Vol. 68.] POST-GLlCrAL CHANGES IN THE LOWER IJEE VALLEY. 191 



flows to Esgob Mill and so into the Dee drainage, whereas the other 

 end of the same ditch sends water into the Severn. 



This opens up the possibility of the pre-Glacial Dee having 

 joined the Severn, but a discussion of this point will be reserved 

 until later. 



One might expect to find in the nature of the Drift that fills the 

 buried valley between Pentre and Chirk a conspicuous cause for 

 the diversion. That it was here that the obstruction took place is 

 proved by the presence of the Ceiriog Valley : for, had the barrier 

 lain more to the south, the Ceiriog would still join the Dee at 

 Chirk. Instead of this both rivers now cross the pre-Glacial valley, 

 and parsue an independent course roughly at right angles to it 

 until they meet farther east. 



Beyond the fact, however, that the Drifts rise to 350 feet above 

 sea-level (about 180 feet above the former surface) nothing very 

 remarkable can be said of them. A.t the same time, they occur at 

 a very sharp bend in the pre-Glacial valley that appears near Cefn 

 to have been deflected southwards, along the outcrop of the soft 

 Lower Coal-Measures, by the escarpment of the Middle Coal- 

 Measures and Coed-yr-allt Series. It is easy to imagine that, on 

 the recession of the ice, the drainage might be induced by a com- 

 paratively small obstacle to avoid so acute a change of direction. 

 We have already seen that in doing so it probably availed itself of 

 a pre-Glacial tributary valley trending eastwards. The peculiar loop 

 of the Dee near Pentre appears almost to coincide with the direction 

 of this tributary valley (see PI. XI). 



(6) The Story of the River Ceiriog. 



At this point, it may be profitable to turn for a moment to a 

 consideration of the liiver Ceiriog. This stream has re-excavated 

 its present course through the Drift that filled up the bottom of 

 a pre-Glacial valley, the straightness and narrowness, depth and 

 steep-sidedness of which have precluded the possibility of diversions 

 such as the Dee has undertaken. 



Prom the village of Glyn ^ downwards, patches of Drift may be 

 seen clinging to the old valley-sides and filling up the tributary 

 stream-courses. But below Chirk, at the point where one would 

 expect that the Ceiriog joined the Dee in pre-Glacial times, the 

 whole gorge is excavated in Drift, and the river flows on Boulder 

 Clay. 



The only post-Glacial change, therefore, in the Ceiriog drainage 

 is its extension to join the Dee near Pont-y-Blew ; and this is 

 effected by a deep valley cut through the Ruabon Marls. 



A series of gravels that accompany the lower part of the present 

 course of the Ceiriog, spread out on the Boulder Clay as high-level 

 flats near Chirk, Brynkinalt, and the Rhyn Park ^ at heights 



"^ Llansantffraid-Glyn-Ceiriog. 



2 The similar terrace at Halton may belong either to the Dee, or to the 

 Ceiriog, or to both. 



