From the sign:
Red Rock Coulee has a diversity of features from bare arid slopes, to native grassland and plateau. The main feature is a large area of exposed bearpaw shale out of which appear dozens of sandstone concretions. These spheroids in size range from 1.5 to 2.5 metres in diameter and are exposed to varying degrees from the soft bedrock.
These concretions were formed in the very sediments upon which they sit. The process by which concretions are formed leaves them harder and more resistant to erosional forces than the surrounding material. When the parent material erodes the spheres are left exposed and they in turn begin to erode.
Photos were taken September 20, 2014.