Some limestone deposits date back to the Jurassic Period, 155 million years ago.
Limestone is a sedimentary rock composed mainly of calcium carbonate. Sedimentary rocks are formed when weather processes break down rocks into a fine material that eventually forms a sediment deposit. The sediment undergoes compaction and cementation over time, and eventually is turned into rock. Limestone is formed by both inorganic and biochemical processes.
Coral Reefs
Coral reefs are an example of limestone formed by a biochemical process. Algae living within the coral secrete calcium carbonate, which acts to cement the non-vertebrate animals together. The limestone is formed in the shape of the coral skeletons. Ancient coral reefs have been found in the large, exposed limestone deposit in the Guadalupe Mountains National Park in New Mexico, and in the mountains of the Australian outback.
Chalk
Chalk is a biochemically produced form of limestone. This type of limestone consists of the skeletal remains of single-celled marine organisms. It is a soft, porous rock that rubs off on your fingers. The infamous White Cliffs of Dover in England are composed of chalk. It is also found in the Paris basin and in the Mississippi River valley in the southern United States.
Dripstone
Dripstone is also known as travertine or Mexican onyx. It is an inorganic type of limestone precipitated from ground waters rich in calcium ions. It consists primarily of calcium carbonate, but can contain impurities that affect the color of the stone. In caves, where dripping water is saturated with calcium carbonate, stalactites and stalagmites are formed.
Oolitic Limestone
Oolitic limestone is formed by an inorganic process. Small, spherical grains called ooids form on particles suspended in warm, agitated marine waters saturated with calcium carbonate. The ooids are white, rounded and form in concentric layers to produce oolitic limestone. Oolites look like small pearls. They are common in areas with high tidal activity. The formation requires a seed or particle, such as a shell fragment, which is washed on the bottom of the seafloor and gathers layers of calcite from the water.
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