Clais Fhearnaig is one of many 'dry-gullies' in the Cairngorms cut by glacial meltwater during the de-glaciation period. Clais Fhearnaig is a hollow that cuts through the hill-mass between Gleann Laoigh and Glen Quoich.
As a place name Clais Fhearnaig means hollow of alder.
Clais Fhearnaig may also be part of a geologic fault running across Scotland, but widened by glacial meltwater. In Alexander (1928) – the author writes :
Some 250 ft. up the hillside on the right is a curious ravine or cut, the Clais Fhearnaig, giving an easy access through to Glen Quoich. The cut is part of the great geological fault which runs across Scotland by Loch Lubnaig, Glen Ample, Loch Tay, and Glen Tilt, and continues in a north-easterly direction along Glen Quoich and into the head of Glen Gairn …
– Alexander (1928) (p 98)
The track through Clais Fhearnaig runs along the northern side of the hollow between Gleann Laoigh, and Glen Quoich.
From the estate road in Gleann Laoigh - Clais Fhearnaig appears to be little more than the steep-sided hollow from which Allt a’ Mhadaidh-allaidh flows. This approach begins a ½-mile upstream from Black Bridge – a short distance before the burn a track leads up the hillside northward. This is the sharp-end of Clais Fhearnaig, and the track climbs steeply before winding down to the floor of the hollow, but it’s only a 200ft. climb, and Clais Fhearnaig is worth it.
Entering the hollow from this end – the track crosses a burn entering the hollow from the north in a spectacular cascade before turning north-east into the first pool. The water from this burn passes through the hollow into Allt Clais Fhearnaig – so there’s no reason for it not to be the upper continuation of Allt Clais Fhearnaig. At the Gleann Laoigh end – the floor of the hollow is dotted with many weathered stumps, and a few weathered trunks. These weathered stumps, reeds, and bog cotton growing from the loch give Clais Fhearnaig its distinctive character.
From the estate road in Glen Quoich - Clais Fhearnaig appears to be little more than a fold in the hillside from which Allt Clais Fhearnaig flows. This approach begins 3-miles upstream from the car park at the foot of Glen Quoich – a short distance beyond Allt Clais Fhearnaig a track leads up the hillside westward.
Entering the hollow from this end - is the easy way, although the ascent is only slightly less from this end, it is more gradual. The track climbs gradually before winding down to the floor of the hollow at the artificial dam built in the early 20th century to create a fishing loch. The old 6-inch map (1869) shows a body of water in Clais Fhearnaig – not quite a loch, more like a wide burn.