I spent a couple days in May 2017 in the Slim Buttes area of Northwestern South Dakota. It’s a unique area with huge clay spires jutting up out of the plains. In areas, it looks like the Badlands of SD, but the spires are much more sparse. There are a couple of formations in the region east of Buffalo SD called the Castles. They have a walking path from a small parking area up to the top of a peak to view them from. It’s a pretty area, and I’d recommend a stop if you are ever in that region of South Dakota.