Professor J. W. Gregory — The Danbury Gravels. 533 



have been introduced into Essex after the Weald en anticline had been 

 sufficiently raised and denuded for the chert beds to have been 

 exposed on the surface. The Wealden anticline has been often 

 referred to as if due to a single uplift in the Oligocene, Miocene, or 

 Pliocene; thus Prestwich (1890, p. 169) assigned it to the late 

 Pliocene. The uplift, however, was the result of a long gradual 

 movement, which began in the Cretaceous, since the upper zones of the 

 Chalk thin out as they are followed from the Thames Valley south- 

 ward toward the Weald. The denudation consequent on the uplift 

 had exposed the Upper Greensand by a very early date in the Eocene, 

 since Upper Greensand debris occurs in the Woolwich and Reading 

 Beds (C. Reid, 1896, pp. 493-4). It would therefore not be surprising 

 if the Lower Greensand had also been exposed in the Eocene ; but, at 

 the period of Dr. Irving' s discussions with Messrs. Monckton and 

 Hemes as to the classification of the* Bagshot Beds, the presence or 

 absence of Lower Greensand cherts was the accepted test whether 

 certain pebble -beds, were Bagshot or redeposited Bagshots. 

 Mr. Bromehead has, however, recently stated (Proc. Geol. Soc, 

 1913-14, p. 86) that Lower Greensand chert occurs in the upper 

 Bagshot pebble-beds (Bartonian); and if so the Lower Greensand was 

 exposed by the beginning of the Upper Eocene, and the chert from 

 it could have been carried into Essex then or at some later date. 

 Lower Greensand chert appears to be quite absent from the Essex 

 Bagshot Beds, which are clearly much older than the Danbury 

 gravels. Unless the country has been disturbed by differential 

 movements the cherts can only have been carried to Danbury, at 

 the level of 360 feet, from the nearest Lower Greensand outcrops, 

 which are in Kent, some thirty miles to the south, when the Lower 

 Greensand was exposed there at the level of not less than 700 feet above 

 the sea ; so that the uplift and denudation of the Weald must have 

 been far advanced before the deposition of the Danbury gravels. The 

 main uplift of the Weald may have been as early as the Oligocene; 

 but it was certainly not later than Miocene, for the Lower Pliocene 

 was a stage of great subsidence, during which the sea spread 

 over the Thames estuary and over the worn-down northern side of 

 the Weald as far south as Lenharn and Wye and almost to Folkestone. 

 How far the submergence extended to the west is doubtful ; some 

 patches of sand on the Chalk Downs to the south of London have 

 been referred to the Diestian, but on evidence which appears 

 inadequate. 



The Lower Greensand chert must have been carried from Kent to 

 Danbury before the Lower Pliocene subsidence. The chert may have 

 been washed into the low-level East Essex gravels at Southminster 

 and Southend in Pliocene or later times, as the slope from the present 

 outcrop of the Lower Greensand cherts near Maidstone would have 

 been sufficient for its transport by river- action into South-Eastern 

 Essex. But it would not have been possible for the river to have 

 carried the Kent cherts on to the top of the now isolated Essex hills 

 at Bayleigh, Langdon, or Danbury, which are as high as the outcrops 

 of chert in Kent, except when the topography of South-Eastern 

 Essex was quite different from the present. 



