Gorges first bored by Subterranean Rivers. 



457' 



as examples : the Mole, one of them, made the gorge, at Box Hill, at right 

 angles to the strike or range of chalk hills, which form an escarpment from 

 Guildford to Reigate, for instance. * In gorges or cuttings through escarpment, I 

 observe that the dip is often with the stream, and thus assists the underground 

 river, which did the difficult part of the excavation. I believe, in the first 

 instance, a river on making a gorge always passes through an upper channel at 

 a high level, and then boreas tunnel or cave passage at a lower level. Some- 

 times this is proved by a part of the arch of rock remaining, as shown in a 



(26) S 



ANCIENr SAA/DQW/V FJC 25 



A _ SFCT/CA/ sliezt/in.(j surjaae 



removed at different- 

 ■periods Zy Denudation 



very beautiful drawing painted on the spot, by Arthur Severn, at Constantine, 

 in Algeria. I have found a case in which both underground and overground 

 channels remain, in Ystrad Vellte, in South Wales, near Hirwain. The rock 

 is hard limestone, at the mouth of the cave 55 ft. high. On one occasion in 

 the last two hundred years, there was such a flood that the water rose forty-five 



Fl C :2 7. ^qtqefiaimfiDJS. CbalMmdemyfmncd. 



a . sever /j osl. 



feet up to the top. I have measured drawings of this interesting spot. The 

 cave is one-third of a mile long. In a wet period, like the gravel or pluvial, 

 the upper and lower channels would be used simultaneously. 



I consider the steepness of the Cheddar Cliffs, and of the limestone gorge, at 



* See Fig. 35, page 473, where the double set of flexures on the escarpment of the Surrey 

 Hills are described. 



