Vol. 68.] POST-GLACIAL CHANGES IIS" THE LOWEE DEE VALLEY. 189 



Although this valley is now obliterated by Drift, the upper slopes 

 of its sides are seen between Pentre and Chirk. On the west the 

 Millstone Grit hills slope gently towards it ; while on the east the 

 Black Park-Chirk Green ridge shuts it in, and is most evidently 

 unconnected Avith the present drainage-system. 



Between these two limits lies a fairly flat spread of Drift, the 

 great thickness of which is shown, not only by the deep-cut valley 

 of the Pen-y-Park stream, but by the evidence from two trial bore- 

 holes, which, in my opinion, prove this diversion conclusively. 



The first bore-hole showed 130 to 140 feet of Drift, and did not 

 even then reach the solid rock. It was situated immediately 

 north-west of Black Park Lodge {No. 1 on the map, PL XI), and 

 the pre-Glacial floor is here at most 170 feet above sea-level, or 5 

 to 10 feet lower than it is near the Viaduct. No. 2 boring was at 

 Black Park Wharf, and not far west of the solid outcrop of the Chirk 

 Green ridge. It proved 142 feet of Drift, and passed into Coal- 

 Measnres at about 200 feet above O.D. Incidentally this shows 

 that the pre-Glacial valley-side here is steep, for it drops 200 feet 

 in less than a quarter of a mile. 



Further evidence as to the position of the buried hollow is found 

 in the valley of the Ceiriog near Chirk. This stream has not yet 

 cut through the iufilling of Drift at the point where one would 

 expect that the pre-Glacial Dee was joined by the pre-Glacial 

 Ceiriog. There are no data as to the thickness of the Drift below 

 the present valley-bottom; but it must descend below 200 feet O.D., 

 for the river passes from Drift on to solid rock at this level near 

 Lady bridge. 



Since the Morlas Brook runs in a similar way on to solid rock 

 at about the same level near the village of Glyn Morlas, these two 

 points help us to locate a north-eastern side to the former Dee 

 Valley: for, as one goes north and east from here, the surface of the 

 solid rock rises. 



The only other direct evidence of the course of the former valley 

 is obtained from an isolated occurrence of solid rock which projects 

 from a wilderness of Glacial Drift near St. Martin's Moor. This 

 must, I think, have formed part of the same north-eastern valley- 

 side, since the Drift-covered escarpment of the Coed-yr-allt Bock 

 offers an apparent!}^ unbroken barrier towards the east from Glyn 

 Morlas to Esgob Mill. 



So great are the changes which have been brought about in the 

 physiography of this district by the Drift, that it is very doubtful 

 whether early post- Glacial lines of drainage form any indication 

 at all of the pre-Glacial lines. But, if the possibility be allowed, 

 we may perhaps see some support for our theory of the position of 

 the pre-Glacial Dee in a remarkable valley, now almost dry, along 

 which the Shropshire Union Canal runs from near Esgob Mill, past 

 St. Martin's Moor, to Hindford. Its direction almost coincides 

 with the supposed course of the pre-Glacial Dee. 



This valley deserves more than passing notice, for it is clearly a 

 continuation of the Morlas Brook valley beyond Esgob Mill (see 



p2 



