I mapped soils in that area back in 1989 and am familiar with that geologic formation. Brought back a couple of boxes of dinosaur bones myself. I have a vivid memory of pulling a clam shell out of the sandstone...the mother-of-pearl was still clear and colorful, although the shell was 70 million years old. It looked like something I just picked up off the beach.
In Montana's cool, dry climate, things just don't rot very quickly.
And what proof is there that that formation is 70 million years old?
Personally, I have my doubts about the age of things like this. It usually goes to some construction of circular logic.