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.