Earthquake-induced slide and alluvial fan on downstream side of Madison River, Montana. Light colored rock is dolomite marble, the more resistant rock that supported weathered, mica-rich, overlying layers. An earthquake in 1959 fractured this dolomite and caused a great avalanche of rock, dirt, and trees to cascade from the steep south wall of the Madison River Canyon, about 395 m above the canyon floor. The avalanche formed a barrier 60-120 m high that completely blocked the gorge and the flow of the Madison River and created a lake. Twenty-eight people were believed to have been engulfed in the 88,000 kg slide.

Image Credit: B. Bradley. University of Colorado at Boulder