Lewisian Gneiss is the oldest unit of rock that can found in the United Kingdom, radiometric dating has the unit aged at 3,000 – 1,700 million years old. The Lewisian Gneiss is concentrated in northwest Scotland especially in the Outer Hebridean islands, also in the mainland of Scotland west of the Moine Thrust Fault. The Moine Trust Fault represents the extent of the Caledonian Orogeny, the rock to the west of this fault have not been affected by Caledonian deformation. To the East of the Moine Thrust Zone there are a few Lewisian type inliers surrounded by the Moine Supergroup. By 1,100 million years ago the gneiss had been uplifted by tectonic activity, erosion took place at the top of the beds, this is where the subsequent unconformity with the over lying Torridonian Group.
The Lewisian Gneiss on the Isle of Rum can be found in several locations across the island. The main locations this report will be interested in outcrop within the Main Ring Fault which encircles the Rum Central Complex. These locations can be found on the north-eastern shore of Long Loch and surrounding the Príomhs Lochs, approximately 450 meters to the east of Long Loch.
The Gneiss outcrop that surrounds Príomh Lochs consist of large outcrops with clear unconformable boundaries between the Fianchanis Gritty Sandstone Member on the eastern side of the Lochs. The outcrops display typical gneissose banding of white (felsic) and black (mafic) bands, this is a very coarse-grained rock which on first glance has the very similar visible characteristics as that of granite.