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Rh 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.

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 ; 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 Claysdistricts 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.

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.