Yol. 6S.'] POST-GLACIAL CHANGES IN THE LOWER DEE VALLEY. 185 



tributary valley on the south side of the gorg-e, and lies about 30 or- 

 40 feet above the river, which here flows in a rocky ravine. 



The Berwyn Gorge can, I think, be regarded as a marginal 

 overflow channel, initiated when the Dee Glacier had shrunk con- 

 siderably and now only occupied the bottom of the valley, while 

 the drainage ran between it and the higher slopes of the hills. At 

 the same time, it must be noted that the drift dam in the loop rises 

 to about 130 feet above the present river, and 90 to 100 feet above 

 the base of the drift in the gorge, and may have determined the 

 direction of flow after all the ice had disappeared. 



(2) Holyhead Road Overflow Channel. 



The pass which the Holyhead Eoad takes about a mile west of 

 Berwyn cuts across the neck of land round which winds the loop 

 of the Dee between E-hydonen and Llantisilio. On the north, a 

 knoll occupying the centre of the loop slopes steeply towards this 

 pass ; while on the south rise high hills forming the upper slopes of 

 the whole vallej'-side. 



The explanation of this curious physiographical feature appears 

 to be that during the Glacial Period the gap formed an overflow 

 channel. One is led to the conclusion that, in order to avoid 

 moving round the long loop which the valley takes here, part of 

 the ice flowed over this col, tearing away the sides and grinding 

 out a fairly flat bottom. At the same time, it is probable that 

 such action was supplemented or was replaced, when the ice was 

 dwindling, by that of water flowing between the edge of the Dee 

 Glacier and the high ground on the south. If this latter hypothesis 

 holds, the minimum thickness of stagnant ice in the valley at that 

 time must have been 230 feet, the height of the pass above the 

 river. 



Had this channel become permanent on the disappearance of the 

 ice there would have been here a gorge similar to, though far 

 deeper than, the one at Berwyn. 



(3) The Pengwern Diversion. 



The Dee follows its pre- Glacial course from Pentre-felin to 

 Llangollen, where it has again been diverted from its original 

 direction, and now flows through the Llangollen Gorge. The 

 history of this diversion is similar to that of the Llantisilio diversion. 



A broad valley leads south-eastwards from Llangollen to Pengwern 

 Hall, where it bends round to rejoin the present Dee Yalley near 

 the Llangollen Golf Links. The Drift forming its wide bottom is 

 of considerable thickness, and is well exposed in the banks of the 

 Cyflymen Brook. 



This broad through- valley I have called the Pengwern Valley, 

 and it is a conspicuous case of ' misfit.' At present there is no 

 through-drainage in it, but a small stream enters it from the south. 

 Part of the stream joins the Cyflymen Brook, while part is led 



