The Pre- Glacial Valleys of Arran and Snoivdon. 163 



streams flowing down a uniform dome. But if the major valleys 

 be regarded as a reticular system formed along fractures which 

 gaped oj)en at the uplift of the country, they could have been made 

 without trunk rivers ; and the depths of the main valleys as com- 

 pared with some of their tributaries is a natural consequence of the 

 more rapid denudation along the tectonic ruptures. 



Professor Davis supports his case by reference to the rounded 

 mountains, or " Moels ". He holds (1909, p. 293) that they were 

 not overridden by northern ice or " overwhelmed by the local 

 ice ", and that their rounded forms have been retained from pre- 

 Glacial times ; and he therefore claims that the adjacent valleys 

 were also rounded then. But if the Moels on the north-west spurs 

 of Snowdon have retained their pre-Glacial forms, they would tell 

 against the view that the mountain has been greatly altered by 

 glacial erosion, since some of them were in positions especially 

 exposed to abrasion by the ice from central Snowdon. 



Durarddu 



Lliwedd 



• 1_^ 



----%■ 





Moel eoch 



C logwyn 

 Valley 



Fig 



Cwm. Qo^wyn. 



8. — Hypothetical sketch of Snowdon from the north-west at the 

 pre-Glacial times, illustrating the valleys formed in consequence 

 Pliocene uplift. 



Llaa 

 Valley 



end of 

 of the 



As Snowdon stands in a position which rendered it subject to 

 a heavy precipitation and is very varied in structure and composition, 

 it appears improbable that it should have suffered so little from 

 stream erosion during the long Pliocene uplift, as is admitted by 

 Professor Davis' explanation. According to his view, on the 

 section from Mynydd Mawr across Llyn Cwellyn to Snowdon, the 

 pre-Grlacial floor of the valley would have been at the present level 

 of about 1,250 feet ; Snowdon and Mynydd Mawr would have risen 

 from the valley in gentle mature slopes along the upper boundary 

 of the shaded area in Fig. 7. The valley below the broken line 

 would have been excavated by glacial action. 



According to the alternative view the pre-Glacial surface was 

 along the lower line ; the valley was a deep trench in the 1,200 

 to 1,400 ft. platform; the bed of the lake is 122 feet lower 

 (Jehu, 1903, p. 436). The greatest removal of solid rock by the 

 ice would have been along the edges of the Llyn Cwellyn Valley, 

 and the amount removed would have been approximately that 



