226 A. C. RAMSAY ON THE PHYSICAL 



great inclined plane over which the Dee ran at a level hundreds of 

 feet above the bottom of its present valley. By and by, as the river- 

 channel deepened, the escarpment began to be formed, its face 

 sloping in a direction at right angles to the general dip of the 

 strata (after the habit of all such escarpments), by a process of 

 recession tbat has often been -explained. The whole was strictly 

 analogous to the manner in which the rivers of the Weald acted at 

 a later date, as described in ' The Physical Geology and Geo- 

 graphy of Great Britain ' — and also for the same reason that the 

 Thames now cuts across the escarpment of the Chalk, as described 

 in my memoir " On the River-courses of England and Wales"*. 

 Escaped into the low country of the New Red series, the history of 

 the Dee becomes simple, and requires no special illustration. 



But this process of ordinary nuviatile erosion is not the only 

 agent that has been at work in Wales ; for in late geological times 

 the Glacial epoch supervened, and the moving ice of thick glaciers 

 exercised a strong abrading power. Then it was that in the moun- 

 tain-region of the west so many lake-basins were scooped out, and 

 among others the rock-bound basin of Bala Lake ; and though the 

 face of the country is always being slowly changed, the time that 

 has elapsed since the close of the Glacial epoch is compara- 

 tively so short that the large essential rocky features of the region 

 traversed by the river have since that time undergone no important 

 alteration. 



I shall conclude with a few words about the length of geological 

 time occupied in the excavation of the valley. 



It may be remembered that in the paper on the excavation of the 

 valley and gorge of the Rhine, I explained that the work of scooping 

 it down to the present levels of the river has been accomplished 

 since the close of the Miocene epoch, and that the direction of the 

 flow of its water then and now is connected with changes of physical 

 geography brought about by a great post-Miocene disturbance of 

 the Alps. 



The height of the old high-level terrace or old bed of the Rhine 

 is about 400 feet above the present river ; and the same is the case 

 with its tributary, the Moselle. The height of the relics of the old 

 plain of marine denudation above the Dee where it passes through 

 the Wenlock strata is, to take a low estimate, not less than from 

 1100 to 1200 feet; and the time occupied in cutting that depth 

 extends from the Permian epoch down to the present day. The 

 depth excavated in that period of time is not less than three times 

 as great as that performed by the Rhine and Moselle ; and I think 

 no geologist will doubt that the time that elapsed between the 

 Permian and Miocene epochs was much more than three times 

 the length of that which has elapsed since the close of Miocene 

 times down to the present day. The Dee is but a petty stream 

 when compared with even the smaller of the two German rivers; 

 and it is obvious that the greater the body of water in a river, 1 aken 



* Quart. Journ. Geol. Soc. vol. xxviii. (1872), p. 148. 



