﻿Vol. 6.1. ] OP THE NORTHUMBERLAND AND DURHAM COALFIELD. 89 



thicker masses of sandstone, such as the ' Grindstone-Sill ' round 

 Newcastle, formed fairly -distinct escarpments in pre -Glacial 

 times. During the Ice-Age many of these masses were planed 

 down, roches moutonnees on a large scale being formed ; and 

 now, if exposed at all, their tops alone rise out of the covering of 

 Boulder-Clay. These phenomena can be clearly observed at the 

 present time in a quarry near Kenton, where the ' Grindstone- 

 Sill' is worked. My attention w r as first drawn, by Dr. J. A. 

 Smythe, to the rounded character of the surface of the sandstone 

 exposed here. The escarpment facing north — which, being due to 

 the southern dip of the rocks, would naturally exist here in pre- 

 Glacial times — appears to have been planed down, and the whole 

 exposure presents a rounded contour, with a slight covering of 

 Boulder-Clay resting upon it. Another point of interest in the 

 same connection is, that nearly all the pit-villages in the South of 

 Northumberland (such as Killingworth, Earsdon, and Backworth) 

 are built upon sandstone, which rises above the level of the super- 

 ficial deposits. 



Since the end of the Glacial Period the present rivers have 

 perforce cut their courses in a varying platform of rock and Bonlder- 

 Clay, and, in. consequence, there is considerable difference between 

 the pre-Glacial and the post-Glacial drainage. Some of the streams 

 flow over the top of the superficial deposits that had been laid down 

 in the already-existing valleys ; while others have left their old 

 channels, and cut through rock or Boulder-Clay ; also several 

 entirely-new streams have been developed on the varied post- 

 Glacial surface. It is remarkable, in this connection, how the 

 post-Glacial rivers have often cut their way through rock rather 

 than Boulder-Clay, and how the rivers in many cases keep to the 

 edge of the pre-Glacial valleys, and have thus denuded the rock 

 along their sides. The most notable case is that of the * Sleekburn ' 

 Valley, which has had developed on either side of it the Wansbeck 

 and the Blyth, both of which are of post-Glacial development, and 

 cut mainly through rock. Also the Wear, in its lower reaches 

 from Durham to near Sunderland, flows for a part of the way along 

 the edge of a minor valley of pre-Glacial origin, running at first 

 mainly over the Coal-Measures, and then, just before reaching the 

 latter town, it breaks through the Magnesian-Limestone escarpment, 

 and the rest of its course is entirely in Permian strata. Phenomena 

 similar to these have been noted by Mr. C. Fox-Strangways (10) 

 and Mr. Cowper Heed (13), in their descriptions of the valleys of 

 North-East Yorkshire. The cause of this is not clear, but I would 

 suggest that the surface of the Drift in the valleys was, immediately 

 after its production, a convex curve following the surface of the ice 

 that produced it, instead of (as at present) a concave one. This 

 view of the contour of the superficial deposits gives a natural 

 explanation of the occurrence of two streams on either side of a pre- 

 Glacial valley, and of the way in which the rivers seem to have 

 developed their courses through rock rather than Boulder-Clay. 



Not only may the surface of the superficial deposits, when looked 



