§ I. GEOLOGY OF THE SOUTH-EAST OF ENGLAND. 361 



The strata of the south-east of England belong to three 

 principal groups or formations. The first consists of the 

 Tertiary sands, clays, and gravel, described in the previous 

 Lecture, which occupy depressions in the chalk. 



The second is the Chalk, (including under this term the 

 White-chalk, Gait, and Greensand,) which forms the most 

 striking feature in the physical geography of the country. 

 The upper division of this formation constitutes the South 

 Downs, which, from the bold promontory of Beachy-head, 

 traverse the county of Sussex from east to west, and pass 

 through Hampshire into Surrey ; from Godalming the 

 chalk hills extend by Godstone into Kent, where they are 

 called the North Downs, and terminate in the line of cliffs 

 that stretches from Dover to Ramsgate. The lowest mem- 

 ber of the chalk, the Greensand, appears as a chain of hills 

 of irregular elevation, skirting the escarpments of the chalk 

 downs ; the Gait occupying the intermediate valley. 



The third group fills up the area between the North and 

 South Downs ; the most elevated masses form the Forrest- 

 ridge, which traverses the district in a direction nearly east 

 and west, and is composed of alternations of sandstones, 

 sands, shales, and clays, with a deep valley on each flank, 

 called the Weald: hence the geological designation of the 

 whole series. From the central ridge of the Wealden, which 

 varies in height from 400 to 800 feet, and stretches from 

 Fairlight Down near Hastings on the east, to beyond 

 Horsham on the west, the strata diverge on each side 

 towards the Downs, constituting an anticlinal axis, and 

 finally disappear beneath the low r ermost beds of greensand. 

 There are conclusive proofs that the Wealden strata were 

 originally covered by the chalk, and that their present po- 

 sition and appearance are attributable to changes which 

 have taken place subsequently to the cretaceous epoch.* 



* See Geology of the South-East of England, chap, xi, 



