524 The Dee. 



for when the Dee began to run, that escarpment had no 

 existence, and the strata of these formations stretched 

 further to the west, ending along some line now un- 

 known in a sort of feather edge, and forming part of 

 the 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. The whole 

 was strictly analogous to the manner in which the 

 rivers of the Weald acted at a later date, and also for 

 the same reason that the Thames now cuts across the 

 escarpment of the Chalk. Escaped into the low coun- 

 try of the New Red series, the history of the Dee 

 becomes simple, and requires no special illustration. 



But this process of ordinary fluviatile erosion is not 

 the only agent that has been at work in Wales, for in 

 later geological times the Glacial epoch supervened, and 

 the moving ice of thick glaciers exercised a strong 

 abrading power. Then it was that in the mountain-region 

 of the west, ice-smoothing, mammillations, and striations 

 were so strongly impressed on the sides of so many 

 valleys, and 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 comparatively so short, that the 

 large essential rocky features of the regions traversed 

 by the rivers have since that time undergone no impor- 

 tant alteration. 



In the ' Journal of the Geological Society ' for 1876, 

 I published the Physical History of the Dee. It is 

 too long, and the necessary diagrams are too large 



