ON TERMINAL CURVATURE IN THE SOUTH-WESTERN COUNTIES. 49 



4. On Terminal Curvature in the South-western Counties. 

 By W. A. E. TTssher, Esq., E.G.S. (Read June 20, 1877.) 



(By permission of the Director of H.M. G-eological Survey.) 



Introduction. 



More than two years ago my attention was drawn to this subject 

 by the perusal of a paper by Mr. Mackintosh in vol. xxiii. of the 

 4 Quarterly Journal.' Having in the interim neglected no oppor- 

 tunity of studying the phenomena in question over a large area, I 

 arrived at conclusions adverse to those advocated by Mr. Mackintosh ; 

 so that the object of this paper is twofold : — first, to controvert Mr. 

 Mackintosh's opinions ; secondly, to put forward more tenable ex- 

 planations. To carry out this object thoroughly is impossible 

 without considering the Pleistocene history of Devon, &c, in some 

 at least of its aspects ; but as the wider subject is reserved for a 

 future communication, I shall here only touch on it as briefly as 

 possible. 



Part I. 



Mr. Mackintosh suggests four explanations of the instances of 

 terminal curvature described in his paper*. They are as follows: — 



1st. " Land-ice filling up the basin of the Bristol Channel " and 

 travelling southward. 



2nd. Moating ice, in bergs or in " any other form." 



3rd. " A swift oceanic current exerting a pressure on the under- 

 lying slates by means of a forcible drifting forward of detritus." 



4th. " A still more violent rush of waters caused by a sudden 

 upheaval or depression of the land." 



The first of these alternative solutions finds most favour with 

 their author. I do not deny its applicability to some instances of 

 terminal curvature within three or four feet of the surface in 

 glaciated districts ; but in the South-western Counties, where evidence 

 of ice-action has not been hitherto detected, this phenomenon can 

 in no wise be cited as a proof of it, for the following reasons : — 



First, such an assumption ignores the great surface-waste and 

 contour-moulding which these counties underwent during Pleistocene 

 times, whilst it entails the preservation of numerous parts of an old 

 glaciated surface of which all accompanying signs have been swept 

 away. 



Secondly, beds exhibiting terminal curvature are frequently 

 visible in situations which, during the Glacial epoch, must have been 

 too far removed from the suppositional ice-bed to have sensibly felt 

 the pressure of an ice-mass. 



Thirdly, if the south-western counties really experienced the 

 rigours of the Glacial epoch, when we consider the nature of beds 



* Quart. Journ. Geol. Soc. vol. xxiii. pp. 326 et seq. 

 Q. J. G. S. No. 133. e 



