Limestone and chalk are sedimentary rocks of organic origin derived from the accumulation of corals and shells in the sea. In its pure state, limestone is made up of calcite or calcium carbonate, but where magnesium is also present it is termed dolomite. Chalk is a very pure form of limestone, white, and rather soft. Limestone is soluble in rain water, which, with carbon dioxide from the air, forms a weak acid. A region with a large stretch of limestone therefore possesses a very distinct type of topography. It is then termed a Karst region, a name derived from the Karst district of Yugoslavia where such topography is particularly well developed.