NPS Photo by John Tyers
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.
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 calcite, sulfur was chemically freed to form either sulfuric or sulfurous acid. These acids dissolved the limestone to form the first cave passageways approximately 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, occuring between 40 and 60 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.
NPS Photo by Tom Bean
Slow moving water was also important in exposing boxwork. At the edges of the former gypsum masses where expansion 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.
Where 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 cave popcorn on cave walls and ceilings. Formations that need more water such as 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.
Photo by Art Palmer
A Very Unusual Cave
Portions of Wind Cave are 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 139 miles of known cave (as of 2012) fit under just 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.