Sunday, 12 June 2011

Glaciers Part 6 - Glaciated valleys

A valley glacier changes the cross profile, plan and long profile of a pre-existing river valley down which it moves. The underlying explanation for all three changes is that the glacier occupies and fills the wholes of the valley floor. Ice erosion takes place wherever the ice is in contact with rock, whereas earlier direct erosion by the river took place only in the small part of the valley where the stream was flowing. When ice is present, meltwater streams are everywhere - within the ice, under it and along its sides; therefore, ersoion is no longer concentrated in one place. River water passes through the system quite quickly, but glacier ice is stored in the system for a much longer time. Ice movement is slow but it is inevitable and unyielding; although ice deforms plastically when it flows as its mass pushes it forward in a straight line wherever possible. Rivers naturally swing form side to side and flow around obstacles forming interlocking spurs. Rivers are less powerful than glaciers. However, the protruding spurs of higher land are cut off, or truncated, by the relentless down valley movement of the ice mass of a glacier. The ice takes away the edges of the valley floor and the lower slopes are eroded by glacier movement which creates the flat-floored, steep-sided and straight valley - the classic  feature of a glaciated valley. The previous positions of the interlocking spurs are marked by higher tops to the valley sides. Today's tributary streams cut their small valleys between truncated spurs before dropping down the stepp valley sides as waterfalls. Above that part of the valley filled by ice, high level benches or shoulders of less steep land are sometimes present.
The irregular long profile of glaciated valleys requires a seperate explanation. The uneveness of the valley floor is partly a reflection of lithological variations, principaly rock resistance and the degree of joitning (a little test to see how much of that 'Geology for Geographers' lesson we had a while ago you can remember!). Soft and well jointed rocks are vulnerable to severe abrasion and plucking. Once a rock basin begins to form, some of the characteristics of ice flow increase its size. For example, extending flow increases ice thickness as it fills the hollow. This increases the weight for abrasion, and leads to more pressure melting at the base of the glacier which encouragesplucking. Another reason for the localized valley deepening is additions of ice. A steep drop in level often occurs towards the head of the valley after cirque glaciers and ice caps increase the size of the glacier in the main valley, theerby giving a increasing its erosive ability.Tributary glaciers joining the main glacier lower down the valley have a similar effect. Hard rock outcrops reduce the valley's width, constricting the glacier, but speding up its rate of erosion as it forces its way through. Where the glacier has melted it can no longer erode and so rock lips or rock bars form (rotational slip also contributes to their formation). Rock bars also form on the seaward edge of a fjord near to the maximum extent reached by the glacier.

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