Geology of Wind Cave National Park
Learn more about the geology of Wind Cave National Park.
Wind Cave is located in the southern Black Hills of South Dakota and is most famous for its large concentration of a fairly rare cave formation known as boxwork, of which more can be found in this park than in all other caves on Earth combined.
This region of the country where Wind Cave National Park is located today was periodically under water during much of the past several hundred million years. About 350 million years ago, this region was covered by a sea with abundant shelled marine life, the remains of which are fossilized in the limestone that makes up Wind Cave. Masses of gypsum (hydrated calcium sulfate, Ca2SO4-2 H2O), formed around the same time as the limestone, as the marine water evaporated and left gypsum salt deposits behind. As water came and went from the gypsum, it shrank and expanded, causing the gypsum and limestone to crack. The gypsum began to make its way into the cracks of the limestone and crystalized. Later, the gypsum transitioned to calcium carbonate (calcite) beginning the formation of the caves.
About 320 million years ago, the first cave passage was formed. The oceans retreated and allowed for acidic fresh water to take its place. This acidic water was the reason the gypsum was able to convert to calcite and create sulfuric acid and sulfurous acid. When the sulfuric acid and sulfurous acid found its way to the limestone, the acids dissolved the rock and created cave passages.
Over time, the seas came again and deposited red clay, sandstone, and more limestone. For about the next 240 million years, the seas came and went, depositing and eroding sediments as through transgressions and regressions. Between 40 and 60 million years ago, the Black Hills were tectonically uplifted, which further opened cavities within the limestone. The water in the cave is believed to have started draining about 40 to 50 million years ago. This process is extremely slow and there is still water in the cave today, continuing to shape the cave.
Park Geology
The Black Hills are a small and isolated mountain range located in western South Dakota and a sliver of Wyoming. The Black Hills are dome-shaped and formed by granite and gneiss at their interior and limestone, shale, and sandstone on their surface. Ever since the uplift that created the mountain range, weathering, erosion, and smaller uplifts have shaped the Black Hills. Rocks that are more resistant to weathering, such as pegmatite and sandstone generally form the ridges and plateaus, while weaker rocks that weather more easily, such as shales, form the valleys.
The oldest rocks in Wind Cave National Park, schists and pegmatites, can be found exposed in the northwest part of the park. Schists are metaphoric rocks that formed with the origin of the Black Hills around two billion years ago. Schists are generally shiny, with abundant muscovite crystals, and are formed through heat and pressure of existing rocks. Pegmatite is an igneous rock, which slowly intruded into the schist about 1.7 billion years ago. The very slow cooling of magma allows for very large (sometimes HUGE) crystals to be left behind. Pegmatites are similar to granite, containing large crystals of gray quartz, pink feldspar, clear to silver muscovite (mica), and black tourmaline. The sedimentary rocks in the park, within which the caves form, are younger than the metamorphic and igneous rocks. The sedimentary rocks of the park formed from about 600 million years ago to about 60 million years ago, deposited by marine sediments and many of them were tilted by tectonic forces.
Cave Formations (Speleothems)
Boxwork
Boxwork is the most well-known feature in the park, where it is the most well-formed and abundant in all the world. Most cave formations form from water dripping down into passages, but boxwork is made of thin blades of calcite that stick out from the walls and ceilings of the waves. The dissolved calcium carbonate crystallized in the cracks of surrounding rock, but when the rock eroded, the resistant calcite crystals remained, forming a box-like, honeycomb pattern.
Frostwork
Frostwork is a cave formation of needle-like aragonite crystals resembling frost. Aragonite is a polymorph (same chemistry, different crystal form) of calcite. The long, needle-like aragonite crystals form into bushes.
Cave Popcorn
When walking through the caves, you may see small, bulbous growths protruding from the walls. These grape-like structures are called cave popcorn and are one of the many cave formations formed with calcite crystals.
Dogtooth Spar
Dogtooth spar are jagged, pointed calcite crystals commonly found in small cracks of limestone walls. These formations form very slowly where there is little water movement.
Helictite Bushes
Helictites are formed by water entering the caves through cracks in the limestone walls. There are many helictites that can be seen in the caves, yet bushes are even more rare. Helictite bushes are usually found on cave floors and are large growths of calcite that branch off in a twisting fashion.
Flowstone/Dripstone
When there is a high concentration of calcite within a crack of the cave wall, dripstone or flowstone occurs. Stalactites and stalagmites, formations commonly found in caves, are rare dripstone features of Wind Cave. Stalactites hang down from ceilings (hang on tight!) and form as drops of water seep through cracks of the cave. When the drop drips from the ceiling, carbon dioxide is lost and calcite deposits where the drop was. This process occurs consistently in the same location and an icicle-like structure forms overtime.
Stalagmites grow the opposite way of stalactites (they “might” reach the ceiling someday). These structures grow in the upward direction and form when drops fall from stalactites. If the two features continue to grow for long enough, a column will form where they meet each other.
Calcite Rafts
Calcite rafts are thin sheets of calcite that can be found on the surface and floor of Calcite Lake where were once filled the floor. These sheets float on the surface of water because of surface tension, sinking to the floor when they become too heavy.
Gypsum Features
Gypsum is a mineral that contains calcium, sulfur, oxygen, and hydrogen. Gypsum features are generally found in the drier parts of the cave and can be found in the shape of clusters of needle-like crystals, puffs of cotton, or curved/coiled crystals.