Vol. 68.] POST-GLACIAL CHANGES IN THE LOWER DEE VALLEY. 183 



day, but their courses liave been seriously altered by the presence' 

 of ice and by the deposition of Glacial Drift. 



This paper only deals with the Dee Valley below Glyn-Dyfrdwy. 

 Prom thereabouts to the Great Western Eailway Viaduct near 

 Cefn, Glacial Drifts can be traced low down in the valley, and, in 

 two cases at least, Boulder Clay has been found below the present 

 alluvium, showing that the Dee has not yet reached its former 

 level. 



This stretch of the valley is therefore plainly pre-Glacial in the 

 main. But, when more closely examined, it is found that in several 

 places the river has abandoned its old course. Some of these diver- 

 sions are ' short cuts,' others the reverse. 



That stupendous physiographic changes could take place in a 

 deep, narrow valley, such as the Dee occupies near Llangollen, was 

 only rendered possible by the serpentine nature of its whole course. 

 This must be a very ancient feature, for it was evidently initiated at 

 a level many hundred feet higher than the present river-bed. The 

 winding gorge of the Dee is, in fact, reminiscent of ' incised 

 meanders ' ; but, if the loops were ever meanders, they must have 

 belonged to a stream of far larger volume than the present Dee. 

 The flat tops of the neighbouring hills may well be relics of a pene- 

 plain, but inequalities on its surface and not meanders may have 

 initiated the bends. 



The form of the valley led to strange happenings when it was filled 

 to overflowing by the ice-sheet, and later when a valley-glacier and 

 its accompanying waters held sway within it. At present one can 

 judge of the power of the ice, as it forced its way down the narrow 

 winding valley, by the marked truncation of the spurs and the 

 uniformly steep slopes which it has left behind. (Similar jjhe- 

 nomena are even better displayed in the Ceiriog Valley.) 



(1) The Llantisilio Diversion. 



The first post-Glacial diversion to be noticed is the ' short cut ' 

 through the gorge at Berwyn (see fig. 1, p. 184). 



A drift-plugged hollow leaving the present valley at Llantisilio 

 Church can be traced northwards for about a mile, where it bends 

 round to the east into the Valle Crucis Valley. At the bend there 

 is a col rising to about 430 feet O.D. The Valle Crucis Valley joins 

 the Dee at Pentre-felin near the lower end of the above-mentioned 

 Berwyn Gorge; and, viewed from near Llangollen, it is clearly a 

 •continuation of that of the Dee. Its pre-Glacial age is evidenced by 

 a considerable thickness of Drift in its lower part. The deserted 

 valley near Llantisilio is also filled with Drift, on the irregular 

 surface of which were kettle-holes that are now marked by stretches 

 of alluvium. A small stream drains through it to the Dee, near 

 the church. 



The broad, drift-filled pre-Glacial valley stands out in strong 

 contrast to the narrow rocky gorge of the present Dee at Berwyn. 

 We have here a clear case of diversion, and one which is specially 



