8 LONDON DISTRICT . 



reaches 278 at Ottershaw. The figures at Southall and White 

 Heather appear anomalous, possibly owing to the difficulty of 

 fixing the level of the base of the Chalk. 



The following is a tabular statement of the deeper borings 

 in the district. Details of eight of these are given in Whitaker's 

 1 Geology of London/ Vol. II. ; the more recent may be found 

 in ' London Wells,' or in the Memoirs on the Water Supply of 

 Kent, Surrey and Essex. (See page 9.) 



CHAPTER III. 



CRETACEOUS ROCKS. 

 CHALK. 



This formation of soft white limestone with flints, in many 

 respects the most important in our district, attains a thickness 

 of about 700 ft., but largely on account of the erosion to which 

 it had been subjected prior to and during the deposition of the 

 Eocene strata, the thickness is subject to considerable variation. 

 The greatest development appears to be at Bushey, near Watford, 

 about 710 ft. ; the thickness at a number of points is given in 

 the table of borings (p. 9). At East Horsley, south-west of 

 Leatherhead and beyond the limits of our area, it reaches 817 ft. 



The area included in the London Maps comprises only a small 

 part of the broad outcrop of the Chalk which forms the rim of 

 the London Basin. The southern margin is to be found in the 

 North Downs, in the scarp which extends from Guildford to 

 Boxhill, Merstham and Otford ; while the northern margin occurs 

 in the Chiltern Hills, the Dunstable, Luton and Royston Downs 

 in both cases beyond the limits of our map. From these heights 

 (600 to 800 ft.) the Chalk dips in Surrey and Kent northward 

 and north-westward, and in Buckinghamshire, Hertfordshire 

 Bedfordshire and Essex southward and south-eastward towards 

 the metropolitan area. 



The nearest point in our district to the North Downs escarp- 

 ment is at Shoreham on the borders of the Darent Valley (the 

 home in his later years of Joseph Prestwich) in the vicinity of 

 which both Lower and Middle Chalk are exposed (see below). 

 Here and along the undulating uplands to Downe (where Darwin 

 long resided) and onwards to the Sanderstead, Purley and Ban- 

 stead Downs, we find the characteristic features of the Chalk, 

 smooth undulating downs, capped on some of the higher grounds 

 by a variable thickness of Clay -with -flints and loam (see 

 Chapter VI.). 



THE ZONES OF THE CHALK AND THEIR FOSSILS. 



Formerly the Chalk was divided, according to its prevalent 

 and conspicuous characters, into Chalk Marl, Chalk -without - 

 flints, and Chalk -with -flints. This grouping is useful from some 



