236 Re])0)'ts and Proceedings — Geological Society of London. 



After poiuting out that the district where the Elver Dee leaves 

 its deep valley in the Welsh hills and enters the lower ground 

 occupied by the Coal-measures is a critical one in connexion with its 

 post-Glacial history, the author briefly describes the composition and 

 local distribution of the Irish Sea and Welsh Drifts. 



The second part of the paper deals with post-Glacial changes in 

 the topography of the Dee Valley. The term ' post-Glacial ' is here 

 applied to events that occurred after the period of maximum 

 glaciation. Manj^ of the changes dealt with may have originated in 

 late Glacial times. 



In the Llangollen district the modifications are dependent on the 

 serpentine course of the whole valley. Glacial phenomena have led 

 to the cutting of overflow channels across the necks of the loops, at 

 great heights above the river. Two of these have become permanent 

 courses of the Dee, with the result that deep gorges have been formed 

 across the rocks while the deserted loops have been left like gigantic 

 oxbows. 



The other diversiojas appear to be due to the fact that the post- 

 Glacial course of the river was determined by the surface configuration 

 of the Drift, and has since been excavated into the solid rock at 

 places where it never flowed before. They are, in fact, cases of 

 superimposed drainage. 



An unimportant diversion at Argoed (near Trevor) is described, in 

 which the river takes a longer course than in pre-Glacial times. The 

 most notable change, however, occurs near Cefn, and involves the 

 cutting of an entirely new course, some ten miles long, between that 

 locality and Eyton near Overton. Details of this gorge are given, 

 and a suggestion is put forward to explain why the river took this 

 course. 



The general direction of this, the present, Dee valley is easterly. 

 The pre-Glacial course has been traced southwards with some 

 precision from near Cefn to Chirk, and then with less certainty in 

 a more south-easterly direction for a few miles. In that direction 

 the Drift is so thick that it has been found impossible to trace the 

 former course until it I'ejoins the deep buried valley which Dr. Strahan 

 discovered in the neighbourhood of Chester. The author's observations 

 have proved the continuation of this buried valley southwards nearly 

 to Bangor-on-Dee. Near here, at Roden's Hall, the rock-surface is 

 30 feet below sea-level. 



The southerly bend of the pre-Glacial valley near Cefn to Chirk, 

 and then towards Whittington, suggests the possibility of a confluence 

 of the Severn and the Dee, and this is shortly discussed. 



It is shown that formerly the Eiver Ceiriog joined the pre-Glacial 

 Dee at Chirk, and that its present course below Chirk is post-Glacial. 

 The gravels of this valley suggest that the post-Glacial drainage 

 always followed this course. It is shown, however, that the Morlas 

 Brook, which is now a tributary of the Ceiriog, was not always so, 

 its early post-Glacial course having been to the Severn. 



The slope of the pre-Glacial valley-floor and the uplift which is 

 indicated by the buried valley, now many feet below sea-level, are 

 briefly discussed. 



