184 



MK. L. J. WILLS ON LATE GLACIAL AND [June I912, 



interesting, as it appears possible to account for the existence of 

 a low col at Eerwyn, which could be seized upon b}^ the river on 

 the retreat of the ice. For, at the present day, the Plas-yn-Yivod 

 stream enters the Dee at Berwyn Bridge, but in pre-Glacial times 

 it joined the latter farther east at Pentre-felin. The erosion of this 

 tributary valley would necessitate a low col near Llantisilio between 



Alluvium 



Lower Gravel-Terrace 



Higher Gravel-Terrace 



v />''^ < 

 ^^v. 



Glacial Drift filling in the 

 bottom of pre-Glacial valley- 



Silurian rocks 



it and the Dee, and it was by the flooding of this col by the post- 

 Glacial waters that the Berwyn gorge was initiated. 



It can be pointed out, in support of this hypothesis, that on the 

 north side of the gorge near the bridge there occurs a small patch 

 of boulder-clay about 100 yards in diameter, Avhich once formed 

 part of the Drift that filled the Plas-yn-Vivod Valley. This boulder- 

 clay rests at about the same level as that Avhich still occupies the 



