Limestone is a sedimentary rock, which means it was formed from small particles of rock or stone that have been compacted by pressure.Sedimentary rock is important because it often contains fossils and gives clues about what type of rock was on the Earth long ago.he second way 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. The area around the Great Lakes, such as Michigan, Indiana, and Illinois, has a large amount of limestone.