Sandstone

Sandstone refers to a sedimentary rock with grains between 1/16 millimeter and 2 mm in size (siltstone is made of finer grains). Therefore sandstone doesn't signify any particular mineral, but in practice, sandstone is usually almost all quartz. Most sandstones, however, have small amounts of other minerals—clays, hematite, ilmenite, feldspar, and mica—that add color and character to the quartz matrix. Sandstones with greater amounts of impurities are classified as wacke or graywacke.

Sandstone forms where sand is laid down and buried. Usually this happens offshore from river deltas, but desert dunes and beaches can leave sandstone beds in the geologic record too. Sandstone does not usually contain good fossils because the energetic environments where sand beds form don't favor their preservation. As a landscaping and building stone, sandstone is full of character, with warm colors, and can be quite durable.

The grains of quartz in sandstone are cemented together by a groundmass of silica (chemically the same as quartz), or calcium carbonate or iron oxide. The tan and brown bands in this sandstone boulder are due to small amounts of iron minerals. The sandstone below, from Pleistocene deposits in central California, shows this dark cement well. The grains are sharp fragments of quartz from the ancient granites of the Sierra Nevada, but the cement is derived from volcanic ash beds of much younger age.

This young rock is rather crumbly. When sandstone is more deeply buried, the pressure of burial and slightly higher temperatures allow minerals to dissolve or deform and become mobile. The grains become more or less closely knit together. With a great deal of heat and pressure, sandstones turn to the metamorphic rocks quartzite or gneiss, tough rocks with tightly packed mineral grains.

See more sedimentary rocks in the Sedimentary Rocks Gallery.

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