202 DE. E. L. SHEELOCK AND ME. A. H. NOBLE ON THE [JuDC I912, 



as evidence of this. In passing, we may note with Mr. Hemes ^ 

 the improbabilitj'' of a convenient swallow-hole being always ready 

 to receive a sarsen. 



If the Tteading Beds originally extended over the country, and 

 their waste added the necessary clay, sand, and stones to the 

 Chalk residue, there are still difficulties, for the unworn flints could 

 onl}' come from the Chalk, and we have seen that they are far too 

 numerous to be accounted for by the solution of the small amount 

 of Chalk-with-Flints which has been dissolved. The swallow-hole 

 theory also does not explain the masses of laminated brick-earth 

 (see PI. XII) which occur in the midst of the deposit. 



If, however, we regard the deposit as of Glacial origin, these 

 difficulties disappear. In the first place, the deposits as seen at 

 Walter's Ash have the appearance of Glacial Drift (see PL XII). At 

 the beginning of Glacial times the Chalk outcrop would be covered 

 with detritus derived from it and Eocene outliers by weathering, 

 just as an ungiaciated area such as Devon is covered by ' head " 

 representing the waste of ages. An ice-sheet coming from the 

 north or north-west would sweep up these materials, and produce 

 the confused mass knoAvn as Clay-with-Elints. The absence of 

 Midland rocks is accounted for by the fact that the ice would have 

 to override the Chiltern Hills and the dirty bottom -ice would be 

 stopped by the escarpment, only clean top-ice surmounting the 

 barrier. It is noteworthy that in some cases, as at Aston Clinton, 

 the Clay-with-Elints does not extend to the verge of the escarpment, 

 although away from the escarpment it always caps the high 

 ground. 



We have spoken of the Chiltern Hills as existing ijj Glacial 

 times, as there are good reasons for supposing that the Chalk 

 escarpment and dip-slope had much the same development then as 

 now. The Dry Valley Gravel is generally accepted as having 

 been formed at a time when the soil- was frozen, that is, during 

 some part of the Pleistocene Epoch. ^ 



The gravels widen out as they approach the Chilterns, as if they 

 had never extended much farther than' they do at present. The 

 Wendover Gap is a good illustration. Although there is now a 

 gap through the escarpment, the Valley Gravel ends a mile south 

 of Wendover in a wide spread, and there is little doubt that it 

 never extended much farther than it does at present. Moreover, 

 the Clay-with-Elints forms a dissected plateau, the surface of which 

 slopes parallel to the dip-slope of the Chalk ; and the Chalk Eock 

 crops out along each valley for miles, parallel at once to the bottom 

 of the valley and to the base of the Clay-with-Elints above. This 

 indicates the existence of the dip-slope before the Clay-with-Elints 

 Avas formed, and the dip-slope implies the existence of an 

 escarpment. But, on any theory, the Clay-with-Elints dates back 



^ E, S. Herries, in ' Excursions to the Cliilterns ' Proc. Geol. Assoc, vol. xix 

 (1905-1906) p. 148. 



■^ 0. Eeid, ' On the Origin of Dry Chalk Valleys & of Ooombe Eock ' Q. J. G. S. 

 vol. xliii (1887) pp. 364-71. 



