The oldest rocks in South Dakota’s Black Hills are Precambrian-era igneous and metamorphic rocks, which formed under heat and pressure nearly 2 billion years ago. During the Mississippian time period, between 345 and 360 million years ago, a shallow sea covered the area. The sea advanced and receded several times. Sediment and calcium carbonate shells accumulated at the bottom of the sea, and over time, were compressed to form the Pahasapa Limestone (regionally known as the Madison Formation). The shells that formed the limestone came from ancient marine animals such as brachiopods. Fossils from Mississippian time are visible in the cave today.
Precambrian-era
Precambrian-era