Limestone can be formed as a result of evaporation, as is the case for stalactites and stalagmites in caves, or through the accumulation of calcium carbonate in marine environments. Limestone is either a biological or a chemical sedimentary rock, depending on its method of formation.
Limestone formations created through evaporation are chemical sedimentary rocks, also known as travertine. They are formed when droplets containing calcium carbonate leak through cave ceilings and evaporate. As each evaporated droplet leaves behind a small amount of calcium carbonate, a stalactite forms. If droplets fall to the floor and then evaporate, a stalagmite extends upward as the calcium carbonate starts to pile up. Many caves are filled with these characteristic limestone teeth.
Biological sedimentary limestone forms most often in warm, shallow marine waters in areas between 30 degrees latitude north and 30 degrees latitude south. This type of limestone is formed as marine organisms with calcium carbonate shells and skeletons die and accumulate layer by layer. Eventually, with extensive pressure and time, that calcium carbonate sediment hardens into limestone. These types of rocks often have fossils present in them. A less common method of limestone formation is through the direct accretion of calcium carbonate from either salt or fresh water.