Most carbonate rocks were deposited from seawater. These sedimentary carbonate rocks are common on every continent and have formed through most of geologic history; they are still forming today in the tropics as coral reefs and at the bottoms of shallow seas. 

Marine limestone forms because seawater has high concentrations of two key dissolved chemicals-calcium (Ca++) and bicarbonate (HCO3-) ions. In the near-surface layer of most oceans, corals, clams, and other sea-dwelling creatures use these two chemicals to make protective shells by combining them to form calcite or "aragonite," which is the same chemical composition as calcite but has a different crystal form. 

Sedimentary limestone deposits can be extensive, covering hundreds of square miles, and can be relatively uniform in thickness and quality. Therefore, limestone quarries can be large and long lived, mining limestone layers that can be hundreds of feet thick over areas of several square miles. Many quarries produce multiple products, and crushed rocks that are not pure enough for certain uses may still be suitable as road aggregate. Marble quarries can also be very large. However, these rocks that were once regularly bedded have been metamorphosed into irregularly shaped bodies that are more difficult and costly to mine. 

In large parts of the United States there are extensive deposits of marine limestone of various ages from a few thousand to more than 350 million years old. Some deposits have chemical grades as high as 95% CaCO3. However, some areas are completely without any suitable limestone deposits. Most of the cost of limestone to the customer is determined by how far away it comes from and how it is shipped. Shipping by barge on water is cheaper than by train which, in turn, is cheaper than shipping by truck.