THE FRESH- WATER LOCHS OF SCOTLAND. 135 



Finally, there is the phase of corrie glaciers, when the glacial detritus 

 was borne for no great distance from the local centres of dispersion. 



During the maximum glaciation, the ice-shed lay round the north- 

 west margin of the Tay basin, from the mountains beyond Rannoch 

 Moor, by Ben Alder west of Loch Ericht, eastwards to the watershed 

 separating Glen Garry from the tributaries of the Spey and the Dee. 



Beginning in the western part of the basin, with the lofty watershed 

 between the head of Glen Lyon and Glen Lochay, in the Mamlorn 

 forest, striae are found at intervals along this ridge for a distance of 

 3 miles, at elevations which in some cases vary from 2700 to 3000 feet, 

 trending E. 20 to 30 S. Further east, about 3 miles north of Killin, 

 on Creag-iia-Caillich at a height of 2250 feet, the direction is about 

 south-south-east. Again, to the west of Ben Lawers, the ice-markings 

 point S. 40 E. about the 2000-feet level. Proceeding northwards to 

 the dividing line between Glen Lyon and Strath Tummel, the evidence 

 is no less remarkable, for on Schichallion, at an elevation of 3000 feet, 

 the trend is E. 30 S. Still further north, on Beinn a' Chuallaich a 

 high mountain between Glen Erichdie and Kinloch Rannoch -the 

 striae point S. 30 E. at a height of 2700 feet. Again, on Ben Vrackie, 

 about 3 miles north of Pitlochry -a mountain which is glaciated to 

 the top the trend is east-south-east. Similar conclusive evidence 

 is obtained on the dividing ridge that stretches eastwards from 

 Schichallion towards Pitlochry and separates Strath Tummel from 

 the upper course of the Tay between Aberfeldy and Logierait. Part 

 of this ridge is composed of the Perthshire quartzite, the glaciated 

 surfaces of which show finely preserved striae, the direction varying 

 from E. 20 S. to E. 45 S. On one of the prominent peaks of this 

 ridge Ben Eagach south of Loch Tummel, ice-markings are found on 

 the top at a height of 2250 feet, which point E. 35 S. Further south, 

 on the dividing ridge between Strath Bran and the valley of the 

 Almond, on Meall nan Caoraich, the direction is E. 30 S., close to the 

 2000-feet contour-line. Additional instances might be given from the 

 mountainous region within the metamorphic area, but the above 

 examples establish the conclusion that during the maximum glaciation 

 there must have been a movement of the mer de glace independent of 

 the valley system in an east-south-east or south-easterly direction. 



During the great extension of the ice, on the broad plateau of the 

 Moor of Rannoch the ice seems to have radiated partly towards the 

 east-south-east or south-east, and partly towards the south-west in 

 the direction of the Tulla and Glen Orchy (see geological map). 



The evidence obtained from the dispersal of the boulders is no less 

 remarkable, for in some cases they have been carried far from their 

 parent source, and over lofty cols. The boulders of diorite or horn- 

 blendic granite from the Moor of Rannoch have been found in Strath 

 Tummel, in Glen Lyon, in Strath Fillan, in Strath Tay, and across the 



