A HISTORY OF ESSEX 



pre-glacial, but that an old hollow or valley was subsequently cut into 

 during the excavation of the Thames valley. River gravel also overlies 

 Boulder Clay in the Blackwater valley near Kelvedon. 



Underlying the valley gravel at Romford, Mr. Holmes has observed 

 a deposit of silt and sand with pebbles of Chalk, and flint, evidently of 

 subsequent date to the Boulder Clay from which it was largely derived. 1 

 He looked upon it as evidence of an ancient silted-up stream whose 

 course he would trace between Warley, Billericay and Maldon on the 

 north and west, and Laindon, Rayleigh and Althorne on the south and 

 east, and into the valley of the Blackwater below Maldon. In any case 

 this silted-up channel, like that in the Cam valley, belongs to a more 

 ancient period than the present Thames valley ; but whether these old 

 valleys which are occupied by Boulder Clay were pre-glacial valleys, or 

 were scooped out by ice-action during the Glacial period may fairly be 

 questioned. There is however no doubt that the main features of the 

 country were formed prior to the glaciation, and therefore we should 

 expect here and there to find traces of old valleys. 



When the Ice age was brought about the surface of the land had 

 long been subject to subaerial waste, the Chalk-tracts were covered with 

 clay-with-flints, and there was generally much weathered material or soil 

 over the land. Before any great movement of ice took place, the accu- 

 mulation of snow led to the formation of much ice, and to the base of 

 this the soil and weathered sub-strata were frozen. 



Eventually, when movement set in and there was coalescing of 

 great sheets of ice which traversed regions of Jurassic rock and Chalk, 

 the base of the ice tore off the frozen soil and debris^ and in some cases 

 great strips of the strata ; in other cases impinging against higher ground 

 the formations were locally disturbed, as may have been the case near 

 Heydon. 



The debris thus removed would rise by overthrusts into higher hori- 

 zons in the ice, and be then carried forward and widely distributed and 

 commingled with local detritus during alternate recessions and readvances 

 of the ice-margin ; the Boulder Clay being deposited, to a large extent, by 

 the melting of the ice, as indicated many years ago by Mr. J. G. Good- 

 child in his account of ice-work in Edenside. 



The abundant chalky detritus was no doubt carried along minor 

 planes of movement in the ice, the chalk lumps being scored by frac- 

 tured flint, and the material being transported far and wide at higher 

 levels in the ice than the bulk of the more local material. In certain 

 instances the soil frozen to the base of the ice-sheet was little if at 

 all moved, being overridden by subsequent ice-movements ; and much 

 Boulder Clay must also have been overridden after deposition, owing to 

 its exceedingly tough character. 2 



1 Quart. Journ. Geol. Sue., vol. 1. p. 443 ; and ' Notes on the Ancient Physiography of South 

 Essex,' Essex Nat., vol. ix. p. 193. 



2 See H. B. Woodward, Geol. Mag., 1897, p. 485 ; J. E. Marr, ibid. 1887, p. 262 ; and J. Geikie, 

 The Great Ice 4ge. 



