limestone
A sedimentary rock formed chiefly by the accumulation of organic remains, as shells and coral, consisting mainly of calcium carbonate, and used as a building stone and in the manufacture of lime. (Ching, 1995) Limestone is both brittle and easily dissolved in water penetrating the rainwater. Its brittleness leads to the formation of multiple cracks. The slight amount of acid naturally found in rainwater then dissolves the soft limestone, and the water soon disappears beneath the surface, forming a new stream bed underground. (Mammoth Cave National Park, 2013) Photo from Chichen Itza, Mexico, 2011.