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Boxwork is a fairly rare cave formation.
It's formed when dissolved calcium carbonate crystallized in cracks in surrounding rock. The surrounding rock then erodes away and leaves the boxlike calcite crystals which appear to be box-like. Here, we see boxwork in Wind Cave, South Dakota, which is the first cave national park and currently the 6th most extensive cave network in the world. It's also famous for its large concentration of boxwork.
Photo credit: Alex Demas, USGS. Original public domain image from Flickr
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