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active rock glacier

a mass of rock fragments and finer material, on a slope, that contains either an ice core or interstitial ice, and shows evidence of present movement.


when a mountain glacier's terminus extends farther downvalley than before; glacial advance occurs when a glacier flows downvalley faster than the rate of ablation at its terminus.

alpine glacier

a glacier that is confined by surrounding mountain terrain; also called a mountain glacier.
Muddy River Glacier carves its way through forested mountains near Frederick Sound in southeast Alaska. Typical of mountain glaciers, it is constrained on all sides by mountainous terrain. (Photo courtesy of U. S. Navy. Archived at the World Data Center for Glaciology, Boulder, CO.)

alpine layers

annual accumulations of snow and dust on a glacier.


sharp, narrow ridge formed as a result of glacial erosion from both sides.

band ogives

alternate bands of light and dark on a glacier; usually found below steep narrow icefalls and thought to be the result of different flow and ablation rates between summer and winter.

basal sliding

the sliding of a glacier over bedrock.


crevasse that separates flowing ice from stagnant ice at the head of a glacier.
Explorer on Skillet Glacier in 1936. Bergschrund is visible as the dark band of ice in the background.

bergy bit

large chunk of glacier ice (a very small iceberg) floating in the sea; bergy bits are usually less than 5 meters (15 feet) in size and are generally spawned from disintegrating icebergs.

bottom bergs

icebergs that originate from near the bottom of a glacier; the color is usually black from trapped rock material or dark blue because of old, coarse, bubble-free ice; they sit low in the water due to the weight of the embedded rocks.