306 TTNEDALE ESCARPMENTS; 



few hours before of the dangerous violence of winds at such 

 heiglits, I was inclined to give some credit to the statement. 

 On the summit of this Assynt mountain the Cambrian sandstone 

 weathers into shreds that hang about its blocks like rags ; and to 

 mark these congeries of grains shivering to the wind is to under- 

 stand by visible demonstration what must unperceivedly bear 

 away many a grain of the harder sandstones of Northumbrian 

 crags, and has in fact formed a covering of sand around some of 

 them. 



lUmtrations of Atmospheric Action in Ti/nedale. — Such being 

 an outline of the action of the wasting "influences of the atmos- 

 phere," one or two illustrations may be given of their actual effects 

 upon rocks. Take first their effects in severing en masse. 



In following, upwards, the course of a Tynedale streamlet, one 

 can seldom go very far without finding the banks drawing toge- 

 ther and becoming more and more precipitous, until the gorge, as 

 narrow, and almost as steep-sided as a high-walled roadway, and 

 all cumbered with fallen blocks, is barred by a waterfall. It is 

 the well known tendency of waterfalls to cut back along the line 

 of the stream course, so as to place themselves at the end of a 

 steeply walled gorge of this kind. The walls at the lower end 

 have stood facing each other since the waterfall cut away the 

 rock from between them, and standing, as they often do, well 

 above the running water, whicli in cases relevant to our purpose 

 must shoot straight out with the incline, they are left to the 

 attacks of frost and rain. Here, then, — as pointed out by my 

 colleague Mr. Goodchild — we have a gauge to measure, rela- 

 tively, the capacity of purely atmospheric forces for cutting back 

 scarped walls of rock. By how much in a given case has each 

 wall receded since left by the waterfall, and what is the ratio be- 

 tween its recession and that of the fall ?* 



A gorge or dene in the Dinley Burn, near JUrtley, supplies 

 sufficient answer for the purposes of an illustration (Woodcut 

 No. 4). The full widtli in front of the waterfall, which was at 

 first formed by a sandstone about ten feet thick and some twenty 



• Oool. Mug., 1875, p. S26. 



