sedimentary rock

The South Rim of the Grand Canyon in Arizona, USA. Erosion has created dramatic configurations in the sedimentary rock formations in and around the gorge.

At Stair Hole in Dorset, England, the sedimentary beds of Portland and Purbeck limestone have been folded into a step- or stair-like shape, by tectonic movements during the Alpine mountain-building period, 30 million years ago. The folding has also fractured the limestone, and the lines of weakness thus created were later exploited by marine erosion, forming small arches in the limestone.

Behind the wire mesh is bare sedimentary rock, exposed by weathering. Many cliff faces remain steep due to rapid breakdown. Weathering of the rock followed by rock falls (a type of mass movement) help to preserve a steep incline. In this picture, layers of sediment in the rock are clearly visible.

Rock formed by the accumulation and cementation of deposits that have been laid down by water, wind, ice, or gravity. Sedimentary rocks cover more than two-thirds of the Earth's surface and comprise three major categories: clastic, chemically precipitated, and organic (or biogenic). Clastic sediments are the largest group and are composed of fragments of pre-existing rocks; they include clays, sands, and gravels.

Chemical precipitates include some limestones and evaporated deposits such as gypsum and halite (rock salt). Coal, oil shale, and limestone made of fossil material are examples of organic sedimentary rocks.

Most sedimentary rocks show distinct layering (stratification), because they are originally deposited as more or less horizontal layers.

Sedimentary rocks are categorized by the grain sizes of the particles: conglomerate rocks may contain rock pieces over 2 mm in diameter; sandstone particles are up to 0.2 mm; siltstone up to 0.02 mm; and claystone less than 0.02 mm in diameter.