Landslides are common geologic hazards throughout Montana. “Landslide” is a term that describes gravitational mass movement of soil, rock or debris downslope. Types of landslides include rock falls, debris flows and slides. The steep slopes of western Montana are the most frequent region of land sliding. Some types of geologic materials, such as shale or glacial till, may be especially susceptible to landslides. For example, the area around the Fort Peck Reservoir has developed extensive landslide complexes since construction of the Fort Peck dam due to saturation and failure of the Bearpaw Shale.
Landslides can be triggered by earthquakes, severe rainstorms and snow melt. Old landslides may be reactivated by these same factors or disturbance of the slide mass by road building or construction activities.