Limestone is formed in two ways. It can be formed with the help of living organisms and by evaporation.
1. Ocean-dwelling organisms such as oysters, clams, mussels and coral use calcium carbonate (CaCO3) found in seawater to create their shells and bones. As these organisms die, their shells and bones are broken down by waves and settle on the ocean floor where they are compacted over millions of years, creating limestone from the sediments and the pressure of the ocean water.
2. Limestone is formed is when water containing particles of calcium carbonate evaporate, leaving behind the sediment deposit. The water pressure compacts the sediment, creating limestone.