Tlie Pre-Glacial Valleys of Arran and Snowdon. 157 



Olen Rosa, along the junction between the schists and the granites. 

 The character of the falls and their rock stejDS show that the lower 

 end of the Garbh AUt is along a modern channel. The fact that the 

 stream has not cut down its bed there more deeply is due to its 

 recent adoption of its present course, and is not evidence of the 

 glacial deepening of Glen Rosa. 



Dr. Mort also claims (pp. 57-8) that Glen lorsa o2)posite Loch Nuis 

 has been glacially deepened by from 500 to 800 feet, since the stream 

 from that loch has made no notch in the bank of Glen lorsa ; but 

 the loch lies on a j^rojecting spur of the thousand-foot platform and 

 the drainage from it is insignificant. The larger streams which 

 join Glen lorsa, as from Loch Tanna and the streams from both the 

 Bens Tarsuinn, have deej^ly incised the sides of the main glen. 



The position of Glen Diomhan as a hanging valley 200 feet above 

 Glen Catacol is advanced by Dr. Mort (pp. 54-5) as evidence that 

 the latter has been deepened by ice ; but as Glen Catacol is one of 

 the tectonic north and south valleys it was probably older than its 

 tributary ; and their difference in level is the natural consequence of 

 the small Diomhan Burn not having been able to deepen its valley 

 as quickly as the larger stream, which was working along a line 

 prepared by a tectonic fracture. 



6. Arran compared with the Mainland of Scotland and 



Snowdon. 



The amount of erosion in Arran attributable to glaciers depends 

 then on the view adopted as to the geographical condition of the 

 island at the beginning of the Glacial period. This principle is 

 applicable to other parts of the British Isles. If the glaciers had 

 found Scotland in the condition of a worn-down country with a 

 mature topography many of the existing features would indicate 

 great denudation during Glacial times by ice, frost, and rivers. If 

 on the other hand Scotland had then an immature and rugged 

 topography the present features could have been produced with 

 insignificant erosion. But as Scotland was u|)lifted before Glacial 

 times at least 1,000 feet, the glaciers had to work on a country with 

 many rugged youthful features, upon which comparatively slight 

 rock abrasion would have |)roduced a very marked effect. The 

 Pliocene uplift also affected England. The movement was smallest 

 in the south ; in North Wales it was as great as in Scotland, for it 

 raised Snowdonia about 1,400 feet.^ 



The Snowdon area has many interesting analogies to Arran. 

 Its major valleys are rectilinear, and they resemble those of Arran 

 by their independence alike of the geological structure and natural 

 lines of drainage. The main valleys of Snowdonia trend from north- 

 west to south-east or from south-west to north-east. Where the 



1 In conversation with Professor 0. T. Jones I was glad to find that he was 

 also of the opinion that the Snowdon valleys had been largely made by pre- 

 ■Glacial rivers eroding the uplifted plateau. 



