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Frequently Asked Questions:
Glossary of Geologic Terms

compiled by Shari Kelley

What is limestone?
Limestone is a bedded sedimentary rock made up primarily of the mineral calcite (calcium carbonate). Limestone starts out as limy mud, calcareous sand, or fossil shell fragments deposited on the ocean floor. Limy mud can form through chemical precipitation of calcite out of oceanic water in a warm, shallow sea. In addition, many organisms like clams and oysters extract calcite out of oceanic water to make their shells, and when these organisms die, their shells accumulate on the sea floor. Sometimes the shells can be broken down to sand-sized particles by wave action. When the limy mud, calcareous sand, and shell fragments are buried and groundwater passes through the sediment, the calcite in the sediment often recrystallizes, thus forming a solid rock known as limestone.
What is sandstone?
Sandstone is a bedded sedimentary rock that often contains sand-sized particles of quartz, but sandstone can contain a wide variety of other minerals, depending upon the source rock from which it is derived.  Sandstones form when pre-existing rocks break down through the erosive action of water, ice, and wind into sand sized particles.  When the transporting agent (e.g., river, glacier, windstorm) losses energy, the sand grains are deposited.  As burial occurs, mineral-enriched groundwater passing through the sediment precipitates minerals in the pore spaces between sand grains, causing cementation of the rock by calcite, clay, quartz, or iron oxides.