The volcanic activity of Mt. Nijo deposited volcanic debris, which then rose to expose tuff, and after 15 million years of weathering and erosion, the rock formations became a group of strange rocks on this rocky mountain, about 150 meters above sea level. Rocks produced here include sanukite and garnet diorite. It is a prefectural natural monument and one of the highlights of Kongo-Ikoma-Kisen Quasi-National Park.