310 TYNEDALE ESCARPMENTS; 



H, or if the rivers are, like the North and South Tyne, conver- 

 gent, the letter A ; and let the single cross har of the letter re- 

 present many bars of hard rock alternating with soft. Any 

 grooves that may be formed between the bars will then descend 

 in each direction on a valley ; each bar will form an arch, with 

 its watershed for summit, and the valloy-slopes, in part, for 

 sides. Now as long as the seaward slope of the river beds occa- 

 sions a ^^gorously deepening flow in them, the convexity of the 

 arch will be maintained, in spite of the attempts of atmospheric 

 waste to flatten its summit. The flow and eroding power of 

 water in the grooves will thus be pro'S'ided for, and ready trans- 

 port for the materials disintegrated in the flattening process will 

 be always at hand. As in the model where free flow is arranged 

 for, the hard bars will stand the best chance of being sculptured 

 into relief. 



1. Development by Streams and Atmospheric Disintegration to- 

 gether. — Let us trace the details of this development. Suppose 

 tlie rivers represented by the limbs of the A or H to have just 

 begun to flow, guided by hollows appreciable to the unerring se- 

 lection of water, llain falls on the higher ground between, and 

 descends upon them. Streams of a hundrecl different sizes flow 

 among the obscure bars, some along, some probably fewer across, 

 at various angles. 



Woodcut No. C— Dlagrnnimatic Hoction of Sandstones and Shales lllu!«tratint; the dere- 

 lojmicnt cif Escarpments nnd Terraces I'V streams. A, H, C, l>, K, transverse 8C«-tion of 

 diffcrcnt-sized channelH on tlie orixiuai surface. Successive stages of excavation are in- 

 dicated by dotted lines, and light and dark shading. 



Suppose the case of five different-sized streams flowing side by 

 side along the bars. "NVoodcut No. 6 is supposed to be a section 

 across outcrops of hard sandstone and soft shale near and paral- 

 lel to one of the river channels represented by tlie limbs of tlie 

 capital letters, which 1 shall begin to call longitudinal vaJlegs in 



