Derry Dam (prehistoric)
In Gleann Doire, on the right-bank of Uisge Doire, a little more than half-way between Derry Lodge and the Derry Dam Footbridge, the track swings back to the river edge. Looking north from this point the pre-historic Derry Dam is apparent - looking very much like a man-made earthwork dam. After following the river for a short distance the track swings north again over the shoulder of the pre-historic Derry Dam through the remaining trees of ‘the grove’ where the trees grow smaller, and thin out.
At 2-miles above Derry Lodge the track swings down towards the river again crossing it over the Derry Dam Footbridge to the left-bank of Usige an Doire.
If a loch formed behind the pre-historic Derry Dam, and I'm pretty sure one did, then it would have been a loch at least 2 miles long before the dam was breached. The breach is evident - Uisge an Doire flows through it, and it looks exactly like you’d expect a breach through a man-made earthwork dam to look - almost dead straight, and symmetrical. It would be relatively easy to recreate the prehistoric 'Loch Doire' by building an artificial dam across the breach, and apparently that’s exactly what was done in the early 19th century by Alexander 'Sandy' Davidson.