Professor J. W. Gregory — TJie Chiltern Wind Gaps. 147 



The ice which (leposited the bouhler-clay extended over the lowland 

 on both sides of the north-eastern end of the Chiltern Hills, but did 

 not cover them or reach the main part of the ranpe. Mr. Harmer's 

 niaf) (1907, pi. xxxiv) shows clearly the relation of the Cliiltern Hills 

 to the boulder-clay, and the only evidence for the glaciation of the 

 Chilterns is that based upon the plateau gravels, which, as I maintained 

 in 1894, are non-glacial and pre-Glacial. 



The view that these high-level gravels are of glacial origin has, 

 however, been adopted by Mr. T. I. Pocock in the memoir on The 

 Geology of the Country around Oxford.^ He has clearly restated 

 the arguments for the glacial origin of the beds ; and he considers 

 that all the Oxford district was covered by ice, and that the country 

 has been lowered during Pleistocene times, mainly by glacial 

 denudation, to the extent in places of 450 feet. The evidence 

 advanced by Mr. Pocock seems inconclusive, and lie quotes the late 

 H. B. Woodward, who was superintending the survey of the 

 district, as " strongly of opinion that none of the plateau drifts can be 

 regarded as the immediate product of land ice, in the sense of being 

 boulder-clay ; but that there are remnants of ' washed drift ' of Glacial 

 age" (No. 6, p. 103, footnote). The distinction between glacial and 

 non-glacial gravels afforded by the presence of Jurassic material is, of 

 course, not available in the Upper Thames Valley ; but other evidence 

 of glacial action should be forthcoming. I go further than H. B. 

 Woodward, by doubting whether there is any reason to regard the 

 plateau drifts as even the wash of glacial beds. Mr. Osborne White 

 (No. 7) and Dr. Salter (No. 8, p. 286) agree as to the fluviatile origin 

 of these plateau drifts. Mr. White, for example, concludes 

 (No. 7, p. 173) that "it seems almost impossible to resist the con- 

 clusion that, despite the great elevation it attains above the beds 

 of the neighbouring streams, this gravel owes its existence to fluviatile 

 agency, operating along the same general lines of drainage as those in 

 existence at the present day". Mr. White adds (No. 7, p. 173), 

 "There is no reason to think that the part played by ice in the 

 formation of this gravel was ever more than a subordinate one." 



It may be said that possibly the Chiltern wind gaps were pre-Glacial 

 while the gorge at Goring was glacial. Mr. Harmer, however, lays 

 stress on the probability that the wind gaps and the Goring gorge 

 were due to the same cause, and therein I fully agree with him. 

 Hence the theory suggested by Sir Joseph Prestwich, that the Upper 

 Thames and Thame once discharged north-eastward into the Wash, 

 appears improbable, and the Chiltern wind gaps may be best 

 explained as the remains of ])re-Glacial valleys, which were excavated 

 by rivers now beheaded bj' the Thame. 



^ F. W. Harmer, "The Origin of certain Canon-like Valleys": Quart. 

 .Journ. Geol. Soc, vol. Ixiii, pp. 470-513, pis. xxxi~v, 1907. 



- P. F. Kendall, " Glacier-Lakes in the Cleveland Hills " : Quart. Journ. 

 Geol. Soc, vol. Iviii, pp. 471-571, pis. xx-viii, 1902. 



^ J. W. Gregory, "The Evolution of the Thames": Nat. Sci., vol. v, 

 pp. 97-108, 1894. 



* W. M. Davis, "The Development of certain English Rivers": Gcog. 

 .Journ., vol. v, pp. 127-46, 1895. 



