202 P. G. H. Boswell — Differential Movement 



to Chelmsford, 27 ft. ; near Bishop's Stortford to Chipping On gar,- 

 36 to 44 ft. (On account of the small scale of the map a few 

 local irregularities in the surface-contours have been smoothed out. 

 The results are still reasonably accurate.) See Fig. 1, p. 201. 



The north-westerly limits of the Lower London Tertiaries (the 

 Thanet Beds and Reading Beds are not differentiated) and London 

 Clay are also shown upon the map. Considering the former beds 

 first, it will be seen that they overstep the Chalk zones and surface 

 contours, their point of deflexion occurring near the Bivers Deben and 

 Aide, again north-east of the change of strike last considered, namely, 

 that of the Chalk surface-contours. Their thickness, as given by 

 field evidence and well-borings, etc., has been plotted on the map 

 and approximate isopachytes (lines joining points at which the 

 thickness of the beds is the same) drawn. An interesting result 

 follows in consequence of the thickening of these beds in both 

 directions from the critical area. Owing to the thinning off of the 

 beds by erosion to a feather-edge on the north-west, 1 these isopachytes 

 are not normal to the strike, but take the form of curves thrown back 

 along the feather-edge, but always convex to the critical area (except 

 in this area itself, where, like the 50-feet isopachyte, they are 

 convex to the south-east). Although the Thanet Beds are thinner in 

 the neighbourhood of the Gipping Valley than elsewhere in the whole 

 district, they show no evidence of shallow-water or shore-line 

 conditions, as do the similar beds in the Sudbury area. Near their 

 surface, however, they appear to yield evidence of planation, and 

 contain lenticles of large wind-polished grains and small pebbles of 

 flint and quartz. It would thus appear that the movement of uplift 

 which spaced out the Chalk surface-contours was in abeyance in 

 Thanetian times, but reasserted itself at their close. The thickening 

 of the Thanet Beds north-eastwards and south-westwards would be 

 accounted for by greater denudation over the central area. 



The Beading Beds, which also thicken on each side of the point of 

 deflexion, consist in the central area of sandy beds with subordinate 

 lenticles of plastic clay. Both on the north-east and south-west, as 

 far as we can rely upon records of borings, etc., they appear to 

 consist of greater thicknesses of mottled clays with subordinate beds 

 of sand. In view of the widespread shallowing in Beading Bed 

 times, a movement which culminated all over the district in the 

 formation of the bed of rounded black flint pebbles (variously referred 

 to the Oldhaven Beds and to the basement-beds of the London Clay), 

 care has to be exercised not to force the evidence. 



Widespread and steady submergence took place during the 

 deposition of the London Clay over the area. The great thicknesses 

 of the deposit recorded in well-borings comparatively near to the 

 feather-edge point to sedimentation under isostatic conditions. The 

 northerly and westerly limit of the London Clay is indicated upon the 

 map, and it is to be observed that the change of strike occurs north- 

 eastwards of that of the Lower London Tertiaries. The boundary of 

 the former deposit retreats from that of the latter in the neighbourhood 



1 It should be noted that this feather-edge constitutes the zero isopachyte. 



