of the Chalk in the London Basin. 297 



so covered, tlie unconformity must liave been accentuated to some 

 extent in consequence of the solvent action of percolating carbonated 

 waters, the evidence of which is familiar to us as "pipes" in the 

 Chalk filled with Eocene strata, and the persistent bed of unworn, 

 green-coated flints (" Eull-head " Eed) which everywhere occurs 

 between our lowest Eocenes and the Chalk; but such secondary 

 action is likely to have been more or less uniform throughout the 

 Avhole area beneath the Tertiarj^ cover. 



Thanks to the steady accumulation of data concerning well-borings 

 and tlie energies of various workers who have given cartographical 

 expression to the information thus made available, Ave have now 

 a good working knowledge of the present contours of the Chalk 

 surface within the London Basin beneath the Tertiary cover. The 

 present configuration of this Chalk surface is, of course, widely 

 different from that which it presented at the tinie of the deposition 

 of our oldest Eocenes, in consequence of the tilting and warping 

 effects of later movements. Nevertheless, in regard to the area 

 Avithin the immediate vicinity of London, we have some knoAvledge 

 of the effects of these post-Cretaceous movements upon the Chalk 

 formation, since here a number of deep borings have completely 

 pierced the Chalk, thus affording us information as to the present 

 level of its base. We are therefore in a position to apply a 

 "correction", so as to reduce the base of the Chalk to a horizontal 

 plane, and, having done this, to observe the form which the contours 

 of the Chalk surface then take. If the correction be applied so as to 

 cause the base of the Chalk to occupy a horizontal plane at present sea- 

 level, the resulting surface-contours are then also the " isopachytes " 

 or lines of equal thickness of the formation. 



The movement of elevation which set in towards the end of 

 Cretaceous times and ushered in the Tertiary era appears to have 

 been a Avidespread and regional one, and it is reasonable to assume 

 that in the area under present consideration the base of the Chalk, 

 at tlie beginning of Tertiary times, did not deviate widely from 

 horizontality. Consequently the contours of the Chalk surface 

 beneath the Eocene cover, when the base of the formation has been 

 corrected to horizontality, may fairly be regarded as giving an 

 approximately true representation of the configuration of the surface 

 upon which our oldest Eocenes were deposited. 



In order to obtain these pre-Thanetian contours of the Chalk 

 surface we proceed as follows : We first plot upon the map the sites 

 of the deep borings (seventeen in number) which have completely 

 pierced the Chalk formation in the area under consideration, inserting 

 in each case the depth below Ordnance Datum at which the base of 

 the Chalk occui-s. We then draw in the present contours in the 

 base of the Chalk at intervals of 100 feet, working upon the available 

 data, and assuming an even gradient between any boring and its 

 neighbours. The total number of borings not being large, the trend 

 of the lines is doubtless influenced somewhat unduly by the accidents 

 of boring sites, but fortunately the sites are fairly evenly distributed 

 throughout the area, and the error involved is not serious. Having 

 prepared this map, we superimpose upon it the map of the same area 



