Colluvium-filled bedrock hollows are the cause of many shallow earth landslides in steep mountainous terrain. They can form as a U or a V shaped trough as local bedrock variations reveal areas in the bedrock which are more prone to weathering than other locations on the slope. As the weathered bedrock turns to soil, there is a greater elevation difference between the soil level and the hard bedrock. With the introduction of water and the thick soil, there is less cohesion and the soil flows out in a landslide. With every landslide more bedrock is scoured out and the hollow becomes deeper. After time, colluvium fills the hollow, and the sequence starts again.