The hanging valley of Watendlath not only demonstrates convincingly that glaciers sculpted the Borrowdale valley but also provides us with an opportunity to estimate the amount of erosion they have caused.
Before the ice age the Watendlath beck would have joined the river Derwent at a point which is now roughly in the middle of Derwentwater. Projecting the present day gradient of the upper reaches of the Watendlath beck to this point, the height of this junction would have been about 200m. The present day height of the lake surface is 74m and the bed of the lake a little below 70m. So the glaciers have lowered the Borrowdale valley at this point by 130m more than they have eroded the Watendlath valley.
The Watendlath Valley is also visibly glacier eroded i.e. U shaped. If, conservatively, we allow for 30m of glacier erosion of the floor of the Watendlath valley then the total glacial erosion, at this point in the middle of Derwentwater, is about 160m or 525ft.
If we suppose that the glaciers have been active for 2 million years during the current ice age (the other 600,000 years being allotted to interglacials) then the average glacial erosion rate is 160m/2 million years = 0.08mm per year or 8mm per century.
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