484 Fishei — Glacial Origin of Denudation. 



key to the last denuding agent. The unstratified character of this 

 trail, the large blocks of stone which it sometimes contains, while 

 the subjacent bed perhaps contains none, the unscattered condition of 

 lumps of clay, crag, and sand, which sometimes occur in it, the force 

 which evidently acted to push it forward, all point to land-ice as the 

 moving agent. 



We need not be surprised at the existence of land-ice at compara- 

 tively recent geological periods, even in the south of England, for 

 Mr. Jukes has described Glacial Striae in Devonshire,^ and in the 

 south-west of Ireland ; "- and the Hon. W. 0. Stanley informs me 

 that Anglesea, which is not high ground, is generally ice-marked. 

 The reason why such markings are rare in the south of England is 

 probably that the surfaces are so often of materials liable to dis- 

 integration and solution. This is exceedingly well illustrated by 

 Mr. Green's remarks, " On the Bloody Stone in Derbyshire,"^ where 

 scratches are preserved on chert, but usually effaced on the con- 

 taining limestone. 



Thus far, then, we see that the general covering of the surface 

 indicates ice-action, and that it is probable, from other independent 

 indications, that land-ice has lately covered the existing surface. 

 The next point is to balance the probable efficacy of ice, as compared 

 with water, to mould the surface to its existing form. 



Conceive the actually existing surface of any district, which con- 

 sists of soft strata, to be intersected by a nearly horizontal plane. 

 The curves of intersection will be full of cusps or sharp angles, run- 

 ning up into the secondary (or tributary) valleys. It seems self- 

 evident that a river gradually deepening its bed (which in the case 

 supposed would be represented by the gradual lowering of the 

 horizontal plane) could not excavate a level area of such a con- 

 figuration. 



Frequently one meets with evident instances of the erosion which 

 a stream has produced, and I never saw a case in which it was not 

 possible to distinguish readily the work of the stream upon the 

 previous configuration of the surface which had been left by the 

 more general ancient denuding agency. Messrs. Foster and Topley 

 rely much, in their paper " On the Denudation of the Weald," upon 

 a river gravel at an elevation of 300 feet. I do not know the 

 locality ; but does it follow that because the river has once flowed at 

 a higher level, therefore the subsequent degradation of the surface 

 is the work of the river ? 



Mr. Maw, at p. 445 of the last number of the Magazine, very 

 justly distinguishes between the excavation by a river at the bottom 

 of a valley, and the denudation of the valley itself. But upon com- 

 paring the absolute quantity of material abraded by the river, with 

 that removed from the valley sides by some other cause, we see 

 how much larger the latter is ; and on the supposition that the 



^ Geological Magazine, Vol. II., p. 473. 



2 Juke's School Manual (1863), p. 326. 



3 Geological Magazine, Vol. II., p. 440. 



