Limestone

Limestone shows some of its variety in this roadcut on Route 20 just west of Sharon Springs, New York, in the Mohawk Valley. Limestone is usually not made of sediment as we think of it—not clay or sand, derived from rocks—but instead is built from the tiny calcite skeletons of microscopic organisms in shallow seas. The Bahama Islands are an example of an area where this process is going on today.

This image, showing about half a meter of thickness, represents many thousands of years of time during the Devonian Period some 350 million years ago. Some limestone layers are nearly pure calcite, and other layers have some silt or clay content that helps them better resist weathering. The dark layer near the top contains a larger fraction of silica-based microfossil skeletons, turning the rock closer to chert. This layer, too, is more resistant to weathering.

Limestone dissolves in rainwater more easily than other rocks. Rainwater picks up a small amount of carbon dioxide during its passage through the air, and that turns it into a very weak acid. Calcite is vulnerable to acid. That explains why underground caverns tend to form in limestone country, and why limestone buildings suffer from acid rainfall. In dry regions, limestone is a resistant rock that forms some impressive mountains.

Under pressure, limestone metamorphoses into marble. Under gentler conditions that are still not well understood, the calcite in limestone is altered to dolomite, changing it to dolomite rock.

See more limestone pictures in the Limestone Gallery.

See more sedimentary rocks in the Sedimentary Rocks Gallery.

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