Limestone, shell, and dolomite are types of marine deposits that have accumulated in Florida over millions of years during times when the state was covered over with seawater or shallow marshes. During this time, limestones formed by chemical deposition and the accumulation of shells from sea creatures. Many invertebrate animals (animals without backbones) take calcite from seawater to construct their shells. When they die, the shells fall to the sea bed. Over the millennia, their remains slowly built up layers of sediment thousands of feet thick. These sediments are the limestone, shell, and dolomite formations that are mined today.