222 A. C. RAMSAY ON THE PHYSICAL 



Beyond Erbistock, where the Dee leaves the hills, and all down 

 the estuary, the valley is undoubtedly preglaeial, in so far that the 

 country is largely shrouded by glacial drifts. 



The question thus arises, by what means was the valley of the Dee 

 excavated, and if it be possible to discover during what geological 

 epoch that excavation began. 



The valley, in some aspects, may be compared to that of the Mo- 

 selle, the physical character of which I have elsewhere indicated*; 

 for, especially east of Corwen, between banks of Wenlock shale or 

 slate, that form a high and much-eroded tableland, the Dee winds 

 in many a curve, two of which, about two miles above Llangollen, 

 form great loops, closely resembling the well-known loops of the 

 Moselle above Alf. In both rivers a steep rocky high bank is apt 

 to be opposed to the concave curve of the stream, while, on the 

 opposite, or convex side, the ground generally rises from the river 

 with a long and gentle slope. This form of ground is characteristic 

 of many rivers that flow through deep valleys, the elevated ground of 

 which on either side is more or less flat-topped, and invariably in- 

 dicates a long-continued course of fluviatile erosion, begun at first by 

 a wandering river on a high and slightly undulating tableland or 

 plain. 



The Silurian rocks of the region now called North Wales un- 

 derwent great disturbance and erosion, both atmospheric and marine, 

 both before and during the deposition of the Carboniferous Lime- 

 stone — one result of this being that as the country was in great 

 part slowly submerged the limestone series was laid in a very 

 unconformable manner on the denuded edges of both the Lower and 

 Upper Silurian strata. 



The high escarpment of Carboniferous Limestone in Denbighshire 

 facing west clearly shows that this limestone once extended a great 

 deal further in that direction across much of the Lower Silurian 

 rocks of the Berwyn Hills and the Wenlock slates and shales that 

 form the high land on either side of the valley of the Dee. This 

 fact, if it needed confirmation, is further proved by the well- 

 known outlier of Carboniferous Limestone that lies, far in among 

 the hills in the valley of the Dee, two miles west of Corwen, ten 

 miles distant from the nearest point of the escarpment, and six 

 miles from the limestone at the southern end of the Yale of Clwyd. 

 This little patch, which occupies an area of something less than 

 three quarters of a square mile, lies quite un conformably on the 

 Wenlock shale on its south-western side, while on the south-east 

 and north-east it is, as it were, dovetailed into the Silurian strata 

 by two faults ; and as its thickness, according to the late Professor 

 Jukes, may be about 2000 feet, it is clear that the Carboniferous 

 Limestone, in force, once spread well across this and the neighbour- 

 ing region. 



Furthermore, everywhere to the east in Denbighshire and Flint- 

 shire, the limestone is overlain by the Millstone Grit and Coal- 



* Physical Geology and Geography of Great Britain, chap. 15. 



