of the Chalk in the London Basin. 299 



the Chalk are points on the 600 contour of the corrected Chalk 

 surface. Similarly, points at which the — 200 contour of the present 

 Chalk surface intersects the — 700 contour of the hase of the Chalk 

 are points on the 500 contour of the corrected surface, and so on. Ey 

 joining up the similarly numbered points so obtained, the contour 

 system of tlie corrected pre-Tertiary Chalk surface is arrived at 

 (see Map 1). The numbers appended to the contours possess only 

 a relative significance. Tlie base of the Chalk might liave been 

 corrected so as to cause it to occupy any horizontal plane, but by 

 selecting present sea-level as the datum-plane we have the additional 

 advantage that the lines serve equally well as the isopachytes of the 

 Chalk formation. 



A glance at the map reveals at once that the information afforded 

 by it well repays the labour- involved in its preparation. It appears 

 that the age of the Streatham-Beckton fault is pre-Tertiary, and 

 from the form of the lines in the neighbourhood of Loughton we 

 may suspect faulting here also. 



The first significant fact which the map brings out with great 

 clearness is that in pre-Tertiary times the Chalk of this area was 

 denuded in sucb a way as to produce escarpments facing inwards 

 towards the centre of the present London Basin, thus indicating 

 plainly that the central area experienced a somewhat greater uplift 

 and suffered greater denudation than the district farther west. 

 "With the exception of a small area between Kentish Town and Mile 

 End, where the Chalk has probably been protected by faulting, it 

 is a noteworthy fact that the Chalk is thinnest over the area where 

 the Gault beneath it rests directly upon the Palaeozoic floor, and the 

 general character of the pre-Tertiary denudation of the Chalk surface 

 is in very close sympathy with the contours of the Palaeozoic floor 

 when the latter is corrected for post-Cretaceous movements. 



The writer has elsewhere put forward the view that the area under 

 present consideration lies on the south-easterly prolongation of the 

 well-known Charnian Axis of Professor P. P. Kendall, and he 

 inclines to the belief that the character of the pre-Tertiary 

 denudation of the Chalk here points to the operation of Charnian 

 posthumous movement in pre-Tertiary times. When we proceed to 

 investigate the question of the zonal composition of the denuded 

 Upper Chalk surface further suggestive evidence is forthcoming. 

 At any rate, additional support is given to the statement made above 

 that the general character of the pre-Tertiary denudation of the 

 Chalk surface is closely associated with the pre-Cretaceous form of 

 the Palaeozoic floor. 



In endeavouring to insert upon the map the positions of the 

 boundaries of the zonal outcrops beneath the Eocene cover we are at 

 once confronted with the difficulty that we have as yet little 

 knowledge of the thicknesses of the various zones of the Upper 

 Chalk in and near this area. It has been possible, in the case of 

 some of the borings, from the details given in the journal, to effect 

 the division between Lower, Middle, and Upper Chalk, and some- 

 times horizons easily recognizable litliologically, such as the Melbourn 

 Eock and the Chalk Rock, can be identified. Apart from this we 



