188 ME. L. J. WILLS ON LATE GLACIAL AND [Jime I912, 



Overton Bridge, the 60-foot terrace can be mapped, lying on the 

 Boulder Clay, for 3| miles to the north-east, and represents the 

 grayel-fan thrown out by the river during the earlier part of its 

 erosion of the gorge. Similarl}^, it is in this neighbourhood that 

 the 40-foot terrace has its chief development. The 20-foot terrace 

 approaches nearly, and may in some cases coincide with the recent 

 alluvium. It may be that these terraces indicate successive uplifts, 

 but the absence of any signs of the 60-foot and 40-foot terraces 

 between Bangor and the sea is against this view. More probably, 

 therefore, they denote stages in the development of the gorge, and 

 depended for their formation rather on changes of climate than of 

 sea-level. 



The re-excavation of the drift-filled Yale of Llangollen and 

 higher reaches of the Ceiriog Yalley almost to their former depth 

 can be matched in many glaciated regions, but the cutting of such a 

 gorge as this Cefn-Overton Yalley, 10 miles loug and 120 to 200 feet 

 deep, affords a striking testimony to the power of erosion and to 

 the great duration of post-Glacial time. Por, not only has the 

 thick covering of Drift been removed, but in some parts of the gorge 

 100 feet or more of solid rock has been scooped out. 



At the suggestion of Mr. Lamplugh, a rough estimate has been 

 made of the amount of rock removed from the post-Glacial valley 

 between, the Cefn Yiaduct and Eyton, near Overton, neglecting 

 the Ceiriog and larger tributary valleys. The estimate does not 

 profess to be at all precise, but it shows in round numbers the 

 removal of some 477 million cubic yards, or -087 of a cubic mile 

 of material, which, if spread out as a uniform sheet 1 foot thick, 

 would cover 462 square miles. 



As the area from which this amount has been denuded is onl}^ 

 about 7 or 8 square miles, the average degradation of its surface is- 

 some 57 feet. An idea is thus gained of the colossal length of time 

 required to effect this, even if we admit that the rate of erosion 

 was much faster at the close of the Glacial Period than now. 



It is interesting to note that, if one assumes that the 60-foot, 

 terrace near Bangor, Eyton, and Overton is, on the average, only 

 6 feet thick, one can claim that it contains 4 per cent, of the total 

 material removed from the gorge. 



Before leaving the description of the gorge, we may notice tho 

 change which the narrow valley undergoes as soon as it reaches 

 the Drift plain. It expands at once, the valley from near Overton 

 Bridge to Holt being several miles wide and excavated entirely in 

 Glacial deposits, until it enters a narrow gorge in the Triassic rocks^ 

 at the latter place. 



Let us now turn to the question of the direction which the pre- 

 Glacial Dee took from near Cefn. 



If we may trust the contour-map (PL XI), the only possible 

 course lay along the outcrop of the soft Lower Coal-Measures 

 towards Chirk, and thence south-eastwards towards Gledrid. 



