sandstone - Origins, Types of sandstone, Gallery

A sedimentary rock composed of grains of sand (usually quartz) cemented together by a matrix, usually silica or calcium carbonate. It may be formed through deposition by water in marine or freshwater environments, or by wind action (as dunes). It is quarried as a building stone.

Sandstone is a sedimentary rock composed mainly of sand-size mineral or rock grains. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow, red, gray and white. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone have been strongly identified with certain regions.

Some sandstones are resistant to weathering, yet are easy to work. Because of the hardness of the individual grains, uniformity of grain size and friability of its structure, sandstone is an excellent material from which to make grindstones, for sharpening blades and other implements.

Rock formations that are primarily sandstone usually allow percolation of water and are porous enough to store large quantities, making them valuable aquifers.

Origins

Sandstones are clastic in origin (as opposed to organic, like chalk and coal, or chemical, like gypsum and jasper). Grain sizes in sands are in the range of 0.1 mm to 2 mm (rocks with smaller grain sizes include siltstones and shales and are typically called argillaceous sediments, as too are clays and rocks with larger grain sizes include both breccias and conglomerates and are termed rudaceous sediments).

The formation of sandstone involves two principal stages. Finally, once it has accumulated, the sand becomes sandstone when it is compacted by pressure of overlying deposits and cemented by the precipitation of minerals within the pore spaces between sand grains.

The environment of deposition is crucial in determining the characteristics of the resulting sandstone, which, in finer detail, include its grain size, sorting and composition and, in more general detail, include the rock geometry and sedimentary structures. Principal environments of deposition may be split between terrestrial and marine, as illustrated by the following broad groupings:

Terrestrial environments Rivers (levees, point bars, channel sands) Alluvial fans Glacial outwash Lakes Deserts (sand dunes and ergs) Marine environments Deltas Beach and shoreface sands Tidal deltas, flats Offshore bars and sand waves Storm deposits (tempestites) Turbidites (submarine channels and fans)

Types of sandstone

Once the geological characteristics of a sandstone have been established, it can then be assigned to one of three broad groups:

arkosic sandstones, which have a high (>25%) feldspar content and a composition similar to granite.

Gallery

Prepared sample of sandstone	A natural sandstone formation composed of cemented quartz sand	Sandstone patterns on an chamber wall in Petra	Arbroath Abbey, showing distinctive sandstone colouring
Arches National Park Sandstone landscape	Parus Rock in Russia.