• Caption: The Tent Rocks. The hills here are made of volcanic ash and debris which is very loosely packed. Simply rubbing your fingers across the rock will wear off some of the material. The tents form underneath large rocks previously buried in the ash but then exposed by erosion. They vary in size from smallish rocks that most men could lift to small boulders. These rocks are actual solid rocks, so they erode much more slowly than the ash, creating tents below the rocks.