166 PROF. J. PKESTWICIl ON A SOUTHERN DRIFT IN THE THAMES 



that at Bricket Wood and Aldenham on the north is on a level of 

 from 150 to 200 feet lower. 



The only outlier belonging to this gronp on the south of the Thames 

 is that on the snmmit of Shooter's Hill, 424 feet high. The shingle 

 is there composed essentially of flint-pebbles, with only a few sub- 

 angular flints and a very few rare quartz-pebbles. The flint-pebbles 

 closely resemble those of Warley Heath. The bed is only 2 or 3 

 feet thick, and is disturbed at the top like that on Brentwood 

 Heath Common and Stanmore. 



Fig. 4. — Section in the Brich-field to the West of Bentley Priory ^ 

 Stanmore^ Middlesex. 



^-'^ .'-f j.*^* ":; '.•:J^\J^ 



a. Unstratified beds of shingly gravel 6 to 7 ft. 



b. Upper loamy bed of the London Clay disturbed by the pressing down 



of bed a. 



It will be observed that on these hills there are either remnants 

 of the Bagshot Sands, or else the London Clay is of such a thickness 

 that little is wanted to bring them in ; or, as the London Clay 

 varies somewhat in thickness, it is quite possible that at Shooters 

 Hill, Havering Atte Bower, and Buckhurst, there were depressed 

 areas in which the Bagshot Sands and pebbles gained at the 

 expense of the London Clay, and that in these cases, as at Brentwood 

 and Billericay, the Pebble-beds are nearly in situ. 



That the Pebble-beds have been disturbed and partly reformed is, 

 however, evident from the introduction of materials foreign to the 

 Bagshots. This may have been effected at the time of the Westleton 

 or Southern Drifts, though further changes in their condition seem 

 to be due to ice-action in the Glacial period. 



4. Early PhysiograjyJiical Conditions of the Wealden Area. 



To understand the relation which the Southern Drift bears to the 

 other Pre-Glacial and Glacial Beds, it is necessary to go back to 

 earlier times to see what were the physiographical conditions pre- 

 viously prevailing in the south of England and the adjacent con- 

 tinental area. 



The denudation* of the great dome of the Wealden anticlinal 

 commenced in the early Tertiary period t ; but though the Chalk, 



* On the general subject of the Denudation of the Weald see chapter 16 in 

 Mr. Topley's ' Geology of the Weald,' pp. 270-301 (1875)^ 

 t The author in Quart. Journ. G-eol. Soc. vol. xiii. 



