Another elaborate memoir treating of the detrital beds within the Wealden escarpment, and their bearing upon the mode in which the denudation was effected, is that of Messrs. Foster and Topley*. The view of these gentlemen was that after a plane of marine denudation had been effected over the original area, the whole of the denudation by which, not merely the valleys proper of the Wealden rivers, but also the great excavation of the Weald itself (or major valley), with its well-known contour, have been accomplished was effected by these rivers, especially the Medway, flowing in their present direction — a view indorsed, apparently, by Prof. Ramsay†.
Many other notices, special and incidental, upon this question have appeared, and among them notices from Mr. Martin, Mr. Godwin- Austen, Mr. Prestwich, and Mr. Mackie ; the first-named of whom has for a long period been a staunch upholder of the marine theory; while Sir Charles Lyell, it is well known, has always adhered, in his ' Elements ' and in his 'Manual of Geology,' to the same hypothesis of marine agency.
In 1866‡ a study of the distribution of the gravel of the Thames, of that of East Essex and its continuation in the lower valley of the Medway, and of that of the heights above Canterbury led me to the conclusion that each of these gravel-sheets had partaken of some of the movements by which the Lower Tertiaries upon which they rest had acquired their present position and outcrop, and had thus been contemporaneous with some portion at least of that earlier part of the Wealden denudation to which the removal of the Tertiaries from the North Downs is due.
I then called especial attention to the circumstance that the position of the Thames and East Essex gravels in their troughs precluded the possibility of a connexion between them and the Thames river, either in its present or any prior condition, because that part of the Thames valley which lies east of Gravesend, instead of being coincident with the gravel-troughs, cuts at right angles through them — a feature also possessed by the next river to the north, the Crouch, the valley of that river, as well as the portion of the Thames valley just referred to, being entirely destitute of gravel or brick- earth.
These features, I pointed out, necessitated an admission that the troughs in question had their seaward terminations in the direction of the Weald, because the trough which contained the Thames gravel was absolutely shut in from the north sea by the lofty ridge which separated it from the East-Essex sheet, that ridge not having been opened for the river Thames to reach the North Sea until such
- Quart. Journ. Geol. Soc. vol. xxi. p. 443.
† Physical Geology and Geography of Great Britain : 1863 & 64.
‡ See papers on the Structure of the Thames Valley and its contained Deposits in vol. iii. of Geol. Mag. pp. 57 & 99, and paper on the Structure of the Valleys of the Blackwater and Crouch, and of the East-Essex Gravel, and on the relation of this Gravel to the Denudation of the Weald, ibid. pp. 348 & 398 ; also on the Postglacial Structure of the South-east of England, in Quart. Journ. Geol. Soc. vol. xxiii. p. 394.
