The evolution of the drainage of the Fionn Loch area, Sutherland

Three lochs, Fionn, Veyatie and Cam, which occupy an elongated rock-basin on the western borders of Sutherland and Ross-shire, were once a continuous sheet of water, here called ‶Loch Suilven.″ Investigation of an extinct river, referred to as the Muirichinn, leads to the conclusion that it was by its channel that the overflow from ‶Loch Suilven″ was discharged.Relics of the beaches of ‶Loch Suilven″ are still extant and their distinctive configuration proves that they were formed after the ice had receded from their neighbourhood. It therefore follows that the River Muirichinn flowed in late-glacial times. There are, however, abundant indications that its channel was severely glaciated. On this and other evidence it is argued that it originated between maximum glaciation and the final retreat of the ice.During the latter period, melt-waters must have filled the above-mentioned rock-basin to overflowing. At several points therefore spillways resulted, one of which,viathe River Kirkaig, was destined to become permanent. For a time, the outflow by the Kirkaig was considerably retarded by a massive dolerite dyke which lay athwart its course. Eventually, enough of the dyke was breached to reduce the loch level sufficiently to cause the Muirichinn to dry up and to isolate Loch Cam, though Fionn and Veyatie still remain united; but further erosion of the dyke resulted in a still further lowering of the water-level and in the ultimate separation of the basin into Lochs Veyatie and Fionn.An additional consequence of this reduction in loch level was the increased headward erosion of certain tributaries and their capture of older streams. Two examples are given.Finally, it is contended that the original drainage of the upper Kirkaig basin was towards the east and that glacial action was responsible for its present westerly direction.