Found these while crawling around in Perkins County South Dakota, Hell Creek Formation. My speculation is that they're poorly conglomerated petrified stumps or possibly tree limbs? The only soil I've seen them in is a very uniform gray and extremely popcorny soil. In some areas I've found as many as 10 of various sizes in an area no bigger than 200 square feet. I don't have a size reference in the photos because I mainly took the pictures thinking they were a neat part of the landscape. They range from sizes of 4" all they way up to 20"+.
Anyone else seen these? Thoughts?