sandstone, sedimentary rock formed by the cementing together of grains of sand. The usual cementing material in sandstone is calcium carbonate, iron oxides, or silica, and the hardness of sandstone varies according to the character of the cementing material; quartz sandstones cemented with quartz are the hardest. Sandstones are commonly gray, buff, red, or brown although green and some other colors are also found. Green sandstones often contain, in addition to sand and glauconite, fossil shells and iron oxides; those that break apart easily are known as greensands and are sometimes used to replenish depleted potash in soils. Sandstones are widely used in construction and industry. Varieties of sandstone include arkose, which contains feldspar and resembles granite, and graywacke, a gray or sometimes greenish or black rock composed of quartz and fledspar with numerous fragments of other rocks, such as shale, slate, quartzite, granite, and basalt. Sandstone may be crushed to the form of loose sand grains, which can then be put to the same industrial uses as sand. See brownstone.

Old Red Sandstone, series of red and brown sandstones, conglomerates, and shales deposited in Wales and Scotland and in England near the Welsh and Scottish borders in the Devonian period of geologic time. The Old Red Sandstone, in contrast to the typical formations of the Devonian, is largely a continental formation, laid down in freshwater and on land as a result of the erosion of the highlands of the Silurian period. It is very thick in Scotland and contains a large assemblage of well-preserved fossils, particularly of the Devonian fishes. The Old Red Sandstone was correlated with the marine Devonian by the British geologists Rodney Murchison and Adam Sedgwick. The New Red Sandstone is a thick red sandstone of Permian and Triassic age found in the British Isles.

New Red Sandstone, name for the thick red layer of the Triassic formation in Great Britain (see Triassic period). It is many thousands of feet thick and is composed chiefly of red sandstones, clays, and conglomerate; the red color and the occurrence of workable quantities of salt and gypsum suggest markedly arid conditions at the time of deposition.

Sedimentary rock formed from sand-sized grains (0.0025–0.08 in., or 0.06–2 mm, in diameter). The spaces between grains may be empty or filled with either a chemical cement of silica or calcium carbonate or a fine-grained matrix of silt and clay particles. The principal mineral constituents of the grain framework are quartz, feldspar, and rock fragments. Sandstones are quarried for use as building stone. Because of their abundance, diversity, and mineralogy, sandstones are also important to geologists as indicators of erosional and depositional processes. Seealso graywacke.

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