200 P. G. H. Bosivell — Differential Movement 



the Gault. Upon a further consideration of the contours of the base 

 of the Gault in this area, in the light of fresh records, Mr. Baker, in 

 the MS. cited, concluded that a strong fault probably existed in the 

 Cliffe area, the effect of which was to give the base of the Gault 

 a downthrow of about 200 feet to the east. From evidence yielded by 

 Tertiarv beds he concluded there had been later movement along this- 

 fault, the prolongation of the line of which met the Medway Gap 

 on the south and the south-north bend of the Thames on the north. 

 As a result of plotting the Chalk surface-contours of the London 

 Basin, the writer independently obtained evidence of the same fault- 

 line, this time traced southwards from Billericay in Essex, 

 approximately along the sudden bend of the Thames, by Cliffe to the 

 Medway Gap. The sub-Eocene Chalk surface has received a down- 

 throw of about 100 to 150 feet to the east. 



This convergence of opinion from many workers engaged upon 

 different problems appears to indicate that with the gradual 

 accumulation of data the time is ripe for preliminary generalizations. 

 The latter will serve to show the directions in which further work 

 and information are required. 



Eor the purpose of discussing any possible Tertiary movements 

 about an unstable axis passing under South-East Suffolk, we must 

 suppose the thick covering of Glacial Drift to be stripped off. The 

 Crag must next be imagined as removed, and the form of the Eocene 

 surface revealed. Finally, the Eocene beds must be stripped away, 

 and the form of the Chalk surface and distribution of the zones 

 realized. Incidentally, it may be pointed out that the change of 

 strike of the Chalk at the Little Ouse-Waveney through-valley, from 

 northerly in Suffolk to north-north-westerly in Norfolk, is hardly 

 indicated by the ' solid ' geology as shown upon the Geological 

 Survey maps, although it is at once apparent by the behaviour of the 

 Chalk zonal outcrops. 



The Chalk zones change strike in South-East Suffolk also. The 

 dip-slope of the Chilterns, south-west of the area, where the Chalk is 

 covered by Eocene beds, consists of the cor-anguinum zone, the strike 

 being generally north-eastwards. At Bishop's Stortford the Beading 

 Beds rest upon Chalk of this zone, but in North-West Essex and South 

 Suffolk, while the Chalk zonal outcrops swing round northwards, the 

 Eocene beds maintain their strike, overstepping the various zones 

 until they rest upon mucronata Chalk in East Suffolk. Dr. A. W. 

 Bowe and Mr. G. E. Dibley tell me that the Chalk of Grays in South 

 Essex is a high horizon in the cor-anguinum zone, and the presence of 

 the Marsupites zone in North -East Kent is significant. The higher 

 Chalk zones (above cor-anguinum) must therefore V up the London 

 Basin under the Eocene beds, but their change of strike occurs 

 south-west of that of the latter in East Anglia. 1 



The Chalk surface-contours change direction also in South-East 

 Suffolk, but cut across the zones as indicated by the map, Fig. 1 ; their 



1 Messrs. H. J. Osborne White and L. Treacher have shown that near 

 Newbury, i.e. on the opposite side of Professor Kendall's Charnian axis, 

 Marsupites Chalk again occurs under Eocene beds, and farther westwards the 

 A. quadratics zone just appears. 



