Late Glacial and post- Glacial Changes in Dee Valley. 807 



beds. Prom the greatly disturbed character of all the Silurian 

 strata to the west of the inlier, as contrasted with the undisturbed 

 condition of the beds on its eastern side, it would appear that this 

 inlier is part of a barrier which has preserved the district lying to 

 the east from the effects of the post-Silurian movements. 



February 28th, 1912.— Dr. Aubrey Strahan, F.P.S., President, 

 in the Chair. 



The following communications were read : — 



1. 'Late Glacial and post-Glacial Changes in the Lower Dee 

 Valley.' By Leonard Johnston Wills, M.A., F.G.S., Fellow of 

 King's College, Cambridge. 



After pointing out that the district where the River 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, tbe author briefly describes the composi- 

 tion 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. Many 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 tbe deserted loops have 

 been left like gigantic oxbows. 



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

 Glacial course of the river was determined by the surface configu- 

 ration 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 10 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 valle}* is easterly. 

 The pre-Glacial course has been traced southwards with some pre- 

 cision 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 rejoins 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. jSTear here, at Poden's 

 Hall, the rock-surface is 30 feet below sea-level. 



