The head of the Loch Avon trough, cut into preglacial slopes with joint-controlled headwater valleys amidst broad granite dome
The meandering course of the middle Avon, a preglacial valley deepened by only ~50 m through glacial erosion
Preglacial valleys
In the Cairngorms, the preglacial form of many valleys can be reconstructed from the alps or valley benches that now stand above the modern valley floors. These fragments can be combined to reconstruct the preglacial drainage pattern. Yet the main valleys have been considerably deepened by the passage of ice and meltwater to give a valley-in-valley form. Only a few headwater streams preserve open valley heads that are suggestive of a preglacial form. These include the streams that cross the deep regolith on the Braeriach plateau and others on Ben Avon. Even here, however, the streams quickly steepen in gradient due to headward erosion in response to Quaternary valley deepening. The lower parts of the streams thus represent canyons of adjustment - Coire Raibert is an excellent example.
Perhaps the best
examples of preserved preglacial valley forms occur outside the core area of
the Cairngorms. The Water of Caiplich, above the elbow of capture at
The
Castle gorge, is a broad, open valley, often less than 50 m deep that flows
across the preglacial basin of the Avon Embayment (Linton, 1950a). In the
hills south of the Dee, the Water of Tarf is a remote W-E trending valley that
has lost much of its former drainage due to glacial diversion and river
capture (Linton, 1950b). Nonetheless, the lack of any strong or persistent
down- valley ice flow has meant that its preglacial form is largely preserved.
Fragments of former valley floor suggest that the preglacial gradient was
around 10 m/km.