What is a sedimentary rock?


Layered Sandstone

Sedimentary rocks are formed by the breakdown older rocks into smaller grains to form sediment through surface processes. Over time these are deposited mainly with the use of water and build up in layers to form sedimentary rocks. In dry, arid environments such as the desert wind blown grains of sand build up and form sand dunes.

In time these are buried and form the sedimentary rock through the process of lithification. This process involves the sediment becoming cemented and then compacted to form the sedimentary rock.

They are commonly layered and contain sedimentary structures such as ripples, laminations (bedding) and grading. These structures help in reconstructing the environment in which the sediment would have been deposited. Not all environments with sediment in are preserved. A good example of are beach environments, which are hardly ever preserved.

Common sedimentary rocks include sandstone, limestone and mudstone.


Halite (Rock Salt) from Cheshire

Fossils are preserved within sedimentary rocks and may even make up a limestone. They can form limestone such as the Portland Limestone which consists of the remains of small microfossils contained within ooids.

Evaporite rocks such as halite and gypsum are classed as sedimentary rocks and have formed by the evaporation of sea water, leaving behind large deposits. Examples of this can be seen in Cheshire and south-east Spain. The deposits in Spain formed when the Mediterranean Sea dried up due to the water evaporating.

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