Saturday, 11 June 2011

Glaciers Part 4 - Glacial Transport

Greatest rates of valley glacier erosion occur in mountainous regions where thick glaciers, well nourished by new supplies of snow, are flowing down steep-sided slopes towards a free outlet, such as a large lowland area or the sea. Some of the pre-existing river valley are often enlarged into spectaaculr glacial troughs, the lower ends of which have been drowned by post glacial rises in sea level to form fjords and many of the world's most rugged coastlines and deepest natural harbours.

The glacial debris removed by erosion or picked up for transport is an unsorted mass of sharp edged rocks and stones, boulders, clay and sand. Some debris is carried on the glacier's surface to produce two distinctive lateral moraines down each side. Frost shattering on rocky peaks above adds to that removed from the valley sides by ice erosion. Lateral moraines join together after tributary valleys have met and several lines of medial moraines may develop on wide glaciers. Close to their snouts many glaciers have a dirty appearance; the presence of surface debris gives the ice and lower albedo and sppeds up melting. A lot of dberis sinks down through the crevasses, some of which is carried englacially, although much reaches the base either carried or moved further down by meltwater. This is added to the sub-glacial material already present having neen removed by abrasion and plucking. Movement reduces the size of eroded pieces of rock and particles; the end product of the operation of the process of erosion is rock flour, fine grained materail carried away by meltwater streas. Under dry condition, it is capable of being picked up and transported by th strong winds.

So, Glacial transport (in short):

-On the surface ---> supra-glacial
Derived from forst shattering of peaks above and lateral moraines meeting after tributary glaciers join.

-Within the ice ---> englacial
Debris falls down through cracks and crevasses in the ice.

-At the base ---> subglacial
This is a mixture of material scarped up from below the glacier and that which has made its way downwards through crevasses, with or without the help of meltwater streams within ice.

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