A hoodoo or toadstool refers to a geologic structure where a large block of erosionally resistant (hard) rock is balanced by more quickly eroding (soft) rock underneath, making a toadstool-like feature. This particular hoodoo is made of a boulder of well-cemented river channel sediment from the Sharps Formation (Oligocene, 30-26 million years ago) balanced on eroding mudstone of the Scenic Member of the Brule Formation (Oligocene, 33.7-32.0 million years ago).