Perennial snowfields in the high country often are mistaken for glaciers. In general a true glacier moves and needs to be at least 100 feet deep and 25 acres in size. The process of more deposition of snow in the cold season than melt in summer, coupled with continued melting and freezing, turns the body into ice. As it thickens more weight is added to the lower ice layers. And when the glacier forms on steep slopes, as is the case in Montana, gravity causes the underlying strata to move. Crevasses are a sign of ice movement. Late spring and early summer is when the maximum new snow load is reached. Movement occurs in summer, and the flow is probably no more than an inch or two a day on average in the Northern Rockies. In coastal Alaska a glacier could form in 10 years, but in the drier Northern Rockies it takes much longer. Check out the website “Glaciers of the American West.”