Japan recently experienced several heavy rain-induced landslide disasters on active volcanic mountains. In July 1990 and July 2012, shallow landslides and their transformed debris flows inflicted severe damage to many settlements in the Aso caldera. These shallow landslides occurred on sediment covered slopes consisting of alternating layers of tephra and “kuroboku” (humus andosols). An outcrop in the Aso caldera shows evidence of the initial stages of the shallow landslide. The outcrop consists of tephra and kuroboku layers deformed by gravitational movement to form shear planes and load marks in the outcrop－the latter are commonly observed in subaqueous sediments. Through observations of the outcrop and soil test results, we conclude that the gravitational deformation of these slope deposits resulted from the “flow” of the highly permeable kuroboku layer over the less permeable tephra layer.