Vol. 68.] POST-GLACIAL CHAISTGES TN THE LOWEE DEE VALLEY. 187 



By means of this dissection the heights above sea-level at which 

 Glacial deposits rest upon solid rock can be determined, and the 

 approximate contouring of the pre-Glacial surface can be deduced 

 as in the accompanying map (PL XI), which shows that before the 

 Glacial Period the river could not have occupied its present valley. 



I now propose to examine some few details of this post-Glacial 

 gorge. 



The lowest point west of the railway viaduct at Cefn at which 

 Boulder Clay is seen resting upon solid rock is about 175 or 180 feet 

 above O.D. As mentioned above, it is not found so low as this 

 until one approaches Erbistock, At most points near Cefn and 

 Black Park it lies at about 250 to 300 feet above sea-level, or even 

 higher ; but at one place in Halton Woods it is as low as about 

 21d feet O.D. Now this occurrence is remarkable, in that solid 

 rock surrounds it at higher levels on every side except where the 

 river flows. One is led to the conclusion, therefore, that this 

 Boulder Clay lies in what was originally a small tributary valley of 

 the Dee that joined it from the east somewhere near Pentre. If 

 this be the true explanation, it suggests a likely reason why, on 

 th6 retreat of the ice, the Dee took its present course. Por here, 

 possibly, would be a low col through which the river could escape 

 to the east ; while, even at the present day, its old course towards 

 Ghirk is blocked with drift up to 350 feet above sea-level. 



The only other explanation of this peculiarly low patch of 

 Boulder Clay inside the gorge postulates an Interglacial Period, 

 during which the river cut through the Drift and through the 

 ••solid rock to within 50 feet of its present level. Then followed a 

 Glacial Period, of which this single patch of Boulder Clay and 

 possibly a few isolated occurrences of high-level gravels lying 

 well within the gorge are the only relics so far discovered. The 

 above-mentioned gravels are certainly puzzling : for, although they 

 resemble Glacial gravels in their moundy mode of occurrence, yet 

 they lie apparently inside the gorge, and often at much lower 

 levels than the rest of the Drift in their neighbourhood. They 

 may possibly be explained as late Pluvioglacial flood-gravels, the 

 terrace-like form of which has been strongly modified by subsequent 

 denudation. 



The early history of the gorge is recorded in the rough terraces 

 at Sodyllt and Gron-wen, and possibly that at Halton, which must 

 be classified as Fluvioglacial gravels of the Dee. Their height 

 above the river is approximately 120 feet. These are comparable 

 to others at Chirk, Brynkinalt, and the Khyn Park, which were 

 deposited by the post-Glacial Ceiriog. 



These Fluvioglacial flood-gravels mark the first stage in the 

 erosion of the gorge, when the waters were flowing almost entirely 

 on Drift. At lower levels on the sides of the gorge alluvial and 

 gravel terraces repeatedly recur at approximately 60, 40, and 20 feet 

 above the river, although its meanderings preclude the possibility 

 of tracing any one continuously for far. But, from the end by 

 Q. J. G. S. iS"o. 270. p 



