsequent deposition across continental interiors. Fluvial environments, such as rivers and streams, have constantly changing energy levels that dictate where material settles. When a river overflows its banks during a flood, the sudden decrease in velocity causes the coarsest sediments to settle immediately adjacent to the channel, building up natural levees. Finer silt and clay particles travel further out onto the floodplain before finally settling.
Within the river channel, deposition occurs on the inside bends of meandering rivers where the current slows, forming crescent-shaped point bars. These deposits exhibit a fining-upward sequence, where coarser sand is overlain by finer silt.
Lacustrine, or lake, environments are low-energy settings where the finest sediments accumulate slowly from suspension. The standing water acts as an effective sediment trap, allowing even microscopic clay particles to settle. In lakes with strong seasonal changes, alternating layers of light and dark sediment form annual layers called varves, providing a precise chronological record of past climate.
When a river reaches a standing body of water, such as an ocean, its velocity rapidly drops to near zero, leading to the formation of a delta. This environment is a massive sink for sediment, often creating deposits that coarsen upward as the delta builds outward over finer offshore muds. The specific shape of the delta is determined by the balance between the river’s sediment supply and the energy of the receiving basin’s waves and tides.
Deposition in Coastal and Marine Settings
Coastal areas are highly energetic transitional zones where river, wave, and tidal processes interact to sort and deposit sediment. Wave-dominated coasts, such as the Nile Delta, feature long, linear barrier islands and beaches. Here, the energy of breaking waves constantly reworks the sediment, pushing finer material offshore while concentrating well-sorted sand into beach ridges.
Tide-dominated coasts are characterized by a large tidal range that generates strong, bidirectional currents. These currents create extensive tidal flats, estuaries, and linear sand ridges oriented perpendicular to the shore. In sheltered, lower-energy zones like estuaries and lagoons, fine muds and organic material settle out, often forming nutrient-rich marsh deposits.
Moving offshore, the continental shelf is the shallow, submerged margin where terrigenous sediment, material derived from land, accumulates. Beyond the shelf break, the continental slope is a steeper environment where gravity-driven processes are dominant. Sediment accumulating here can become unstable, sliding down the slope in dense, turbulent masses known as turbidity currents. These currents carve out submarine canyons and deposit characteristic graded beds of sediment, called turbidites, onto the flatter abyssal plains of the deep ocean floor.
Deposition in Dry and Icy Climates
Wind and ice are powerful transporting agents in arid and cold climates. Aeolian deposition, driven by wind, occurs mainly in deserts and coastal areas where vegetation is sparse and loose sediment is abundant. The dominant transport mechanism for sand-sized particles is saltation, where grains bounce along the ground, dislodging other grains and moving them downwind.
This process builds up sand dunes, which migrate as sand moves up the gentle windward slope and cascades down the steep leeward slip face. Finer silt and clay particles, carried in suspension by the wind, travel vast distances before settling to form thick, unstratified deposits known as loess. The Loess Plateau in China is a dramatic example of wind-deposited silt.
Glacial deposition involves the movement and eventual melting of large masses of ice. Glaciers carry a massive, unsorted mixture of sediment, ranging from fine clay to huge boulders, collectively referred to as glacial till. When the glacier melts, this load is dropped in place, creating chaotic, unstratified deposits.
The most common landforms resulting from this process are moraines, which are ridges or mounds of till deposited at the glacier’s edges.
- A terminal moraine marks the farthest extent of the ice sheet.
- Lateral moraines form along the sides of a valley glacier.
- Ground moraine is a blanket of till left over a broad area.
- Streamlined, egg-shaped hills of deposited till are called drumlins.
The Role of Time: From Sediment to Rock
The depositional environments discussed are temporary resting places for sediment, which, over geologic time, undergo lithification to become sedimentary rock. As layers of sediment accumulate, the weight of the overlying material causes compaction, squeezing out water and reducing the pore space between grains.
Following compaction, cementation occurs as mineral-rich groundwater flows through the remaining pore spaces. Dissolved minerals, such as silica or calcite, precipitate, acting as a natural glue that binds the individual sediment grains together.
The resulting sedimentary rock preserves the characteristics of its original depositional environment. Thick, long-term accumulations of sediment in these widespread locations are known as sedimentary basins, representing the ultimate destination for material eroded from the continents.