ORIGINAL ARTICLES. 



The Pre-Glaclal Valleys of Arran and Snowdon.' 



By J. W. Gregory, F.R.S., University, Glasgow. 



Contents. 



1 • The Valley System of North Arran. 



2. Evolution of the Existing Topography. 



3. Tectonic Clefts and Shatter-belts. 



4. Amount of Glacial Excavation in the Valleys. 



5. Garbh Allt and the Glacial Deepening of Glen Rosa. 



6. Arran compared with the Mainland of Scotland and Snowdon. 



Introduction. 



rpHE history of the British Isles in the Pliocene has an important 

 -^ bearing on their glacial geology and on the origin of their existing 

 geographical features. Physiographic evidence indicates that the 

 Lower Pliocene was in Britain an epoch of depression, during which 

 the sea covered parts of South-Bastern England, whereas the Middle 

 and Upper Pliocene were marked by an uplift which excluded the 

 sea from the British Isles, except in East Anglia and Western 

 Cornwall. It is generally agreed that this uplift raised the low- 

 lying plains that had been formed during the Lov/er Pliocene into 

 plateaus from 500 to 1,400 feet above sea-level. The uplift lasted 

 through a long period of time ; and such a slowly rising land would 

 naturally be greatly denuded by rivers. Plateaus and platforms 

 due to the Pliocene uplift have been described by Dr. Mort for 

 Arran and by Professor W. M. Davis for Snowdon ; yet in their 

 discussion of the present topography of those districts they attribute 

 a very slight influence to stream erosion during the long pre-Glacial 

 elevation. 



The Island of Arran, amongst its many other attractions, offers 

 much clear evidence on the history of this plateau and its bearing 

 on the problem whether the Scottish valleys were made or merely 

 moulded by glacial action. Dr. Mort has recently so attractively 

 restated the case for the glacial origin of these valleys, that it may 

 be advisable to summarize the evidence in favour of pre-Glacial 

 rivers having been the more important agent in their formation. 

 Though Dr. Mort's memoir appears to attribute to ice too large a 

 share in the formation of the Arran valleys, it includes several such 



1 Since this paper was written, shortlyafter arevisit to Snowdon in September, 

 1915, the Pliocene jalateau of Carnarvon has been described by Mr. Dewey, 

 who demonstrates the jjre-Glacial age of the Snowdon valleys : " On the 

 Origin of some Land-forms in Caernarvonshire, North Wales" : Geol. Mag., 

 1918, P15. 145-57, PI. VII. The publication of the present paper has been 

 delayed by pre-occupation with other matters and two years' absence abroad ; 

 it is issued as originally written excejrt for a few verbal changes. I am greatly 

 indebted to Dr. A. Scott, who during my absence kindly re-drew the sections 

 for press. 



