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Glacial and Periglacial Processes and Landforms

Snow line

A temporary line marking the elevation where winter snowfall persists throughout the summer; seasonally, the lowest elevation covered by snow during the summer.

Alpine glacier

A glacier confined in a mountain valley or walled basin, consisting of three subtypes: valley glacier (within a valley), piedmont glacier (coalesced at the base of a mountain, spreading freely over nearby lowlands), and outlet glacier (flowing outward from a continental glacier).


A scooped-out, amphitheater-shaped basin at the head of an alpine glacier valley; an erosional landform.

Continental glacier

A continuous mass of unconfined ice, covering at least 50,000 km2 (19,500 mi2); most extensive at present as ice sheets covering Greenland and Antarctica.

Ice sheet

An enormous continuous continental glacier.

Ice cap

A large, dome-shaped glacier, less extensive than an ice sheet although it buries mountain peaks and the local landscape; generally, less than 50,000 km2.

Ice field

The least extensive form of a glacier, with mountain ridges and peaks visible above the ice; less than an ice cap or ice sheet.


Snow that has persisted through a summer season in the zone of accumulation; snow of a granular texture that is transitional in the slow transformation from snow to glacial ice.

Firn line

The line that is visible on the surface of a glacier, where winter snows survive the summer ablation season; analogous to a snow line on land.

Glacial ice

A hardened form of ice, very dense in comparison to normal snow or firn.


Loss of glacial ice through melting, sublimation, wind removal by deflation, or the calving of blocks of ice. (See Deflation.)


A vertical crack that develops in a glacier as a result of friction between valley walls, or tension forces of extension on convex slopes, or compression forces on concave slopes.

Glacier surge

The rapid, lurching, unexpected forward movement of a glacier.


Mechanical wearing and erosion of bedrock accomplished by the rolling and grinding of particles and rocks (tools) carried in a stream, removed by wind in a �sandblasting� action, or imbedded in glacial ice.


A sharp ridge that divides two cirque basins. Derived from �knife edge� in French, these form sawtooth and serrated ridges in glaciated mountains.


Formed by two headward-eroding cirques that reduce an ar�te (ridge crest) to form a high pass or saddlelike narrow depression.


A pyramidal, sharp-pointed peak that results when several cirque glaciers gouge an individual mountain summit from all sides.


a crevasse or wide crack that has opened along the headwall of a glacier; most visible in summer when covering snow is gone.


A small mountain lake, especially one that collects in a cirque basin behind risers of rock material or in an ice-gouged depression.

Paternoster lake

One of a series of small, circular, stair-stepped lakes formed in individual rock basins aligned down the course of a glaciated valley; named because they look like a string of rosary (religious) beads.


A drowned glaciated valley, or glacial trough, along a seacoast.

Glacial drift

The general term for glacial deposits, both unsorted and stratified.


Direct ice deposits that appear unstratified and unsorted; a specific form of glacial drift.

Stratified drift

Sediments deposited by glacial meltwater that appear sorted; a specific form of glacial drift.


Marginal glacial deposits (lateral, medial, terminal, ground) of unsorted and unstratified material.

Lateral moraine

Debris transported by a glacier that accumulates along the sides of the glacier and is deposited along these margins.

Medial moraine

Debris transported by a glacier that accumulates down the middle of the glacier, resulting from two glaciers merging their lateral moraines; forms a depositional feature following glacial retreat.

Terminal moraine

Eroded debris that is dropped at a glacier's farthest extent.

Till plain

A large, relatively flat plain composed of unsorted glacial deposits behind a terminal or end moraine; low-rolling relief and unclear drainage patterns are characteristic.

Outwash plain

Area of glacial stream deposits of stratified drift with meltwater-fed, braided, and overloaded streams; occurs beyond a glacier's morainal deposits.


A sinuously curving, narrow deposit of coarse gravel that forms along a meltwater stream channel, developing in a tunnel beneath a glacier.


A small hill of poorly sorted sand and gravel that accumulates in crevasses or in ice-caused indentations in the surface; a depositional feature of glaciation


A steep-sided hole, frequently filled with water; formed after a glacier retreats and leaves an isolated block of ice in a ground moraine, an outwash (till?) plain, or a valley floor

Roche moutonne'e

A glacial erosion feature; an asymmetrical hill of exposed bedrock; displays a gently sloping upstream side that has been smoothed and polished by a glacier and an abrupt, steep downstream side.


A depositional landform related to glaciation that is composed of till (unstratified, unsorted) and is streamlined in the direction of continental ice movement—blunt end upstream and tapered end downstream with a rounded summit.


Cold-climate processes, landforms, and topographic features along the margins of glaciers, past and present; these characteristics exist on more than 20% of Earth�s land surface; includes permafrost, frost action, and ground ice.


Forms when soil or rock temperatures remain below 0�C for at least 2 years in areas considered periglacial; criterion is based on temperature and not on whether water is present.

Active layer

A zone of seasonally frozen ground that exists between the subsurface permafrost layer and the ground surface; subject to consistent daily and seasonal freeze-thaw cycles.

Patterned ground

Areas in the periglacial environment where freezing and thawing of the ground create polygonal forms of arranged rocks at the surface; can be circles, polygons, stripes, nets, and steps.

Ice age

A cold episode, with accompanying alpine and continental ice accumulations, that has repeated roughly every 200 to 300 million years since the late Precambrian Era (1.25 billion years ago); includes the most recent episode during the Pleistocene, which began 1.65 million years ago.


An ancient lake, such as Lake Bonneville or Lake Lahonton, associated with former wet periods when the lake basins were filled to higher levels than today.

Arctic region

Area north of the 10 deg.C isotherm for July; coincides with the the boundary between the northern forests and tundra (treeline)

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