The landscape of Wind Cave National Park is formed by the rock types, their structure, and how they were weathered and eroded. Gently tilting layers of sedimentary rocks lie under most of the park.
The Oldest Rocks
The oldest rocks are exposed in the northwest part of the park. These are schists and pegmatites. The schists are metamorphic rocks which formed under heat and intense pressure during an early episode of mountain building, about 2 billion years ago. They have almost parallel bands, or foliation, caused by the growth of mica crystals under pressure.
Pegmatites are made of large crystals of glassy-gray quartz, pink feldspar, silvery micas, and shiny black tourmaline. Pegmatite is an igneous rock, similar to granite. It hardened from magma and hot fluids. In places, the pegmatite intruded into the schists. This proves the pegmatite is younger than the schists, but still very old at 1.7 billion years. The emplacement of the pegmatite probably occurred during another mountain building event.
NPS Photo by John Tyers
To the southeast of the igneous and metamorphic rocks, progressively younger layers of sedimentary rocks are at the surface. They span a time from the origin of abundant sea life, about 600 million years ago, to the end of the age of dinosaurs, about 60 million years ago. During these years, seas advanced and retreated over this region many times. Periods of deposition of sediments alternated with periods of erosion. About 65 million years ago forces within the earth produced another period of mountain building, raising the "modern" Black Hills.
Shaping the Landscape
Since that uplift, weathering, erosion and minor uplifting have been shaping the Black Hills. Sediments produced by the erosion filled some valleys within the park and spilled outside the Hills to the east, forming the layers now visible at Badlands National Park. Rocks more resistant to weathering and erosion, like pegmatite, limestone, and sandstone, form ridges or plateaus. Weaker rocks, like schists and shales, form valleys. Examples of limestone, sandstone and shale are visible in Beaver Creek Canyon, Wind Cave Canyon, and Red Valley. Schists and pegmatites are visible along State Route 87.