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moving mass of ice that survives year to year, formed by the compacting of snow into névé and then into granular ice and set in motion outward and downward by the force of gravity and the stress of its accumulated mass. Glaciers are usually found in high altitudes and latitudes.


Glaciers are of four chief types. Valley, or mountain, glaciers are tongues of moving ice sent out by mountain snowfields following valleys originally formed by streams. In the Alps there are more than 1,200 valley glaciers. Piedmont glaciers, which occur only in high latitudes, are formed by the spreading of valley glaciers where they emerge from their valleys or by the confluence of several valley glaciers. Small ice sheets known as ice caps are flattened, somewhat dome-shaped glaciers spreading out horizontally in all directions and cover mountains and valleys. Continental glaciers are huge ice sheets whose margins may break off to form icebergs (see icebergiceberg,
mass of ice that has become detached, or calved, from the edge of an ice sheet or glacier and is floating on the ocean. Because ice is slightly less dense than water about one ninth of the total mass of a berg projects above the water.
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). The only existing continental glaciers are the ice sheets of Greenland and Antarctica, but during glacial periodsglacial periods,
times during which large portions of the earth's surface were covered with thick glacial ice sheets. In the Pleistocene epoch, in the Carboniferous and Permian periods of the Paleozoic era era, and in Huronian time of the Precambrian, the earth experienced an
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 they were far more widespread. Glaciers may be classified as warm or cold depending on whether their temperatures are above or below −10°C; (14°F;).

Geological Impact

Glaciers alter topography, and their work includes erosionerosion
, general term for the processes by which the surface of the earth is constantly being worn away. The principal agents are gravity, running water, near-shore waves, ice (mostly glaciers), and wind.
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, transportation, and deposition. Mountain glaciers carve out amphitheaterlike vertical-walled valley heads, or cirques, at their sources. They transform V-shaped valleys into U-shaped valleys by grinding away the projecting bases of slopes and cliffs and leveling the floors of the valleys; in this process tributary valleys are frequently left "hanging," with their outlets high above the new valley floor. When the tributary valleys contain streams, waterfallswaterfall,
a sudden unsupported drop in a stream. It is formed when the stream course is interrupted as when a stream passes over a layer of harder rock—often igneous—to an area of softer and therefore more easily eroded rock; the edge of a cliff or plateau; or the
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 and cascades are formed, such as Bridal Veil Falls of Yosemite National Park. Elevations over which glaciers pass usually are left with gently sloping sides in the direction from which the glacier approached (stoss sides) and rougher lee sides. Humps and bosses of rock so shaped are known as roches moutonnées.

The debris from glacial erosion is carried upon, within, and underneath the ice. The debris frozen into the underside of the glacier acts as a further erosive agent, polishing the underlying rock and leaving scratches, or striae, running in the direction of the movement of the glacier. Glacial deposits are often known as till or driftdrift,
deposit of mixed clay, gravel, sand, and boulders transported and laid down by glaciers. Stratified, or glaciofluvial, drift is carried by waters flowing from the melting ice of a glacier.
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. The melting of the ice in summer forms glacial streams flowing under the ice, while the retreat of a large glacier sometimes leaves a temporary glacial lake, such as the ice age Lake AgassizAgassiz, Lake
, glacial lake of the Pleistocene epoch, c.700 mi (1,130 km) long, 250 mi (400 km) wide, formed by the melting of the continental ice sheet beginning some 14,000 years ago; it eventually covered much of present-day NW Minnesota, NE North Dakota, S Manitoba, central
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. Fjordsfjord
or fiord
, steep-sided inlet of the sea characteristic of glaciated regions. Fjords probably resulted from the scouring by glaciers of valleys formed by any of several processes, including faulting and erosion by running water.
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 generally owe their origin to glaciers.

Glacial Movement

A glacier moves as a solid rather than as a liquid, as is indicated by the formation of crevasses (see crevassecrevasse
, large crack in the upper surface of a glacier, formed by tension acting upon the brittle ice. Transverse crevasses occur where the grade of the glacier bed becomes suddenly steeper; longitudinal crevasses, where the glacier spreads over a wider valley or plain.
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). The center of a glacier moves more rapidly than the sides and the surface more rapidly than the bottom, because the sides and bottom are held back by friction. The rate of flow depends largely on the volume of ice in movement, the slope of the ground over which it is moving, the slope of the upper surface of the ice, the amount of water the ice contains, the amount of debris it carries, the temperature, and the friction it encounters. Glaciers are always in movement, but the extent of the apparent movement depends on the rate of advance and the rate of melting. If the ice melts at its edge faster than it moves forward, the edge of the glacier retreats; if it moves more rapidly than it melts, the edge advances; it is stationary only if the rate of movement and the rate of melting are the same.

The causes of glacial movement are exceedingly complex and doubtless are not all operative on the same glacier at the same time. Important elements in glacial movement are melting under pressure followed by refreezing, which may push the mass in the direction of least resistance; sliding or shearing of layers of ice one on top of the other; and rearrangement of the granules when pressure causes melting. Sudden, rapid movements of glaciers, called glacier surges, have been observed in Alaskan and other glaciers, with evidence for such abnormal movements as the crumpled lines of surface debris found on them. It is thought that the relatively sudden movement and melting of glaciers may be indicative of climateclimate,
average condition of the atmosphere near the earth's surface over a long period of time, taking into account temperature, precipitation (see rain), humidity, wind, barometric pressure, and other phenomena.
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A mass of land ice, formed by the further recrystallization of firn, flowing slowly (at present or in the past) from an accumulation area to an area of ablation.


a slowly moving mass of ice originating from an accumulation of snow. It can either spread out from a central mass (continental glacier) or descend from a high valley (alpine glacier)
References in periodicals archive ?
No cryptic species were discovered through this study, although distinct geographic and genetic structuring was revealed, likely due to current geographic isolation of the sampled waterbodies, as well as the locations of historical glacial refugia.
Variation in mitochondrial DNA reveals multiple distant glacial refugia in black spruce (Picea mariana), a transcontinental North American conifer.
Glacial refugia influence plant diversity patterns in the Mediterranean Basin.
Glacial refugia would have sheltered Tilia populations in Iberian coastal and mountain areas (BREWER & al.
2008--Species persistence in northerly glacial refugia of Europe: a matter of chance or biogeographical trait?
Third, to assess the expansion distance for dice snakes to reach the central German sites, we take two likely glacial refugia based on low lying fluvial areas in southeastern Europe, (a) around Belgrade, Serbia, at the southern end of the Pannonian Plain, and (b) the plain around Skopje, northern Macedonia.
tessellata in central Asia, we suggest that either the glacial refugia of N.
st] values observed in the Olympic Peninsula and Vancouver Island indicate that these populations may have been separated from the remaining northern population relatively recently; therefore, these areas may have served as additional glacial refugia during recent glacial cycles.
If historical factors were important, the direction and extent of introgression should vary according to putative dispersal routes and glacial refugia, regardless of distance among localities.
These features make this species an ideal organism to test the glacial refugia hypothesis using both a phylogeographic approach and ecological niche modelling.
John River drainage did not support the hypothesis that they all originated from the secondary contact between two monophyletic whitefish groups that evolved allopatrically in the Acadian and Atlantic glacial refugia during the last glaciation events (Bernatchez and Dodson 1990a).
The time elapsed since recolonization is short in comparison with the rate of the mtDNA clock, suggesting that much of the extensive mtDNA variation observed in field voles today was brought by the founders originating from glacial refugia in southern or eastern Europe.