Deposition

Discovering Geology – Geological processes

Deposition is the laying down of sediment carried by wind, water, or ice. Sediment can be transported as pebbles, sand & mud, or as salts dissolved in water. Salts may later be deposited by organic activity (e.g. as sea-shells) or by evaporation.

Wind

Sand dunes at Prestatyn, North Wales
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An inland line of sand dunes marks a former shoreline near Prestatyn, North Wales – BGS © UKRI

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The term saltation describes the process by which sand grains are picked up and transported by the wind. Sand grains bounce along the ground in the wind, when the wind stops or slows down the sand is deposited, and may build sand dunes.

Ice

Erratic perched blocks at the side of Llyn Idwal.
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Erratic perched blocks at the side of Llyn Idwal, Snowdonia – BGS © UKRI

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Glaciers are not static objects, they move albeit very slowly, flowing under their own weight, and they grow and shrink depending on the climatic conditions. As they move they carve the landscape below them, picking up sediments and rocks of all sizes. In fact, glaciers can carry the largest of sediments, boulders that can sometimes reach enormous sizes for very long distances. When the glaciers melt and retreat, they leave them behind in what are known as ‘glacial erratic boulders’, made of rock that has no relation with the local geology where they have come to rest. The typical deposit of a glacier is known as till, which is an unsorted mixture of all the debris that the glacier was carrying at that moment, a mixture of clay, sand, pebbles and boulders. The typical glacial sedimentary features known as moraines are composed of tills.

Rivers

Flowing water picks up and moves particles of soil and rock. When the water slows down, for example by reaching flatter land, it starts to drop the particles it is holding. It drops the largest particles first and the smaller ones as it slows down even further.

Alluvial fans at Andes Mountain. Photo by Eurico Zimbres.
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Alluvial fans in the Andes, Argentina. Photo by Eurico Zimbres.

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Alluvial fans are a feature typically created when sediment carried by a mountain stream is deposited due to a rapid change in slope from a high to a low gradient. When the slope angle is high the stream flows with a high velocity and is able to transport larger pieces of sediment such as pebbles and sand. When the slope angle is lower the stream loses the energy it needs to carry these larger pieces of sediment and they are deposited. The deposited materials eventually spread out, creating an alluvial fan.

 Water may also carry dissolved material – mostly ions that have minerals it has dissolved from solid rock.  These ions may be deposited en-route to the sea or may reach the ocean and contribute to its saltiness.

Sea

Hengistbury Head, Dorset
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Hengistbury Head is a spit at the entrance to the Avon–Stour estuary near Christchurch, Dorset – BGS © UKRI

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In coastal environments, sediments are deposited along or near a coastline by low energy waves that can no longer support their sediment load. Material carried by the sea is washed up by the water and begins to build up along the coastline, creating beaches and other coastal features such as spits and shoals. Beaches change and move by the combined action of tides, waves and currents and can loose or gain sand from season to season, in some cases with sand disappearing altogether leaving behind heavier cobbles.

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Alluvial fans at Andes Mountain. Photo by Eurico Zimbres.

Deposition

Deposition is the laying down of sediment carried by wind, water, or ice.

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