40 LONDON DISTRICT. 



Somewhat similar sands occur at Headley Heath, about 

 6 miles south of Ewell, and 5 miles beyond the limits of our 

 district ; but there no fossils have as yet been found. 



The fact, however, of the occurrence of these shelly beds at 

 Netley Heath is most important as indicating a submergence 

 probably of parts of our district, certainly of adjacent areas, 

 during Pliocene times; and the depression may have amounted 

 to 840 ft. or more, according to Clement Reid's estimate that 

 the depth of water for the Lenham fossils was not less than 

 40 fathoms (op. cit., p. 52). 



We have no certain indication of the extension of the Lenham 

 Beds, and it is possible that any further deposits that may 

 originally have covered parts of our present district have been 

 either destroyed or reconstructed and decalcified. 



Before the identification of such relics can be discussed we 

 must indicate briefly the physical changes to which the district 

 has since been subjected. These changes have not only recon- 

 structed the remains of Tertiary beds, but have introduced newer 

 deposits which are frequently difficult to distinguish from each 

 other and from the pre-existing materials. 



That elevation of the area took place in Newer Pliocene 

 times is generally admitted, and that this may have been accom- 

 panied by some relative depression along the line of the Thames 

 Valley, giving direction to a late Pliocene river, the precursor 

 of the Thames, appears probable. 



With this upheaval of the land, the district was again 

 subjected to the erosive action of rain and rivers ; a process 

 which has practically continued, with sundry changes of level 

 and climate, to the present time. 



The result has been a complex series of gravels and other 

 accumulations, some directly connected with the Thames and 

 its tributaries, others having no immediate relation with the 

 present valley system. Mr. Whitaker remarked in 1890 that 

 1 there was no problem more difficult in the geology of the south 

 of England than the classification of various deposits. of gravel,' 1 

 some of which may be of any age between Bagshot Beds and 

 Boulder Clay. Since that date much attention has been paid 

 to this complex subject, but few undisputed conclusions have 

 been reached. 



On the maps the superficial deposits are shown by seven 

 distinct colours ; the newest three divisions of Alluvium, Valley 

 Brickearth and Valley Gravel form a fairly distinct series asso- 

 ciated with the past or present river systems ; they are described 

 in Chapters VIII and IX. The Boulder Clay is a definite deposit 

 with which are associated certain gravels, described here as 

 Glacial, either underlying or apparently washed out of it. These 

 gravels are characterised by the presence of certain constituents 

 foreign to the Thames Basin, and brought in by glacial action ; 

 they are coloured pink on the maps and described in the indexes 



1 Quart. Journ. Geol. Soc., vol. xlvi, 1890, p. 180. 



