Red Valley
Geology
The Landscape

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.

Geologic Cross Section

Black HillsThe 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.

FossilsSedimentary Rocks

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.

Geology MapShaping 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.

The Pahasapa Limestone

Wind Cave formed in the Pahasapa Limestone. This limestone was deposited in a warm shallow sea about 350 million years ago and is composed mostly of fragments of calcium carbonate sea shells. Coinciding with the accumulation of limestone, bodies of gypsum (calcium sulfate) crystallized from the sea water, when arid conditions caused evaporation. The gypsum formed irregular shaped masses within the limestone.

The Role of Gypsum

The gypsum masses were unstable. Their volumes increased and decreased as they absorbed and expelled water. This caused fracturing to occur within 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.

The Cave Develops

Since acid-rich water dissolves limestone, a chemical change in the groundwater had to occur for the cave to form. The oceans receded allowing fresh water into the region. As gypsum was converted to limestone, sulfur was chemically freed to form either sulfuric or sulfurous acid. These acids dissolved the limestone to form the first cave passageways some 320 million years ago.
     After the first period of cave formation, seas again advanced over this area. About 300 million years ago, layers of red clay, sandstone and limestone of the Minnelusa Formation were deposited above the Pahasapa Limestone. Some of this sediment washed into and filled early-formed cave passageways. These "paleofills", are visible in higher levels of the cave, near the Garden of Eden and Fairgrounds rooms.

A Complex Cave

Seas continued to advance and retreat over the area for the next 240 million years. Deposition of sediment alternated with erosion. Development of the cave was probably slow until the most recent Black Hills uplift about 65 million years ago. This opened more fractures in the limestone allowing more cave to form. The waters that made the cave probably sat in the limestone for long periods of time. Water did not flow through the cave like a river. The water had plenty of time to dissolve passageways along the many small cracks, thus developing the complex maze-like pattern.

BoxworkBoxwork

Slow moving water was also important in exposing boxwork. At the edges of the former gypsum masses where the expansions had formed cracks, limestone was dissolved. This dissolving of the surrounding limestone, left the previously deposited crack fillings standing in relief. These exposed crystal fins are called boxwork. A river might have eroded boxwork from the cave.

Underground LakeWhere is the Water Now?

Geologists believe that the water began slowly draining from the cave 40 to 50 million years ago. Today the water level is about 500 feet below the surface at an area named "the Lakes". Water, however, is still changing the cave. Slow seepage of water produces frostwork and popcorn on cave walls and ceilings. Formations that need more water, like flowstone or dripstone deposits (stalactites and stalagmites), are rare in Wind Cave and are limited by the dry climate and semi-permeable clay beds above the cave.

Speleothems


A Very Unusual Cave

Wind Cave is over 300 million years old, making it one of the oldest in the world. Besides extreme age, other features make Wind Cave unique. The cave is large and extremely complex, the 113.05 miles (181.936 kilometers) of known cave fit under just over one square mile of land. The boxwork is rare and found in few other caves. Wind Cave has undergone many geological changes and the processes continue. Geologists have many questions yet to answer before we can fully understand the rich, incredible world below our feet.

See Palmer and Palmer 1999 for the latest theory on the Origin of Wind Cave.

Wind Cave National Park Nature & Science

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Page Last Updated: Saturday, June 5, 2004 12:11 PM
Web Author: Jim Pisarowicz