GEOLOGY 



or pebble beds derived from the Reading Beds. Other greywethers 

 have been derived from the Bagshot Sands, which are locally solidified. 



In these accumulations we see the waste of an old land-surface in 

 the true Clay-with-flints, but it is so intermingled with the extraneous 

 loamy and gravelly deposits that we can only look upon these wide- 

 spread Drifts as in the main a wreck of Eocene deposits. 



GLACIAL DRIFT 



Extensive sheets of gravel occur on the lower dip slopes of the 

 Chalk tracts from Amersham and Cheneys, southwards over the uplands 

 east of Chalfont St. Peter. They occur also at Chalfont St. Giles, south 

 of Penn, at Flackwell Heath and west of Great Marlow, and they extend 

 over the Eocene tracts from Beaconsfield to Gerrard's Cross, and over 

 Burnham, Stoke and Fulmer Commons to near Iver. 



To what extent these Drifts were connected with certain stages 

 in the development of the Thames Basin is a question difficult to decide, 

 but the subject has been ably discussed by Mr. H. J. O. White 1 ; and 

 there is little doubt that the Thames belongs to a very early system of 

 drainage, modified from time to time by various physical changes. 



In the northern part of the county we find Drift gravels and sands 

 and Chalky Boulder Clay, all distinctly connected with the Glacial 

 period. The Boulder Clay contains many fragments of glaciated chalk, 

 much flint, large boulders of Oolitic rocks and fossils derived mainly 

 from the Oxford and Kimeridge Clays, such as Ammonites, Belemnites 

 and Gryphaea, It is spread over the Great Oolite Series and over 

 portions of the vale of Oxford and Kimeridge Clays districts formerly 

 more richly wooded than they now are, but of which traces remain in 

 Whittlewood Forest, Stowe Park and Salcey Forest, on the borders 

 of Northamptonshire and Buckinghamshire, and in Whaddon Chase. 

 Patches of Boulder Clay occur on the western end of the Woburn 

 Hills, and thence southwards from near Leighton Buzzard across the 

 Vale of Aylesbury to Long Crendon, and through the region of the 

 Claydons to the north of Bicester. 



Here and there beneath the main mass of Boulder Clay, as at 

 Shalstone, we find beds of sand and gravel of irregular thickness and 

 extent ; and both Boulder Clay and gravel contain much chalk and 

 fragments of limestone, which are dissolved away at the surface, giving 

 rise to irregular furrows or pipes like the Chalk itself. Fine sections 

 of Boulder Clay were exposed along the Great Central railway at 

 Chetwode, and at Rosehill farm, where it rests on buff sands. 2 



The ice sheet to which this Boulder Clay owes its origin may have 

 covered the northern portions of the county, but it did not overspread 

 the main escarpment of the Chiltern Hills, although it extended east- 

 wards into Hertfordshire and the northern parts of Middlesex. 8 



1 Proc. Geol. Assoc. xv. 157. 8 H. B. Woodward, Geol. Mag. p. 105 (1897). 



3 See S. V. Wood, jun. Quart. Journ. Geol. Sac. vol. xxxviii. map 4, pi. 26. 



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