208 DE. E. L. SHEELOCK AXD ME. A. H. KOBLE OX THE [June I912, 



streams flowing over it regardless of the ^ solid' geology. Thus 

 the Misbourne below Chalfont St. Peter leaves the Chalk for the 

 Eeading Beds, and farther on returns to the Chalk. If the drainage 

 is post-Fluvioglacial, this is readily accounted for. 



Certain other peculiarities in the drainage receive an easy expla- 

 nation on our hypothesis. T^^e have already explained the sharp 

 bends of the Thames at Bourne End and the Colne at Eickmans- 

 worth. If we reconstruct the old channel of the Thames through 

 Beaconsfield and Chalfont to Kickmansworth, we find that north of 

 this line its tributaries, the AY ye and the Misbourne, entered the 

 river normally. After the old channel was filled by Drift and the 

 ice had retreated, the drainage of the Wye, seeking the nearest way 

 of escape across the gravel-flat, turned sharply westwards in the 

 remarkable curve at Wooburn. Similarly, the Misbourne turned 

 eastwards into the Colne. 



The great floods of water from the melting ice account for another 

 peculiarity of the Thames drainage — that is, the sudden expansion 

 of the valley below Taplow and Bray. The valley widens rapidly 

 from about a mile and a half at Maidenhead to about 4 miles at 

 AYindsor, and, after passing the Chalk ridge of Windsor, which 

 probably ofi'ered an obstacle to denudation, there is a further in- 

 crease to 8 miles at Iver. The position of this great flat is exactly 

 opposite to the termination of the glacier, where the floods from 

 the melting ice, aided by the Thames and the Colne, have worn 

 away the soft Tertiary clays. 



It is possible that the southern end of the Fluvioglacial Gravels 

 is buried under the later Thames gravels. J. Allen Brown ^ 

 noticed, at Durley and elsewhere, gravels with quartzite and 

 other far-travelled pebbles under gravels which but very rarely 

 contain them. 



In conclusion, we wish to express our indebtedness to Mr. 

 Clement Eeid, P.E.S., for help and encouragement in the pre- 

 paration of this paper. 



Y, SrMMAEY. 



(1) The Clay-with-Flints was formed by an ice-sheet from the 

 north or north-west. 



(2) The Glacial Gravels of the Old Series Geological Survey 

 Map have been separated into two divisions, Gravelly Drift and 

 Fluvioglacial Gravels. 



(3) The Thames in early Pleistocene times flowed from Bourne 

 End to Watford along the foot of the Eocene escarpment. It was 

 deflected into its present course by the ice-sheet which produced the 

 Clay-with-Flints. 



(4) The Colne, south of Eickmansworth, is a new river draining 



^ 'Ifotes on the Higb-Level Eiver Drift between Hanwell & Tver' Proc. 

 Geol. Assoc, vol. xiv (1895-96) p. 159. 



