Speleothems (Cave Formations) - Wind Cave National Park (U.S. National Park Service) Most of the speleothems in the cave form by similar processes. The water passes downward through the soil above the limestone, absorbs carbon dioxide, and becomes acidic. As a weak acid, the water is able to dissolve a small amount of the limestone rock as it passes through cracks and pores on its journey down into the cave. As this water drips into the air-filled cave, dissolved carbon dioxide is given off. Because the water has lost carbon dioxide, it cannot hold as much dissolved calcium. The excess calcium is them precipitated on the cave walls and ceilings to make up many of the different kinds of formations. Most calcium is precipitated in the cave as the mineral calcite (CaCO3).