50 W. A. E. USSHER ON TERMINAL CtTRVATTJRE 



capable of mechanical flexure near the surface, and couple with it 

 the extensive waste of Pleistocene times, the survival of glaciated 

 surfaces composed of shales would he little short of miraculous, 

 considering that hard grit and igneous rocks nowhere in these 

 counties present moutonnks or striated surfaces. 



In following the course of the Exe above Dulverton, not far from 

 Barlynch Abbey, I noticed a face of hard grit crossed by parallel 

 grooved lines. This, I believe, is the spot visited by the late Prof. 

 Jukes, and rightly, I think, described by Mr. Whitley as a slicken- 

 side *. It does not resemble a moutonnee surface. 



Fourthly, the intrusion of extraneous substances, as well as the 

 formation of extensive moraines of local materials, would accompany 

 the nascent flow and dwindling away of so powerful an agent as 

 land-ice in the form advocated by Mr. Mackintosh. Yet we have 

 no clear indications of moraine-matter in Devon. 



The Boulder-gravels of the Dartf and Teign % valleys might be 

 the relics of old moraines carried down to lower levels in the further 

 excavation of the valleys, having been re-sorted and well water- 

 worn in the process ; but the proximity of granitic or slaty rocks, 

 of which they consist, and the concession of greater force and volume 

 to the former representatives of the present streams would equally 

 entitle one to refer them to fluviatile deposition alone. 



Notwithstanding the paucity of possible signs of Glacial action 

 in Devon, I am inclined to think that the absence of deposits com- 

 mensurate with the great Pleistocene denudation experienced by 

 the South-western Counties § may be due, in the first place, to 

 some powerful denuding agent in the form of a local ice sheet or 

 glacier system, and, in the second, to the great force and volume of 

 surface-water likely to be liberated at the close of a period of 

 Arctic severity. 



But the instances of terminal curvature given by Mr. Mackintosh 

 do not seem to be ascribable to land-ice in any form. The absence 

 of terminal curvature on the northern slopes of the Brendon Hills 

 is against the idea that the phenomena were produced by the sur- 

 face-ruffling action of a hill-ignoring ice sheet or the pressure of 

 ground-moraine between slaty laminse ; for the greatest force would 

 be expended on the slopes affording most resistance to the southerly 

 movement of the mass, whilst on the coincident declivities its effect 

 would be proportionately less. To regard the northern slopes as 

 unmoulded whilst the hilltops and southern declivities present their 

 present forms is a way out of the difficulty that cannot well be en- 

 tertained. 



Mr. Mackintosh's remaining alternatives postulate great changes 

 in the physical geography of the country. To ascribe the produc- 

 tion of terminal curvature on the Brendon Hills to the action of 



* Geol. Mag. vol. v. p. 31. 

 t Trans. Dev. Assoc, for 1876, p. 427. 



\ Quart. Joum. Geol. Soc. vol. xxiii. p. 418, and Geol. Mag. vol. vi. p. 40. 

 § Comp.Buckland and DelaBeche "On the Geology of Weymouth," Trans. 

 Geol, Soc. 2nd ser. vol. ir., and Pcngelly, Geol. Mag. vol. iii. p. 573. 



