Deposition is a fundamental geological process where sediments, soil, and rocks are added to a landform or landmass. This constructive action builds up new landscapes and modifies existing ones across the Earth’s surface. Deposition acts as the final stage of the erosional cycle, following the breakdown of material by weathering and its subsequent transport. Wind, water, ice, and gravity move these materials, collectively known as sediment. The process culminates when the energy required to keep the sediment moving is no longer sufficient, causing the material to settle out and accumulate.
The Core Mechanism: Loss of Transport Energy
The physical principle driving deposition is the reduction of kinetic energy within the transporting medium, whether it is air, water, or ice. A moving fluid, such as a river or the wind, possesses a certain capacity to carry sediment, which is directly related to its velocity. When this flow velocity decreases, the fluid loses its ability to exert the necessary upward force to counteract gravity, forcing the suspended particles to fall out.
This mechanism involves a balance between the downward pull of gravity on a particle and the upward drag and turbulence of the fluid. A particle’s settling velocity is the constant rate at which it falls through a still fluid, achieved when the gravitational force equals the fluid drag. When the fluid’s velocity and turbulence drop below the threshold needed to keep a particle in suspension, the downward gravitational force becomes dominant, and the particle is deposited.
The concept of competent flow describes the maximum size of sediment a fluid can transport at a given velocity. As the flow’s competence decreases, larger, heavier particles are the first to be dropped, followed by progressively finer material as the energy continues to decline. This sorting mechanism ensures that sediment is deposited in predictable sequences. Fine particles like clay require a high velocity to be picked up but remain suspended at very low velocities once in motion.
Deposition by Specific Agents
Fluvial and Marine Deposition
Different agents lose their transport energy through distinct mechanisms, leading to varied depositional patterns. In fluvial (river) and marine (ocean) environments, energy loss typically occurs when a stream’s gradient lessens or when the water body widens. A river deposits its load significantly when it enters a standing body of water, such as a lake or the ocean, where its velocity is rapidly neutralized, causing sediment to drop out. This results in a sequence where the coarsest sediment is deposited first near the mouth, with finer silts and clays carried farther out into the basin.
Eolian Deposition
Eolian deposition, driven by wind, occurs primarily when wind velocity decreases or when the airflow meets a physical barrier. Sand grains are transported mainly by saltation, a bouncing movement, while finer dust can be held in long-term suspension. When the wind slows down, the pressure and drag forces on the particles weaken, causing them to fall to the surface and accumulate.
Glacial Deposition
Glacial deposition differs fundamentally because ice itself is the transporting medium, not a fluid. The primary mechanism is the melting or sublimation of the ice, which directly releases the entrapped sediment. This material, known as till, is characteristically unsorted and unstratified because it is dropped en masse without the sorting action of water or air. Meltwater streams flowing from glaciers, however, engage in glaciofluvial deposition, where the water’s energy loss results in sorted deposits like outwash plains.
Landforms and Structures Created by Deposition
Depositional Landforms
Deposition creates a wide range of recognizable features on the Earth’s surface. Prominent landforms include deltas, fan-shaped bodies of sediment built where a river enters a larger water body, and alluvial fans, cone-shaped deposits formed where a steep mountain stream emerges onto a flat valley floor. The abrupt loss of gradient causes the stream to dump its entire sediment load, with the coarsest material settling closest to the source.
Glacial and Eolian Features
Glacial deposition leaves behind distinctive landforms such as moraines, which are ridges of unsorted till marking the former edges or terminus of a glacier. Drumlins are elongated, streamlined hills of till molded parallel to the direction of ice flow. In arid or coastal environments, dunes form as hills of sand sculpted by eolian processes, migrating as the wind continually moves and redeposits the grains.
Sedimentary Structures
Internally, deposition creates several key sedimentary structures that record the history of transport. Stratification, or bedding, is the most common feature, representing distinct layers of sediment laid down sequentially over time. This layering often reflects changes in the source material or the energy of the depositional environment. Cross-bedding is a smaller-scale structure characterized by inclined internal layers within a larger bed, resulting from the migration of ripples or dunes.