The exact cause of the landslide is still debated. One theory is that the soft shales below are too weak to support the weight of the heavy sandstone above and collapse under it, or that, because water can run through gritstone but not shale rock, trapped water may have "lubricated" the rock to the point where one layer slid over another, causing the landslide. A further possibility is that a valley glacier steepened the sides of the valley, leaving unstable slopes that failed after the glacier melted, causing the landslide. However, immediately upstream is a normal river valley so any glacier would have been small.