GOODCHILD: GEOLOGICAL HISTORY OF UPPER SWALEDALE. 245 



Much later on followed yet another long period of disturbance, 

 upheaval, and consequent denudation. This episode was followed 

 by one of prolonged and steady subsidence, during which the Upper 

 Cretaceous Rocks were spread out over nearly the whole of the 

 British Isles. Not until long after that did any of our mountains — 

 or indeed any other of our great natural features — begin to appear. 



When at last the River Swale began to flow, which was in late 

 tertiary times, the surface rock was probably the Chalk, and the 

 river took its rise, not where it does now, but far away to the west, 

 where now are the lowlands of Edenside. Underneath the Chalk 

 in Upper Swaledale extended the thin edge of the New Red Rocks, 

 now denuded back to the Vale of York. Prolonged exposure to 

 denudation stripped off first the Cretaceous Rocks, then the New 

 Red, and finally exposed the plateau, gently inclined to the east, 

 upon which these last rocks had been laid down. At this time the 

 plateau just mentioned was probably not at a very high level above 

 the sea. But after the valley of the Swale had been, so to speak, 

 outlined, in the higher part of the plateau, great volcanic disturbances 

 affected all the north-western parts of the kingdom. The volcanic 

 rocks of the Western Islands of Scotland were formed, and these 

 disturbances were accompanied, over a much larger area, by earth 

 movements of great importance. Vast upheavals of strata took 

 place, the last great dislocation along the line of the Pennine Fault 

 occurred, the Swaledale Massif was upheaved to more than 2,500 ft. 

 above the level of the sea, and the present order of geographical 

 features generally was instituted. But the most important result, so 

 far as Swaledale is concerned, was that due to the action of thermal 

 springs, which uprose towards the surface over large areas as the 

 great volcanic episode was on the wane. It was these thermal 

 springs, rising through the old faults, where these were stretched open 

 during the upheaval of the district, that carried upwards the solutions 

 containing the metallic sulphides, which, when the rising currents 

 cooled, were left in the old fissures as the valuable lead veins for 

 which North-west Yorkshire has so long been famous. 



The present summit-level joining the highest fell-tops of Swaledale 

 represents the modified descendant of the old plateau referred to 

 above, and it is out of tliis old plateau, by the prolonged action of 

 Subaerial Denudation, that the present valleys have been carved in 

 the course of long ages. 



Long after the valleys and all the larger natural features had been 

 carved out by denudation into nearly the same form they exhibit at 

 the present day, set in the commencement of that long succession of 

 periods of increasing cold, which culminated in the climax of the 



August 1800. 



