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 Mor. 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 Mor 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 Mor, 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 Cr aggie 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. 



Lnch an, Daimh 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. 



