Glaciers and snowfields cover about 80 percent of the mountain above the level. The glaciers declined by an average of 34 percent from 1907-2004. Glaciers on Mount Hood retreated through the first half of the 20th century, advanced or at least slowed their retreat in the 1960s and 1970s, and have since returned to a pattern of retreat. The neo-glacial maximum extents formed in the early 18th century.