284 Reports & Proceedings — Geological Society of London. 



B. 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 Tliames-Kennet River was formed along the axis of the 

 syncline of the Thames Basin. The Thames, therefoi-e, 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 (probably 

 Upper Eocene) and during the deposition of the higlier-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 jSTorthern 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 conglomerates 

 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 the 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-movements 

 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 Danburj'- 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 Blackwater and Chelmer Rivers. These rivers, 

 after the diversion of the Thames to its present line, must have 

 extended their old valleys south-eastwards; the Blackwater and the 

 Colne excavated the estuary of the Blackwater, 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 Rodiug, have had to excavate post-Glacial 

 valleys, 



(5) A post-Glacial change led to the deepening of the Chelmer 

 Valley between Chelmsford and Maldon, whereby the Chelmer was 



