and thirdly, that many terraces and small cliffs may now lie concealed beneath a talus of detrital matter. _Denudation of the Weald Valley._--No district is better fitted to illustrate the manner in which a great series of strata may have been upheaved and gradually denuded than the country intervening between the North and South Downs. This region, of which a ground plan is given in the accompanying map (fig. 252.), comprises within it the whole of Sussex, and parts of the counties of Kent, Surrey, and Hampshire. The space in which the formations older than the White Chalk, or those from the Gault to the Hastings sand inclusive, crop out, is bounded everywhere by a great escarpment of chalk, which is continued on the opposite side of the channel in the Bas Boulonnais in France, where it forms the semicircular boundary of a tract in which older strata also appear at the surface. The whole of this district may therefore be considered geologically as one and the same. [Illustration: Fig. 252. Geological Map of the south-east of England and part of France, exhibiting the denudation of the Weald. 1. Tertiary. 2. Chalk and upper greensand. 3. Gault. 4. Lower Greensand. 5. Weald clay. 6. Hastings sand. 7. Purbeck beds. 8. Oolite.] [Illustration: Fig. 253. Section from the London to the Hampshire basin across the valley of the Weald. 1. Tertiary strata. 2. Chalk and firestone. 3. Gault. 4. Lower greensand. 5. Weald clay. 6. Hastings sands.] [Illustration: Fig. 254. Highest point of South Downs, 858 feet. Anticlinal axis of the Weald. Crowborough Hill, 804 feet. Highest point of North Downs, 880 feet.[243-A] Section of the country from the confines of the basin of London to that of Hants, with the principal heights above the level of the sea on a true scale.[243-B]] The space here inclosed within the escarpment of the chalk affords an example of what has been sometimes called a "valley of elevation" (more properly "of denudation"); where the strata, partially removed by aqueous excavation, dip away on all sides from a central axis. Thus, it is supposed that the area now occupied by the Hastings sand (No. 6.) was once covered by the Weald clay (No. 5.), and this again by the Greensand (No. 4.), and this by the Gault (No. 3.); and, lastly, that the Chalk (No. 2.) extended originally over the whole space between the North and the South Downs. This theory will be better understood by consulting the annexed diagram (fig. 253.), where the dark lines represent what now remains, and the fainter ones those portions of rock which are believed to have been carried away.