12 Remarks on the Formation of Alluvial Deposites. 



are marked, at frequent intervals, either by vast cylindrical im- 

 pressions, or by the cleavages which these impressions have in- 

 tersected *. 



It is manifest that this peculiar action of stones in water de- 

 pends upon the structure of the channel. The distinct concre- 

 tions of the rock, and its less yielding portions, project in salient 

 angles, which drive the current, with its load of stones, against 

 the opposite wall, and, by repeated blows, it is chiselled into 

 the forms which we now survey. 



Another characteristic circumstance of gorges thus formed, 

 is their astonishing depth and narrowness. In various alpine 

 valleys we see them some hundred feet deep, while the opposite 

 rocks are as near each other at the top as at the bottom of the 

 gorge, sometimes nearly touching, and never many yards asun- 

 der. This, however, manifestly results from the grinding ac- 

 tion of the stones, which, as they occupy the bottom of the 

 stream, must always tend to deepen the channel, not to widen it. 



Here the question arises. If alluvial action produces only 

 deep and narrow gorges, is it not necessary to assign some other 

 cause for those more expanded valleys, at the bottom of which 

 they are situated f P 



Without presuming to deny the possibility of other modes of 

 action, it appears to me, that the formation of more expanded 

 valleys above the gorges (as represented in the section. Fig. 8.) 

 is a necessary consequence of the process which we are consider^ 

 ing. Notwithstanding the extraordinary form of these gorges, 

 when excavated in remarkably hard and solid rocks, the time 

 must come when their walls, not only intersected by seams and 

 cleavages, but subject to the action of numerous lateral torrents, 

 and of powerful atmospherical influences, will collapse by their 

 own weight ; and, as we see banks of sand, clay, or gravel, fall 

 in large flakes, as the stream undermines them, so the solid and 



" See also the paper of Mr Strangways above referred to, Geological 

 Transactions, vol. v. p. 341, and Plate xviii., fig. 1. and 2., F. Sir T. D. 

 Lauder, in his instructive and valuable " Account of the Great Floods of 

 August 1829," 2d edition, p. 3C5, mentions the appearance of these " circular 

 holes " in the mica-slate Oi" the Cuach, a tributary of the Dee in Aberdeen, 

 shire, " the shaking of the rocks " at une of the falls of this stream, and the 

 removal by it of" immense masses" from the walls. 



t De la Beche's Geological Notes. London, 1R30. 8vo. No. HL 



