• A glacier is a thick mass of ice originating on land as a result of the compaction and recrystallization of snow, and it shows evidence of past or present flow. Today, valley or alpine glaciers are found in mountain areas where they usually follow valleys that were originally occupied by streams. Ice sheets exist on a much larger scale, covering most of Greenland and Antarctica.

• Near the surface of a glacier, in the zone of fracture, ice is brittle. However, below about 50 meters, pressure is great, causing ice to flow like plastic material. A second important mechanism of glacial movement consists of the entire ice mass slipping along the ground.

• The average velocity of glacial movement is generally quite slow but varies considerably from one glacier to another. The advance of some glaciers is characterized by periods of extremely rapid movements called surges.

• Glaciers form in areas where more snow falls in winter than melts during summer. Snow accumulation and ice formation occur in the zone of accumulation. Its outer limits are defined by the snowline. Beyond the snowline is the zone of wastage, where there is a net loss to the glacier. The glacial budget is the balance, or lack of balance, between accumulation at the upper end of the glacier, and loss, called ablation, at the lower end.

• Glaciers erode land by plucking (lifting pieces of bedrock out of place) and abrasion (grinding and scraping of a rock surface). Erosional features produced by valley glaciers include glacial troughs, hanging valleys, pater noster lakes, fiords, cirques, arêtes, horns, and roches moutonnées.

  

• Any sediment of glacial origin is called drift. The two distinct types of glacial drift are (1) till, which is unsorted sediment deposited directly by the ice; and (2) stratified drift, which is relatively well-sorted sediment laid down by glacial meltwater.

• The most widespread features created by glacial deposition are layers or ridges of till, called moraines. Associated with valley glaciers are lateral moraines, formed along the sides of the valley, and medial moraines, formed between two valley glaciers that have joined. End moraines, which mark the former position of the front of a glacier, and ground moraines, undulating layers of till deposited as the ice front retreats, are common to both valley glaciers and ice sheets. An outwash plain is often associated with the end moraine of an ice sheet. A valley train may form when the glacier is confined to a valley. Other depositional features include drumlins (streamlined asymmetrical hills composed of till), eskers (sinuous ridges composed largely of sand and gravel deposited by streams flowing in tunnels beneath the ice, near the terminus of a glacier), and kames (steep-sided hills consisting of sand and gravel).

• The Ice Age, which began about two million years ago, was a very complex period characterized by a number of advances and withdrawals of glacial ice. Most of the major glacial episodes occurred during a division of the geologic time scale called the Pleistocene epoch. Perhaps the most convincing evidence for the occurrence of several glacial advances during the Ice Age is the widespread existence of multiple layers of drift and an uninterrupted record of climate cycles preserved in seafloor sediments.

  

• In addition to massive erosional and depositional work, other effects of Ice Age glaciers included the forced migration of organisms, changes in stream courses, adjustment of the crust by rebounding after the removal of the immense load of ice, and climate changes caused by the existence of the glaciers themselves. In the sea, the most far-reaching effect of the Ice Age was the worldwide change in sea level that accompanied each advance and retreat of the ice sheets.

• Any theory that attempts to explain the causes of glacial ages must answer two basic questions: (1) What causes the onset of glacial conditions? and (2) What caused the alternating glacial and interglacial stages that have been documented for the Pleistocene epoch? Two of the many hypotheses for the cause of glacial ages involve (1) plate tectonics and (2) variations in Earth's orbit.

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