Glacial erosion

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Glacial landforms


Striations are the most common and are fine lines inscribed on bedrock by an angular boulder carried in the basal ice. Crescentic fractures, cracks and gouges are sometimes collectively known as friction cracks and are concave downstream.

On a larger scale, roches moutonees also give clear evident of passing ice. These are small asymmetrical rode knobs which have abraded stoss (upstream) sides and plucked lee sides. This change of process within the one feature may be related to small irregularities in the topography of the bed-rock floor, which are picked out by the ice, or to the jointing pattern in the rocks.

The well-known U shape of a glaciated valley is formed because erosion occurs throughout the base of the glacier, becoming progressively less at the sides, rather than being concentrated at one point, as in a river valley. Well-developed glaciated valleys are known as glacial troughs: at their upper end they may have a steep head wall, known as a trough end. Above the headwall, several tributary glaciers may have fed into the main trough. Glaciated troughs typically possess truncated spurs, rode strand basins. Rock steps are similar to, although much larger than, roches moutonees and have abraded lips and plucked lee sides: again, steps may be related to jointing to resistant lithologies.

Where a main glacier was formerly joined by a smaller tributary, a hanging valley may be created. The relative size of the two valleys is directly related to the power of erosion of the glaciers occupying them. In the reverse situation, where a powerful trunk glacier overspills the valley confines, thus breaking the old watershed, a glacial breach is formed. On deglaciation, this will be left as a broad col connecting one highland valley to the next.

Cirques, corries or cwms (pronounced kooms; a place name from Wales), when fully developed, have an amphitheater-like form with steep backwalls and small rock basins often occupied by lakes after the ice has disappeared. The basic form arises from the rotational movement in the ice induced by the steep surface gradient of the glacier. Abrasion appears to be the principal process which excavates the basin, but the backwall has a fretted appearance and is developed by freeze-thaw action in the bergschrund, a deep crevasse at the back of the glacier. The headward coalescence of adjacent cirques on a mountain mass may cause their sides to intersect and form aretes and Pyramidal peaks.

All cirque glaciers develop from snow banks. Once the snow has become firm, the process of nivation (erosion by snow) starts to deepen the hollow through diurnal freezing and thawing. Some cirques do not proceed beyond the firn stage and are known as nivation cirques. These do not basins or moraines, but may have frontal mounds of protalus material, built by debris sliding down surface of the snow patch.

Erosion by large ice-sheets of lowland areas produces a scoured, generally debris-free landscape. On old, hard rocks glacier ice picks out rock knob and depressions: in northwest Scotland, glaciation of the Lewisian gneiss has produced a landscape known locally as knock-and-lochan topography. This variation in erosive effect is known as glacial selectivity. The capability of ice to erode depends largely on the thermal characteristics of the ice and the gradient over which it is flowing. A sluggish polar-type glacier in a flat lowland area may therefore achieve little erosion in quantitative terms, whereas a temperate-type glacier flowing down the relatively steep gradient of a mountain valley will be much more effective.

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