80 LONDON DISTRICT. 



CHAPTER X. 



ECONOMIC GEOLOGY. 

 WATER SUPPLY. 



In the economic geology of the district, Water Supply holds 

 by far the most important position. The subject is dealt with 

 fully in the special memoirs of the Geological Survey, which 

 cover practically the whole area here dealt with. Those 

 interested in the subject from the practical point of view should 

 not fail to consult these volumes, while the student of geology 

 will find the thousands of well-sections recorded invaluable, more 

 particularly as aids to deciphering the detailed structure of the 

 area. A special atlas of the 6-in. maps of the County of London, 

 on which the exact sites of many wells are marked, is available 

 for reference in the library of the Geological Survey. 



In these circumstances a brief outline of the subject will 

 suffice for our present purpose. The chief source of supply, apart 

 from the Thames and other rivers, is the Chalk. The yield, 

 however, varies considerably in quantity, in the depth at which 

 it is obtained, and in quality. Chalk when saturated is capable 

 of holding two gallons of water in a cubic foot, but even when 

 the plane of saturation is reached, it is only along joints, fissures 

 and cracks that it parts readily with its water. 1 Such openings 

 are naturally more abundant where the Chalk is near the surface 

 than where it is covered by a great thickness of Tertiary Beds. 

 Consequently, heavily loaded areas like Richmond and Wimbledon, 

 Hampstead and Highgate, the Norwood Hills and much of Essex 

 yield little water, either from the Chalk or from the Thanet 

 Sand, which also tends to be compacted. The minor structural 

 folds appear to have little direct influence on the yield of water, 

 but the effect of the fault running from Deptford to Raynes 

 Park (see p. 37 and the New Series Map Sheet 270, South 

 London) is considerable. Water coming from the surface- 

 outcrops in the south is impeded by this fault, which brings the 

 impervious London Clay down against the Chalk. Wells sunk 

 on the north-west, or down-throw side have a much smaller 

 yield than those to the south-east; in this part of the Wandle 

 Valley many wells used to overflow at the surface ; now the 

 water-level is a few feet down. 



The level of the water in Chalk wells is also largely affected 

 by pumping. If the Chalk under London be regarded as a 

 reservoir we may say that the contents are being used at a rate 

 in excess of the supply by percolation from the outcrops. This 

 subject receives special attention in ' Records of London Wells,' 

 where three small-scale maps show the contours of the water- 

 level in 1878, 1890 to 1900, and 1911 respectively. We may 



1 Prestwich, Sir J., Quart. Journ. Qeol. Soc. t vol. xxviii, 1872, p. Iviii. 



