Mass Balance

A measure of the change in mass of a glacier at a certain point for a specific period of time. The balance between accumulation and ablation. Also called Mass Budget.

July 1979 photograph of a 12-foot deep snow pit dug into the 1978-1979 thick snow accumulation that fell on the surface of the Taku Glacier, Juneau Icefield, Tongass National Forest, Coast Mountains, Alaska. The thickness and density of the new snow is being measured to determine its volume and water content, as well as to study its metamorphism to glacier ice.

August 1968 southwest-looking photograph of the surface of the Bucher Glacier, an outlet glacier of the Juneau Icefield, showing several glacier tables, several conical mounds of sediment-covered ice, and several small piles of sediment left on the ice surface marking the position of former mounds, Coast Mountains, Tongass National Forest, Alaska.

Moraine

A general term for unstratified and unsorted deposits of sediment that form through the direct action of, or contact with, glacier ice. Many different varieties are recognized on the basis of their position with respect to a glacier.

·Ablation Moraine

An irregular-shaped layer or pile of glacier sediment formed by the melting of a block of stagnant ice. Ultimately, ablationa moraine is deposited on the former bed of the glacier. Also called Ablation Till.

August 1997 north-looking oblique aerial photograph of the moraine-covered surface of a stagnant piece of the former terminus of the Malaspina Glacier, Wrangell-Saint Elias National Park, Saint Elias Mountains, Alaska. The ice mass is more than 1/4 mile long and has a maximum sediment thickness exceeding 3 feet.

·Ground Moraine

A blanket of glacier till deposited on all of the surfaces over which a glacier moves, typically by moving ice.

North-looking, July 1998, photograph showing about a pair of glacially-sheared trees growing on a till-covered surface, Bering Glacier, Chugach National Forest, Chugach Mountains, Alaska. Bering Glacier flows through Wrangell-Saint Elias National Park.

·Ice-cored Moraine

A moraine ridge consisting of a drape of sediment overlying a mass of stagnant ice.

July 1968 northeast-looking photograph of the moraine covered surface of a large mass of stagnant ice, formerly part of the lateral moraine of the Herbert Glacier, Juneau Icefield, Coast Mountains, Tongass National Forest, Alaska. Note the person to the upper left of the exposed ice, for scale.

·Lateral Moraine

A sediment ridge, located on a glacier’s surface adjacent to the valley walls, extending down glacier to the terminus. It forms by the accumulation of rock material falling onto the glacier from the valley wall, rather than by water deposition.

September 2000 northwest-looking oblique aerial photograph of an Talkeetna Glacier, showing a well preserved former lateral moraine towering above the surface of the rapidly thinning and retreating glacier. The moraine is more than 200 feet high, southeast Talkeetna Mountains, Alaska.

July 1968 north-looking oblique aerial photograph of the confluence of the Gilkey and Bucher Glacier, showing numerous medial moraines and a pair of lateral moraines joining to form the large medial moraine in the foreground, Juneau Icefield, Coast Mountains, Tongass National Forest, Alaska.

·Medial Moraine

A sediment ridge, located on a glacier’s exposed ice surface, away from its valley walls, extending down glacier to the terminus. It forms by the joining of two lateral moraines when two glaciers merge.

·Push Moraine

A ridge or pile of unstratified glacial sediment that is formed in front of the ice margin by the terminus of an advancing glacier, bulldozing sediment in its path.

August 1989 north-looking photograph of a five-foot-high push moraine formed by an advance of Harriman Glacier, Harriman Fiord, Chugach National Forest, Prince William Sound, Alaska. The moraine had a horizontal extent of ~ 350 ft.

·Recessional Moraine

A ridge of glacial sediment that forms when the terminus of a retreating glacier remains at or near a single location for a period of time sufficient for a cross-valley accumulation to form.

September 2002 northeast-looking oblique aerial photograph of a > 100-foot-high moraine formed by a retreating unnamed glacier, Harris Peninsula, Kenai Fjords National Park, Kenai Mountains, Alaska. An end moraine is located ~ .5 mile down valley from this moraine, off the photograph to the lower left.

·Terminal Moraine

A cross-valley, ridge-like accumulation of glacial sediment that forms at the farthest point reached by the terminus of an advancing glacier. Also called an End Moraine.

September 2002 north-looking oblique aerial photograph of Nellie Juan Glacier and the 2-mile-long, seawater-filled lagoon, exposed by the 20th century retreat of the glacier. The east end of the lagoon is contained by an arcuate terminal moraine formed during the first quarter of the 20th century, Port Nellie Juan, Chugach National Forest, Prince William Sound, Alaska.

July 1988 near vertical photograph looking into the top of a moulin, located on the surface of the Malaspina Glacier, Wrangell-Saint Elias National Park, Saint Elias Mountains, Alaska. This moulin is more than 2 feet in diameter and penetrated more than 20 feet before it changed direction.

Moulin (Glacier Mill)

A narrow, tubular chute or crevasse through which water enters a glacier from the surface. Occasionally, the lower end of a moulin may be exposed in the face of a glacier or at the edge of a stagnant block of ice.

August 1976 northeast-looking photograph of a moulin, exposed in longitudinal section, at the terminus of LaPerouse Glacier, St Elias Mountains, Glacier Bay National Park, Alaska.