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



The transport of the Lower Greensand chert on to the Essex hills 

 was therefore pre-Diestian, and the Danbury gravels cannot be later 

 than the end of the Miocene. 



It may be suggested that Miocene gravels would not have lasted so 

 long in such a situation. But as patches of Bagshot and even older 

 pebble-beds occur in similar positions in the Thames Yalley there is 

 nothing very surprising in the survival of a Miocene gravel at 

 Danbury. The Danbury hills owe their existence partly to the slow 

 rate of denudation in a mature low-level country, and partly to the 

 mantle of redeposited gravel on the hill-sides having protected the 

 underlying clay from wind and rain. 



The Mid-Essex Fault. — The age of the Danbury gravels is, however, 

 complicated by the possibility that their present elevation may be due 

 to uplift, for if so they might have been laid down in Pliocene times- 

 by some southern river, contemporary with that which deposited the 

 Southminster gravels. 



If the Danbury gravels stood alone this complication would seriously 

 affect the question, but fortunately the high-level gravels atRayleigh 

 (alt. 260 feet) also contain Lower Greensand chert ; and there is 

 nothing to suggest their differential uplift. As the gravels in the 

 Bayleigh hills must have received their Lower Greensand chert before 

 the Diestian subsidence, the Danbury gravels must have received their 

 scantier supply before that event. 



That the Danbury gravels have been upraised by an earth- 

 movement was first suggested by S. V. Wood, jun., who held that 

 a bed of London Clay near Biffhams, to the north of Danbury, which 

 occurs between two beds of the Danbury gravels, owed its position to 

 an overthrust fault. The section is now obscure and overgrown, and 

 in its present condition does not give any evidence in support of 

 Wood's conclusion. The outcrop of the bed is now so weathered that 

 it might be a redeposited clay. But as Wood examined this section 

 when it was clear, and he was not likely to have mistaken rainwash 

 for London Clay in situ, his interpretation of this section is not to be 

 lightly dismissed, especially in view of the fresh evidence for a mid- 

 Kainozoic dislocation along the line indicated by him. Thus the 

 chalk surface under this part of Essex is not a uniform plane, as 

 represented in Geological Survey Horizontal Section, No. 84, 1871 ; 

 it is disturbed by well-proved irregularities. These irregularities in 

 Mid-Essex are due to a disturbance, which was either a fault or 

 a sharp fold along the line indicated by Wood. Among the evidence 

 for this dislocation is the steep dip in the London Clay at Perry 

 Wood, beside the Kelvedon railway ; this dip has been attributed by 

 Holmes to a fault. The Colchester earthquake of 1884 was probably 

 due to a slip on the same or a parallel fault (Meldola, 1885, p. 185). 

 Evidence for differential movement across the Chelmer Valley and for 

 the extension of the fault from Billericay southward to the Thames 

 near Cliff has been recently advanced by Mr. Boswell (1915, 

 pp. 200-2, 205). 



The most sticking evidence is afforded by the Wickham Bishop 

 bore, which proved that the beds at the base of the London Clay 

 were inverted and repeated. This bore began at 234 feet O.D. and 



