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Sedimentary Rocks - Page 2

© Geoff Habiger

Limestone can also be formed by the precipitation of the shells of dead animals, from microscopic animals called foraminifera, to bivalves and corals, in the ocean. This is the third type of sedimentary rock, those formed from organic sediment. Limestone formed in this way can contain large organisms like bivalves, crinoids, trilobites, and corals. When these animals die, their shells are broken up and cemented together. This type of limestone is known as coquina. When microscopic organisms like foraminifera die, their shells become compacted and form chalk. Another organic sedimentary rock forms from the carbon left over from dead plants. These plants are killed and then buried under sediment from rising sea level. These carbon rich sediments eventually form coal.

Sedimentary rocks form from a variety of sources, from clastic sediments or by precipitation from either chemical means or organic detrious. They are the most common types of rocks, covering most of the ocean floor and three quarters of the land. The next time you are driving and see layers of limestone, shale, or sandstones, take a moment to consider the processes that it took to form them.

Glossary
Chalk: rock formed from microscopic organisms.
Clast: a particle that is transported and deposited in a sediment.
Claystones: sedimentary rock composed mostly of clay-sized (less than .004 millimeters) material.
Coal: sedimentary rock formed from plant and organic remains.
Conglomerates: a coarse-grained (greater than 2 millimeters) clastic rock composed of gravel.
Coquina: limestone composed of sorted fossil debris that were transported before being deposited.
Diagenesis: chemical and physical processes that transform loose sediment into a hard sedimentary rock.
Foraminifera: microscopic organisms whose shells are one component of limestone.
Limestone: sedimentary rock consisting almost entirely of calcium carbonate.
Sandstones: sedimentary rock in which sand-sized (.0625 to 2 millimeters) material predominates.
Siltstones: sedimentary rock in which silt-sized (.004 to .0625 millimeters) particles predominate.

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