THE FRESH-WATER LOCHS OF SCOTLAND. 
307 
or Orcadian and the upper divisions of this formation are represented, 
the latter occurring between Tain and Tarbat Ness and northwards 
along the shore by Dornoch. 
Gorm Loch Mdr . — This lake, situated in the high plateau east of 
Ben More, lies in a rock basin formed mainly of Cambrian quartzite. 
Part of the floor, where the Garbh Allt enters the loch, may be composed 
of thrust Lewisian gneiss underlying these quartzites. The deepest 
sounding is 91 feet, and at the outlet the water flows over ledges of the 
higher or “ pipe-rock ” zone of the quartzite. Around the lake, the 
traces of glaciation are extremely abundant. Both the striae and the 
disposition of the carried boulders prove that, during the greatest 
extension of the later glaciers, the ice radiating from the east side of 
the Ben More range crossed the ridge in a north-east direction beyond 
Gorm Loch Mbr and overflowed into Loch Shin. At a later stage, the 
glacier that issued from Coire a’ Mhadaidh curved round Cailleach an 
t-Sniomha on the west side of Gorm Loch Mbr, and moved north-west 
by Glen Beg to the head of Loch Glencoul. The quartzite plateau in 
the east part of the lake is dotted over with moraines, which there form 
the islands. 
Loch Aihh is a shallow lake — the greatest depth being 24 feet — 
partly enveloped in drift and solid rock. It rests on various zones of 
Cambrian age, including the quartzite, Fucoid beds, serpulite grit, 
and limestone with intrusive igneous materials, all overlying the Ben 
More thrust-plane. From the covering of drift, it is uncertain whether 
this lake is a true rock basin. Its surface level is 498-5 feet, and the 
rock first appears at the outlet at a height of 490 feet above Ordnance 
datum line. 
Loch Graggie is a true rock basin, the deepest sounding being 40 feet. 
The rocky barrier is formed by siliceous schists and mica-schists that 
are well exposed in the stream below the outlet and by the side of the 
road along the north bank of the lake. The height of the surface of 
the water above sea-level is 505-95 feet, and that of the solid rock where 
the bridge spans the Craggie burn below the outlet is 505 feet. The 
direction of the ice-movement during the later glaciation was parallel 
with the long axis of the lake. 
IjOcIi an Dainih lies along a line of dislocation or fault that has been 
traced for a long distance in the crystalline schists south-westwards 
towards the head of Loch Broom. In the streams draining the hill 
slope on the northmost side the strata are exposed, which there consist 
of quartzose granulites with intercalations of mica-schist. On the 
higher part of the declivity the beds dip at gentle angles to the south- 
east, but on approaching the lake they are thrown into rapid folds 
parallel with its long axis, and are much crushed and shattered. At 
its lower end the lake is invaded by cones of alluvium brought down by 
the streams on either side. 
