Limestone is formed in the calm, warm seas from the hard remains of corals and mollusks, among other marine organisms. Limestone is a sedimentary rock and consists mainly of calcium carbonate (CaCO3). Sea water is rich in calcium ions (`Ca^(2+)` ) and bicarbonate ions (`HCO_3^-`). Corals, mollusks, and other marine organisms use these two ions to form protective shells. Such shells are made up of calcite, which is purely calcium carbonate, or aragonite.
These sedimentary rocks are continually formed by the action of corals and mollusks. At many places, the fossils of corals (especially, skeletal remains) and shells of mollusks can be found in these sedimentary rocks and help us date these rocks. A large fraction of such rocks was made in the past, over thousands of years, when the sea level was higher than it is today.
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