As snow accumulates at the start of the glacier it compresses the layers below it. Once the snow reaches a certain depth, the bottom layers begin to compress. Snow flakes compact to granules and eventually super-compacted ice. This deformation is necessary for the glacier to begin spreading, or flowing, outward. Presently, Greenland and Antarctica are covered almost entirely by this type of glaciation.
Glaciers can also occupy more enclosed areas.
Mountainous areas can contain valley and cirque glaciers, which form on high mountains and flow downslope. As recently as 12,500 years ago, Mt. Mansfield and Camel's Hump were just beginning to poke their summits out of the surrounding glacier ice. As the ice receded, changes in the land began to show. Material embedded within the ice was deposited as melting occurred, and new landforms were created. Many of these deposits are visible today and, to a somewhat trained eye, they can tell us more about the last glaciation.