The Evolution of the Essex River- System. 159 



II. Judged from the distribution and dates of appearance of the 

 non-local constituents in these gravels, the evolution of the Essex 

 river- systems appears to have been as follows : — 



(1 ) The Thames-KennetRiver was formed along the axis of 

 the syncline of the Thames Basin. The Thames, therefore, began 

 its existence in the Upper Eocene and Oligocene Periods, and then 

 flowed north-eastwards and passed out of Essex to the north of 

 Clacton. Tributaries down the northern slope of this syncline 

 introduced the quartzite-drift from the Midlands, while flints from 

 the Chalk were washed down both slopes. 



(2) As denudation lowered the upraised borders of the Thames 

 Basin, the Lower Greensand was exposed in the North Downs, and 

 chert from it was carried into Essex. The chert began to reach 

 Mid-Essex after the deposition of the Brentwood Gravels (pro- 

 bably Upper Eocene) and during the deposition of the higher-level 

 Danbury Gravels (360 feet). As the Upper Greensand had been 

 exposed in the Weald during the Lower Eocene Period, it is most 

 probable that the Lower Greensand would have been exposed during 

 the Oligocene Period. Its exposure in the Miocene Period is 

 indicated by the occurrence of its cherts in the Crags, combined 

 with the existence of a plain in Northern Kent before the oldest 

 Pliocene (Diestian). As the oldest Danbury Gravels give no 

 evidence of redeposition they are, at the latest, Miocene. 



(3) Further denudation exposed the Lower Greensand con- 

 glomerates of Cambridgeshire, and felsites from them were carried 

 into Mid-Essex, where they arrived later than the highest-level 

 Danbury Gravel but during the deposition of the Braintree Gravel 

 on an old plain, at about 200 feet above present sea-level. The 

 Braintree Gravels were not later than the end of the Miocene 

 Period, although many of them have probably been redeposited in 

 the Pliocene. 



(4) The Upper Miocene, or at least pre-Diestian, earth-move- 

 ments lowered the Weald, and led to the deposition of the Diestian 

 Sands on the worn -down northern edge of the Wealden anticline. 

 This subsidence deflected the Thames southwards to its present 

 line. Its former route was closed by the uplift of Mid-Essex by 

 the reversed fault or fold along the Mid-Essex Range. The latest 

 date for this movement is indefinite. It has been suggested that 

 the great height of the Danbury Gravels is due to uplift. The 

 most definite evidence as to the age of the earth-movement shows 

 that it was later than the existence of the Blaekwater and Chelmer 

 liivers. These rivers, after the diversion of the Thames to its 

 present line, must have extended their old valleys south-eastwards ; 

 the Blaekwater and the Colne excavated the estuary of the Black- 

 water ; and the Chelmer, following a parallel route, discharged 

 through the Crouch. These rivers and most of the larger Essex 

 river-valleys were pre-Glacial, but the smaller valleys in Northern, 

 Central, and Western Essex were completely filled during Glacial 

 times, and so the streams, such as the Upper Boding, have had to 

 excavate post-Grlacial valleys. 



