of the Chalk in the London Basin. 303 



tlie precise manner in which this pre-Thanetian denudation of the 

 Chalk was effected, but it appears likely that land existed here in 

 Montian times and that strike streams ran at the foot of the escarp- 

 ment, their direction being influenced by the presence of the ridge 

 which proceeded from west to east by way of Southall and Chiswick. 

 It is perhaps more than coincidence that the course of the present 

 Tliames shows a tendency to adhere to the same direction. 



Interesting light is thrown on the question of the source of the 

 material composing the Lower Eocenes of the London area. It 

 seems clear that the material must have come from the east; it can 

 hardly have been carried from the west over the escarpment. We 

 liave good reason to believe, too, that at this time, to the south of 

 the present area, the Wealden uplift had already been initiated, and 

 the interesting question is raised as to whetlier the denudation 

 of the Chalk had been carried, by Thanetian times, to an extent 

 sufiicient to expose to eroding influences an adequately large area of 

 the Lower Greensand to furnish the arenaceous material of the 

 Lower London Tertiaries. 



A simple movement of submergence appears all that is necessary 

 to explain the overlapping of the Thanet Sand by the Reading Beds 

 in our area, and with regard to the development of the Lower 

 London Tertiary Pebble-beds (Blackheath Pebble-beds) in the 

 Woolwich, Blackheath, and Bromley districts, it appears feasible to 

 suppose that these accumulated as banks against the easterly 

 prolongation of the Southall-Chiswick ridge. 



In view of the notable scarcity of borings which have completely 

 pierced the Chalk in Essex, we are very ignorant concerning the 

 present level of its base in tliis county and are, in consequence, 

 unable to adopt the present method of investigation here. In 

 Suffolk we have more data to work upon, but the area of Chalk yet 

 remaining beneath the Eocene cover is very limited for our present 

 purpose. Nevertheless, when the base of the Chalk is corrected to 

 horizontality, several interesting points are brought out. The Chalk 

 surface in this county is then seen to slope away east, west, and 

 south from an ai'ea of maximum elevation (over 1,200 feet) situated 

 between Beccles and Bungay. This area of maximum elevation 

 extends north-westward to a spot a little east of Norwich, and is 

 evidently the sole relic of an elevated Chalk area which once 

 extended much further to the west. In fact, the downward westerly 

 slope from this district, beneath the Crag deposits, is clearly the 

 result of post-Eocene and pre-Crag denudation of the Chalk. The 

 southerly and easterly slopes are, however, facts of deeper significance, 

 since these are found beneath the area where the Eocene cover yet 

 remains. There is a drop of 200 feet between Beccles and Aldeburgh 

 (wliich is only another way of saying that the Chalk is 200 feet 

 thicker in the former place than in the latter), and hence we have in 

 this area the last remaining evidence of the escarpment (now become 

 very gently sloping although more elevated) which faced south- 

 eastward and stretched away many luiles to the west and south. 

 Tlie easterly slope of the corrected Chalk surface from Beccles to 

 Lowestoft (or thinning of the Chalk formation in that direction) i& 



