THE FRESH- WATER LOCHS OF SCOTLAND. 47 



of Ardchullarie More, and strikes the west margin of the lake near 

 the spit of sand to be referred to presently (see Plate II.). The steep 

 gradient on the west side of the lake to the north and south of this spit 

 of sand coincides with the course of the Loch Tay fault. About a quarter 

 of a mile to the west of the Loch Tay fault a minor dislocation, with a 

 similar trend and downthrow, crosses the lake and follows the channel 

 of the Dubh Shruith burn. Now the lower deep basin lies to the east 

 or upthrow side of the Loch Tay fault, and the upper deep basin is 

 on the west or downthrow side of the Dubh Shruith fault. These dis- 

 locations doubtless produced brecciation of the strata along the lines of 

 movement, which led to more rapid disintegration of the materials. 



Close to the north-west limit of the upper basin enclosed by the 

 100-feet contour-line the loch shallows to 20 feet, and from thence 

 north-westwards to a point opposite Bienacreag the depth increases to 

 62 feet. Here there is a small basin enclosed by the 50-feet contour- 

 line. 



At the lower end of the loch, on the east side, there is a steep 

 gradient which coincides with a line of fault, having a downthrow to 

 the west (see Plate II.). As already indicated, this dislocation together 

 with the Loch Tay fault may have determined in part the course of the 

 river in remote geological time. But an impartial consideration of the 

 evidence furnished by the soundings shows that the faults cannot 

 account for the erosion of the lake basin. The striking fact that the 

 lower deep basin of Loch Lubnaig coincides with the upthrow side of the 

 Loch Tay fault the most powerful dislocation traversing the crystalline 

 schists of this area shows that this rock-basin must be ascribed to an 

 erosive agent acting independently of the lines of fault. It has further 

 been shown that Lochs Voil and Doine must have been originally con- 

 tinuous with Loch Lubnaig. The deepest sounding in Loch Voil is 

 98 feet, and in Loch Lubnaig 146 feet, and it is obvious that their 

 erosion must be ascribed to a common cause. The upper part of Loch 

 Lubnaig coincides roughly with the trend of the ice-sheet during the 

 great glaciation, which, from the evidence adduced in the foregoing 

 pages, must have attained a minimum thickness of 3000 feet. But the 

 basin must have undergone further erosion by the large valley-glacier. 



About half a mile to the south of Ardchullarie More, on the west 

 margin of Loch Lubnaig, there is a prominent spit of sand extending 

 into the lake for about 100 yards. It occurs not far to the south of the 

 bend in the lake, at the meeting-point of the waves produced by the 

 prevalent westerly winds. By the action of the waves the sand is 

 steadily borne outwards on both sides of the spit, and from the soundings 

 it is clear that this feature projects far into the lake. Further, it must 

 have been in process of formation when the loch stood at a higher level, 

 for a section appears in the adjacent railway cutting, which shows the 

 sloping layers of sand coinciding with the form of the spit. 



