1867.] WOOD SOUTH-EAST OF ENGLA]!fD. 397 



dation towards the north brow of the Thames valley, a much less 

 thickness than 160 feet remains in that part. 



Although these diagrams, in strictness, relate to the Glacial struc- 

 ture, it is essential to the realization of the Postglacial structure, 

 which has resulted from the denudation commencing at the elevation 

 of the Upper Glacial clay, that the position of this clay along the 

 northern brow of the Thames valley should be clearly defined. 



It is from this great deposit of Upper Glacial clay, the result of a 

 wide -spread submergence, that the valley of the Thames is cut down 

 at Grays to a depth (measured by the strata denuded in the process) 

 of 600 feet. Owing to the upcast imparted to the beds during the 

 denudation of this 600 feet, the brow occupied by the Glacial clay 

 stands at a height of from 250 to 350 feet only above the Thames river. 

 The manner in which the valley is, on its north side, cut down from 

 the Glacial clay, is indicated by the strong lines in fig. 3; and actual 

 sections drawn to the longitudinal scale of the Ordnance map will 

 be found in the manuscript memoir before referred to, taken in various 

 directions through Sheet 1, illustrative of all the features displayed 

 by the above diagrams. While the northern side of the Thames is 

 occupied by a slope which is cut down from the Glacial clay, in the 

 manner shown by the strong lines, for a distance of 40 miles east of 

 London (without reckoning sinuosities), we do not anywhere on the 

 south side, or over the counties of Kent, Surrey, and Sussex*, en- 

 counter the faintest trace of the Glacial clay, although we have in 

 that area all elevations from the sea-level up to 800 feetf. 



The extensive deposit known as the Thames gravel occupies, but 

 in a complex way, the trough thus cut down from the Glacial clay 

 along that part of the Thames valley, east of London, which is west 

 of Fobbing in Essex — a point 13 miles west of the Thames' mouth 

 at Shoeburjness : and while in this part the gravel rests against the 

 denuded surface represented by the strong lines of fig. 3, on the north 

 side of the river only, that arm of it which stretches up the Lea 

 valley for more than 15 miles has the Glacial clay on both sides of it, 

 the trough containing the gravel being there cut in a simple way 

 through the Glacial clay. The following diagram sections (figs. 4 

 and 5) illustrate the position of the Thames gravel in either case. 



Numerous actual sections corroborative of these diagram sections 

 throughout Sheet 1 will be found in the memoir referred to. 



The position occupied by the Upper Glacial clay along the north 

 side of the Thames appears to me due to the circumstance of its 

 having formed the southern edge and emerged portion of the 



* Nowhere over the south or south-east of England, indeed ; but I have, in 

 a paper in the Greological Magazine, called attention to the probability that a 

 search over the highest Tertiary beds of Surrey and Hampshire might, perhaps, 

 yield traces of it. 



f It is important to observe that the depressions in which the Glacial beds 

 are shown, in figs. 1, 2, and 3, to have been deposited have no relation to 

 our present valleys, except that, where in a few instances (as in that of the 

 Roding about Navestock) the present valleys happen to pass such a depression, 

 they appear (but for a very short distance usually) to have the Glacial beds rest- 

 ing within their brows. 



^ 2e2 



