Toppling phenomena were observed along the cut of forestry road where rhyolitic welded tuff of the Nohi rhyolitic rocks is distributed. The rock mass of the welded tuff is separated into platy segments at the site by joints that strike parallel with the slope surface and dip 80° to 90°. The joints decrease in their number and the rocks near joints change in their texture with increasing vertical distance from the slope surface. These facts show that the joints have developed by weathering action from the slope surface.
The toppling phenomena are inferred to have developed through the following process. Firstly the development of joints separated the rock mass into platy segments with narrow width, and decreases the strength of segments against bending and buckling. Secondly segments began to topple when they are buckled mainly by their own weight or bended by the force exerted by adjacent segments.