Fig 1. (1) Head of glacier; (2) firn or névé; (3) region of ground moraine deposition; (4) terminal moraine; (5) drumlin; (6) braided stream; (7) kettle; (8) medial moraine; (9) lateral moraine; (10) U-shaped valley; (11) arête; (12) hanging valley; (13) cirque; (14) tarn; and (15) ice fall.
Fig 2. Bering Glacier, Alaska. Credit: NASA.
Fig 3. The Matterhorn.
Fig 4. Southwest-looking oblique aerial photograph showing a number of snow-covered cirques, cut into the summit of a Coast Mountains ridge along the Alaska-Canada border, east of Glacier Bay National Park, Alaska. Credit: USGS.
Fig 5. The flank of a drumlin composed of glacial till. The length of this drumlin is about 1/8 mile. Note the two people for scale. Cranes Beach, MA. Credit: USGS.
Fig 6. Glacial grooves, East Sister Island, Lake Erie, Ontario, Canada. The glacial grooves shown here are roughly 385 million yr old, and were created from the gouging action of ice flowing in the Lake Erie basin. Source: Natural Resources Canada / R. N. McNeely.
Fig 7. Bridal Veil Falls, Yosemite, and a hanging valle.
Fig 8. North-looking oblique aerial photograph of a complex sediment mass deposited by the retreating Malaspina Glacier in Malaspina Lake. Included are meandering eskers, linear crevasse fills and a massive ice cored kame. Wrangell-Saint Elias National Park, Chugach Mountains, Alaska. Photo credit: USGS.
Fig 9. Aug 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. Credit: USGS.
Fig 10. The surface of the ground in this area adjacent to Sherman glacier is covered with a coarse glacial till. Chugach Mountains, Chugach National Forest, Alaska. Credit: USGS.
Fig 11. Northeast-looking photograph of a symmetrical, unnamed U-shaped valley cut into the crystalline rocks of the Coast Mountains, Tracy Arn, Tracy Arm-Ford's Terror Wilderness, Tongass National Forest, Arkansas. The valley walls are more than 2,000 ft high and the valley is about 1/4 mi wide.
A glacier is a large mass of ice that can survive for many years. In most cases, glaciers are heavy enough to flow downhill under their own weight. They are significant agents of erosion. Three main types of glacier are recognized: ice sheets and caps, mountain or valley glaciers, and piedmont glaciers.
Glaciers form wherever conditions are such that annual precipitation of snow, sleet, and hail is greater than the amount that can be lost through evaporation or otherwise. The occurrence of a glacier thus depends on the latitude and also on local topography: there are several glaciers on the equator.
Glaciers account for about 75% of the world's fresh water, and of this the Antarctic ice sheet accounts for about 85%.
Mountain glaciers usually result from snow accumulated in cirques coalescing to form glaciers; and piedmont glaciers occur when such a glacier spreads out of its valley into a contiguous lowland area.
Features associated with glaciers
An arète is a sharp ridge formed by erosion where the heads of two glaciers meet. A well-known example is the Matterhorn on the Swiss-Italian border (Fig 3).
A bergschrund is a deep, wide crevasse or a series of parallel narrow crevasses in a glacier, produced by tension within the ice, often at the point where the moving ice pulls away from the rock slope at the head of the glacier cirque.
A cirque, also known as a corrie (in Scotland) or cwm (in Wales), is a bowl-shaped, steep-sided hollow formed by glacial erosion, usually occupied by a lake where the glacier has retreated, or by névé where the glacier is still present (Fig 4). Typically, a cirque has a lip at its lower end. The term is French and is derived from the Latin word circus. See also erosion.
A col is a low smooth depression in a mountain range, often forming a pass through it. It is generally caused by the action of two opposing glaciers, the heads of which are commonly separated by an arête a sharp-crested ridge between the two cirques (valleys) formed by the glaciers.
A crevasse is a deep crack in a glacier. It is the result of stress within the glacier or the movement of the glacier over uneven terrain.
Sedimentary drift is a general description of surface debris carried either by a river or glacier.
A drumlin is an elongated hillock, formed of till, found usually in swarms in lowland areas formerly occupied by a glacier (Fig 5). Drumlins usually taper away from a steep slope that faced the incoming ice.
Glacial drift is sediment and rocks transported by glaciers and deposited directly on the land or indirectly in streams, lakes, and oceans. It consists of a heterogeneous mixture of clay, silt, sand, gravel, and boulders ranging in size and shape.
A glacial groove is a deep, wide, usually straight furrow cut in bedrock (Fig 6). It is caused by the abrasive action of large rock fragments dragged along the base of a moving glacier. The grooves are larger and deeper than glacial striations, ranging in size from a deep scratch to a glacial valley.
A hanging valley is a tributary valley that ends high up the face of a larger main valley, possibly with a stream running through it and ending in a waterfall (Fig 7). Hanging valleys are usually associated with U-shaped valleys, and result from a tributary glacier having flowed into a glacier of larger volume. The main glacier eroded a deep U-shaped valley with nearly vertical sides while the tributary glacier, with a smaller volume of ice, made a shallower U-shaped valley. Since the surfaces of the glaciers were originally at the same elevation, the shallower valley appears to be hanging above the main valley.
A well known example is the hanging valley in Yosemite National Park over which Bridal Veil Falls plummets.
kameKame is a deposit formed by a subglacial stream near the terminal margin of a melting glacier (Fig 8). Streams flowing down mountain sides onto glaciers deposit sand and gravel at the point where the stream first reaches the ice, or where the water goes down into a crevasse. The accumulation of sand and gravel gradually fall down to the valley floor as the ice melts.
Kames are generally small features, 20–30 m (65–100 ft) in length or breadth. Shapes include hills, mounds, knobs, hummocks, or ridges.
Moraine is an accumulation of boulders and rock fragments carried or deposited by a glacier (Fig 9). Ground moraine is drift left in a sheet as a glacier retreats. Terminal moraine are ridges deposited when the ice is melting prior to the glacial retreat; a series of ridges may mark pauses in the retreat. Lateral moraines are formed of debris that falls onto the glacier: when two glaciers merge their lateral moraines may unite to form a medial moraine.
Till, also known as boulder clay, is the unsorted material left behind on the land after the retreat of a glacier (Fig 10). Till is a heterogeneous mixture of different sized material deposited by moving ice (lodgement till) or by the melting in-place of stagnant ice (ablation till). After deposition, some tills are reworked by water.
A U-shaped valley is a valley with a parabolic or 'U' shaped cross-section, steep walls, and generally a broad and flat floor (Fig 11). Formed by glacier erosion, a U-shaped valley results when a glacier widens and over-steepens a V-shaped stream valley.