Fisher — Glacial Origin of Denudation. 483 



II. — On the probable G-lacial Okigin of certain Phenomena of 



Denudation. 



By the Eev. 0. Fishek, M.A., F.G.S, 



THE pages of tlie Geological Magazine have lately contained 

 some interesting articles on denudation. I wish to add some 

 further remarks on the subject, which I think will, if duly con- 

 sidered, add important elements to the discussion of that problem. 



In my paper -'On the Denudation of Soft Strata," read before the 

 Geological Society in 1861, 1 combated the view then generally held, 

 that the present contour of the country was due to the ordinary action 

 of the sea, as the land was slowly elevated above the waters. At 

 that time I had perceived that the denuding agent must have flowed 

 from higher to lower levels, and I suggested the flowing off of 

 water owing to a sudden elevation of the land. I am not pre- 

 pared entirely to recant those views, for I believe we shall err 

 in this, as we are liable to do in all physical problems, by sum- 

 marily excluding any possible explanation of the phenomena. 



Messrs. Foster and Topley did me the honour to quote my 

 argument against the possibility of escarpments being Old Sea 

 Cliffs, in their paper "On the Denudation of the Weald ;"^ and 

 Mr. Topley has repeated the argument in the last number of this 

 Magazine.^ His explanation of the formation of escarpments in 

 that article is very good, and, in general, I believe the true one, in 

 as far as he has shown how a denuding agent has worked to cut out 

 the valleys and leave the escarpments. But the point I wish to 

 discuss at present is what that denuding agent has been. 



Rain and rivers are the agents now chiefly relied upon. They 

 have, no doubt, done much, and are still at work : so that we can 

 judge of their past labours by their present effects. But I believe I 

 have succeeded in proving in my paper " On the Warp," in the 

 forthcoming number of the Journal of the Geological Society, that 

 the present surface of this country has been eroded into channels 

 by a plastic mud or gravel derived from higher grounds, and 

 forcibly indented into the surface. The materials in these furrows 

 (and sometimes we have extensive coverings of it, not mere furrows^) 

 I have called " trail." The junction of the trail with the subjacent 

 bed, where that is clay, is constantly marked with "slickenside ;" and 

 my own impression is, that pebbles near the surface of junction are 

 somewhat polished. I do not believe it possible that any theory of 

 denudation can be of the least value which ignores the existence of 

 this almost universal coating of the surface. It is necessarily the 



\Quart. Joiir. Geol. Soc, Vol. xxi. p. 463. _ 2 Geol. Mag. Vol. III. p. 435. 



^rAn excellent exhibition of the trail, under this aspect, will he found in the brict- 

 pit at Uphall, near Ilford, the same pit from which came the head and tusks of 

 Elephas primigenius in the British Museum. Here I saw from the trail an angular 

 block of grey weather sandstone, weighing about twenty-five pounds. 



