My next post about the Geology of the National Parks Through Pictures is from our move across the country from Utah to New York. Along the way we visited 13 National Parks as well as some other sites. This was the 8th National Park along the way.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
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Obligatory entrance sign
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| Geological map of the Black Hills, including Wind Cave National Park (noted just below the southernmost point of the purple rocks). Image courtesy of the NPS. |
Like the Mount Rushmore National Memorial and the Crazy Horse Memorial, Wind Cave National Park is located within the Black Hills dome. But unlike those parks, Wind Cave National Park is not located within the Harney Peak Granite. The rocks in this region started forming about 1.6 billion years ago, during the Proterozoic. This is when magma began to work its way up through the surrounding rocks in this area at the time. While still well below the surface of the earth, that magma cooled slowly to form the Harney Peak Granite. Due to the hardness of the granite, from its formation at 1.6 billion years to about 500 million years ago, several rocks formations were likely deposited on top of it and then eroded away leaving no trace. However, starting around 500 million years ago, new rocks that were deposited on the Harney Peak Granite have been preserved. These rocks include the green "Limestone Plateau" rocks as pictured in the map above.
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| Geological map of Wind Cave National Park. Map courtesy of the NPS. |
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| Legend for the Wind Cave NP Geological Map. Image courtesy of the NPS. |
The "Limestone Plateau" can be subdivided into numerous different rock formations, once we zoom in on the part of the dome within Wind Cave National Park. Deposited directly on top of the Harney Peak Granite is the Ordovician Age Deadwood Formation (~480 million years old). This is a mixture of sandstones and limestones, as well as some other rocks, that represent a transgression, where the sea level started to rise and the oceans started to cover this region. On top of that is the Mississippian Age Englewood Limestone (~ 363 –358 million years old). The Englewood represents a shallow marine environment.
And on top of the Englewood Formation is the primary rock of interest for Wind Cave National Park, the Pahasapa Limestone (AKA Madison Limestone). This is a Mississippian Age limestone that dates to about 358 to 341 million years old. This is the limestone from which Wind Cave is formed within. The Pahasapa Limestone is up to 420 feet thick and formed from the deposition of seashell fragments within a warm shallow sea across the region. Following deposition of the Pahasapa Limestone, the seas started to recede away ~320 million years ago. This is when the first parts of Wind Cave started to form.
When the seas started to come back, (~300 million years ago) rocks were deposited on top of the Pahasapa Limestone. This time they were made up of limestones, sandstones, and red clay shales. This formation, called the Minnelusa Formation, created a semi-permeable barrier on top of the Pahasapa, limiting water flow down through the rock from the surface. An interesting thing to note is that the Minnelusa Formation can actually also be seen within Wind Cave as well, with some of the red clay visible in higher parts of the cave near the Garden of Eden and Fairgrounds rooms. The inclusion of the Minnelusa Formation within Wind Cave is how scientists know that the cave started forming over 300 million years ago, making it one of the oldest known cave systems in the world. Following deposition of the Minnelusa Formation, sea levels continued to rise and fall, slowing dissolving out the cavern system. This transgression-regression cycle of the seas continued until around 70 million years ago when the entire area started to be uplifted into the Black Hills dome. As the dome was being uplifted, fracturing of the rocks allowing more water to flow through the rocks, quickening the pace of cave formation.
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| Cross section of the Black Hills. Image courtesy of A Textbook of Geology. |
In geological terms, a dome is an anticlinal structure where the rocks dip gently away from the center in all directions. After folding, fracturing, and faulting, this causes the overlying rocks to break apart in the middle, allowing for easier erosion of the them. Once these younger rocks have eroded away, the older rocks are exposed with the oldest rocks exposed in the center. As before, due to the extreme hardness of the Harney Peak Granite, they withstood erosion and remained around much longer. Their hardness is also why the Black Hills have these granitic mountain peaks that have not eroded away.
Most limestones are comprised primarily of the mineral calcite, which is a variety of calcium carbonate (CaCO3). Calcite is the mineral that most sea shells are made out of. Calcite has a physical property that it will dissolve in slightly acid waters, which is where you get the formations of caves. One of the unique properties of the Pahasapa Limestone though, is that when it formed, it had significant quantities of gypsum, another type of mineral, within the rock. Gypsum (CaSO4) has the ability to absorb large quantities of water, and when it does so it, expands and then contracts when that water is expelled. Over time, as water entered and left the limestone, the gypsum expanded and contracted, fracturing the surrounding rocks. As time progressed, the water that flowed through these fractures within the limestone slowly started to replace the gypsum with calcite.
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| Boxwork Formations |
Geologically speaking, Wind Cave is most well known for its boxwork speleothems (cave formations) (as seen in the image above). The boxwork is a result of these gypsum fractures that were refilled with calcite. The calcite crystals that filled in the fractures were more resistant to dissolution than the surrounding limestone was, so as the limestone rock dissolved away, revealing the cave, the calcite crystals that filled the gypsum fractures remained behind. These calcite crystals are what primarily form the boxwork speleothems. The fracture pattern, and therefore the speleothem pattern, forms a very angular, boxy configuration, giving the speleothems the term boxwork.
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| Map of Wind Cave |
Even looking at a map of the cave itself, it also forms an angular and boxy configuration, producing a fractal pattern (where a pattern is repeated over different scales). Overall, the cave descends 643 feet and extends over 167 miles (as of 2024). As the dome was uplifted, the water started to drain out of the cave about 40 to 50 million years ago. The current day water levels are located about 500 feet below the surface in an area of the cave known as "the Lakes".
Although the most well know, boxwork isn't the only cave formation within Wind Cave. There are other formations such as popcorn ceiling and frostwork. However, formations such as flowstones and dripstones, like stalactites and stalagmites, are more rare here due to the drier climate and the semi-permeable Minnelusa Formation overlying the cave that limits the amount of water flowing through.
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