THE GEOLOGY OF THE 



LONDON DISTRICT. 



CHAPTER I. 



INTRODUCTION. 



The four maps which it is our purpose to explain in this 

 Memoir represent the surface geology of the London District. 

 In this respect, however, they are not intended to depict the 

 soils which are often but a few inches thick, and for the most 

 part of too variable a nature to be denned on any plan. The 

 maps show the extent of the strata or geological formations 

 which occur immediately beneath the soil. They represent in 

 fact the subsoils, a term commonly applied to the more or less 

 weathered portions of the strata near the surface. The subject 

 of soils and subsoils in this area is fully considered in a separate 

 Memoir. 1 



In the present area the strata consist of gravel, sand, loam 

 or brickearth, clay, marl, limestone, and peat; and they are 

 grouped according to age, position and mode of origin, under 

 names that denote the different geological formations. Examples 

 are found in the Thames Valley Gravels, the Bagshot Sand, the 

 Thames Valley Brickearth, the London Clay, the Boulder Clay 

 (a chalky clay or marl), the Chalk (limestone), and the Peat 

 which occurs in the Alluvium of the marshlands. These terms 

 indicate for the most part the leading lithological characters, 

 but, as will be learnt from the detailed descriptions, all the 

 formations are subject more or less to changes in composition. 



The formations represented in the area are tabulated in 

 sequence on Sheet 4; and their order of succession has been 

 proved by observation in pits and quarries, and by the records 

 of wells and borings. 



The thick mass of the Chalk forms the foundation of the 

 entire area. It is in places overlain by the Eocene series, which 

 is represented in and around London by the Thanet Sand and 

 higher divisions up to the Bracklesham and Barton Beds. 2 This 

 series occupies a shallow trough formed by the uplift and bend- 

 ing of the Chalk so as to constitute what is known as the 

 London Basin; thus it covers that formation from Dartford 



1 * Soils and Subsoils from a Sanitary Point of View, with especial reference 

 to London and its Neighbourhood ' (Mem. Geol. Surv.), by H. B. Woodward. 

 2nd Ed., 1906. 3rd Ed. in preparation. 



3 These last occur at St. George's Hill, south of Weybridge, but they are 

 grouped with the Bracklesham Beds on the map. 



A 4 



