Laying the Foundation
Wind Cave is formed in the Madison Limestone formation. Locally, the Madison formation is called Pahasapa Limestone. (Pahasapa is the Lakota word for "Black Hills.") This limestone was deposited in a warm shallow sea about 350 million years ago and is composed mostly of fragments of calcium carbonate seashells. Coinciding with the accumulation of limestone, bodies of gypsum (calcium sulfate) crystallized from the seawater when arid conditions caused evaporation. The gypsum formed irregular shaped masses within the limestone.
The gypsum masses were unstable. The size of these masses increased and decreased as they absorbed and expelled water. This caused fracturing to occur with the gypsum and in the surrounding limestone. Like thick toothpaste, some gypsum squeezed into these cracks and crystallized. At a later time, water rich in carbonate ions converted all of the gypsum to calcite (calcium carbonate). This set the stage for the cave and boxwork to form.