Limestone is a sedimentary rock composed largely of calcium carbonate. It forms from skeletal fragments of marine organisms and makes up about 10% of all sedimentary rocks. Limestone erodes over thousands to millions of years through water and weak acids, forming caves and karst landscapes. It has many uses including building material, road base, fillers, and feedstock for other materials. Stalactites form on cave ceilings through dripping water depositing calcium carbonate over time.