Learn about this unique formation found in Wind Cave
- Credit / Author:
- NPS video by Matthew Chuvarsky
- Date created:
- 2011-03-21
“Simple calcite rock. But something looks different…. Is this an ancient spider web or petrified honeycomb? Use your imagination. This unique cave formation has taken on many different names to many different people, but to us this is boxwork”
“Welcome to the Wonderful Wind Cave. I’m Matthew, a ranger at Wind Cave National Park and I bet that you’re wondering, ‘what on earth is boxwork?’ It may not look much like a box to you, but to some of Wind Cave’s earliest explorers the formation all around me looked like post office boxes, and the name has stuck for over a hundred years. However, our understanding of boxwork is constantly evolving. And Wind Cave National Park is the perfect place to study boxwork because, believe it or not, Wind Cave contains nearly 95% of the entire world’s known boxwork formations!
“So what does make boxwork so unique and how did Wind Cave end up with more boxwork than any other cave? Well, I personally think that we just got geologically lucky. You see, there are many different factors that have to all come together in order to form boxwork. Let’s start with water.
“A long time ago, the land that is now Wind Cave National Park would have been covered in water and I would be swimming! This was a salt water sea and it was full of sea creatures. The critters’ shells, along with sea mud, became the limestone rock around me; while the salt became pockets of gypsum salt in between the layers of limestone.
“The next step is that those pockets of gypsum needed to get wet. You see, gypsum is like a sponge, when it gets wet it expands. The problem is, in our case, the gypsum was surrounded by solid rock, where could it expand to? Nowhere! That is, until the water-logged gypsum pushed so hard against the surrounding limestone the limestone was shattered. The gypsum was then able to ooze into the cracks in the limestone.
“Now this is where it gets interesting but also chemically complicated. You see, over time, all that water began to chemically change the gypsum into calcite, which is just a little harder than either gypsum or limestone. The calcite then filled up all those cracks in the rock. Think of this new formation like a fossil, a fossilized cave crack.
“Finally, the ever important, ever changing water became acidic. Now, we’re talking about a weak acid called carbonic acid, but over long periods of time this acid is able to slowly dissolve limestone; and that’s exactly what happened here at Wind Cave (in fact, that’s what created the cave!). The important thing is, since calcite is just a little harder than limestone, while the acid was able to dissolve the limestone, it wasn’t quite strong enough to dissolve the calcite. Think about it like a brick and mortar wall. The limestone would be the bricks; the calcite the mortar. Now imagine that just like the water dissolves the limestone, you remove the bricks. What would be left? What would it look like? A lot like the boxwork behind me, wouldn’t it?
“But, unlike mortar, boxwork is infinitely varied. It changes depending on the size of the original crack, or the mineral inclusions in the calcite, or as other formations like popcorn and frostwork grow on top of it. This makes each piece of boxwork even more unique. And boxwork is also old; in fact, the boxwork had started forming before the cave itself had. This means it can’t grow any more. What you see here and now is it. This is all the boxwork we will ever have. Therefore, at Wind Cave National Park we not only want to teach you a little about the boxwork, but we also need your help to protect it. After all, if anything happens to this boxwork, it’s gone forever.
“So I’d like to thank you for spending a little time with me in the Wonderful Wind Cave and learning something about our unique boxwork formations. And if you ever find yourself in South Dakota, stop by to see us and some of this lovely boxwork in person!”
