Geology of Wind Cave National Park
Wind Cave National Park is home to a whole labyrinth of caves and passages with superb examples of a unique boxwork formation (check out the photo).
Located within the Black Hills of Dakota in the Madison limestone formation, this limestone laid down around 350 million years ago – mostly made up of seashells from the warm seas. Just as the limestone was accumulating there was also particles of gypsum crystallizing from the water caused by evaporation during the extreme dry periods. This is how the irregular shapes were formed in the limestone. These gypsum deposits, however, were largely unstable as they grew and decreased periodically absorbing and then expelling water. Some of this thick gypsum was forced into the cracks where it crystallized later becoming transformed into calcite by water rich in ions. This forms the basis of the unique boxwork features.
Gradually the seas receded and fresh water arrived in the area, and over a period of time (a very long time) the water became rich in acid from the calcite which dissolved the limestone and began to create the first caves – around 320 million years ago. Of course, nothing stays the same for long and eventually the area was once again covered by seas which deposited layers of sandstone and red clay over the limestone. Some of this was washed into the cave passageways. Over the next 240 million years or so the seas continually advanced and retreated bringing with it sediments then eroding the sediment away. During the uplift of the Black Hills (around 50 million years ago) more fractures were formed within the limestone resulting in the formation of even more caves. The water didn’t take the usual route of flowing river-like through these caves, instead it just hung around gradually dissolving the passages in a series of small cracks – this is how the unique boxwork formation was created.
So where did all the water go? Well, the most likely theory seems to be that it began to drain away around 40 or 50 million years ago, it is presently around 500 feet beneath the surface at the Lakes.
Some of the caves at Wind Cave National Park are more than 300 million years old and that’s old, even by cave standards – making them some of the oldest caves throughout the world. Much of the complex remains unexplored although at present there are around 139 miles of known cave beneath Wind Cave National Park.