Boulder Glacier is a small glacier in Montana’s Glacier National Park, north of Boulder Peak and west of the Continental Divide. In recent decades it has shrunk rapidly. Between 1966 and 2005, it lost more than 75 percent of its surface area, and by 2005 it covered only 13 acres, less than the 25 acres often cited as the minimum size for an active glacier. Photographs taken in 2007 by U.S. Geological Survey researchers showed it had nearly vanished, a dramatic change from 1910 images that show a much larger mass of ice.