I really like this part of the park. It’s the Yellow Mounds area, aptly named, and it says so much geologically. The yellow is the oxidized Pierre Shale [pronounced Pier] that was exposed to air and plants once the Western Interior Seaway left the middle of the continent some 60 or 70 million years ago. I can’t remember specifically what the purple is, but it’s pretty! The grey is the Chadron formation, which is around 40ish million years old, has such a plastic consistency that it bends down slopes, sometimes covering the underlying layers. Because of the plasticity, the tops of the Chadron form rounded hills, called haystack mounds or haystack buttes. Coool, eh?
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