A large mass of bedrock or regolith sliding downhill is known as a landslide. Large, disastrous landslides are possible wherever mountain slopes are steep.

Landslides can also result when the base of a slope is made too steep by excavation or river erosion.

Landslides range from rockslides of jumbled bedrock fragments to bedrock slumps in which most of the bedrock remains more or less intact as it moves.

In a rockslide, rubble travels down a mountainside at amazing speed.

Geologists think this speed is possible because there is a layer of compressed air trapped between the slide and the ground surface. This air layer reduces friction, so the rubble can move faster, and landslide takes place.