2 LONDON DISTRICT. 



and Croydon on the south-east to Watford and Rickmansworth 

 on the north-west. 



At one time Eocene strata extended over the entire area, 

 and their present limits are due to the effects of erosion. Within 

 the basin, however, there is evidence of the impersistence of two 

 of the divisions, the Thanet Sand and Blackheath Pebble Beds, 

 which do not extend underground through the north-western 

 portions of the Basin, being overlapped or overstepped respec- 

 tively by the Woolwich and Reading Beds and by the London 

 Clay. Otherwise the sequence is maintained, and the absence 

 of any member is due to denudation. Thus the Bagshot Beds, 

 which at one time covered the whole of the area, excepting 

 possibly the south-eastern parts, have been so much eroded 

 that they now occur only in outlying patches. 



All these formations of Chalk and Eocene, like the many 

 other series which enter into the main structure of our country, 

 constitute what is sometimes termed ' bed-rock.' They have 

 otherwise been designated the ( Solid ' formations, in distinc- 

 tion from the ' Drift ' or superficial accumulations, such as the 

 Boulder Clay and Valley Gravels, which belong to the later 

 periods of Geology, and rest irregularly and indifferently on any 

 of the so-called Solid strata. 



In ordinary geological maps of England and Wales, on a 

 small scale, the Drift deposits are omitted, owing to the 

 difficulty of representing them and at the same time of showing 

 the main structure of the country. 



In the New Series Geological Survey map on the scale of an 

 inch to four miles, both Drift and Solid editions of the eastern 

 and south-eastern portions of England have been published, and 

 a comparison of the two maps of the country around London is 

 instructive. The more striking differences are in parts included 

 in our present Sheets 2 and 3, where, in place of the large areas 

 of Valley gravel and brickearth, the map of the Solid geology 

 shows for the most part only London Clay. 



For both practical and scientific purposes the Drift edition 

 is the appropriate geological map of the London District. 



The nature of the soil is of consequence only to the farmer 

 and market gardener, and to them the subsoil is of equal 

 importance. It is with the subsoil, not the soil, except in the 

 case of artificial accumulations of made ground, that the 

 architect, the physician, and the house-hunter are concerned. 



Apart from the economic products of the area, the chapters 

 in geological history that can be deciphered from the strata in 

 the London District are of much greater interest than at first 

 might be supposed. The ordinary citizen who has delved in his 

 back-garden finds as a rule either interminable stones or a stiff 

 unproductive brown clay. Of these two subsoils the brown 

 clay is known as the London Clay; the stones that occur in 

 such abundance belong for the most part to the Thames Valley 

 Gravel. 



