'A mass of snow ice and rock that falls down the side of a mountain?

Falling snow is called an avalanche. If falling snow entrains air then it is called a powder snow avalanche. Falling ice is called an ice slide. Falling rock is called a rock slide. Falling mud is called a mud slide. There may be some other special terms used.
en.wikipedia.org/wiki/Avalanche
More: www.google.com/#q=avalanche