Volcanic ash from Black Hills overlying Pierre Shale.These veins throughout the park turned out to be the most interesting features.Trolls live in these caves.Can’t get over the clay-filled fractures.No smiling. This is serious geology.Arroyo behind our campsite.My favorite pic (so far).Note the thin soil profile. The quarter-section (“starvation claims”) farmers soon found out that it takes at least 1,000 acres to feed the family.