Ch.XIX.] inland chalk-cliffs in NORMANDY. 267 



CHAPTER XIX. 



DENUDATION OF THE CHALK AND WEALDEN. 



Physical geography of certain districts composed of Cretaceous and "Wealden 

 strata — Lines of inland chalk-cliffs on the Seine in Normandy — Outstanding 

 fillars and needles of chalk^Denudation of the chalk and Wealden in Surrey, 

 Kent, and Sussex — Chalk once continuous from the North to the South Downs 

 — Anticlinal axis and parallel ridges — Longitudinal and transverse valleys — 

 Chalk escarpments — Rise and denudation of the strata gradual — Ridges formed 

 by harder, valleys by softer beds — At what periods the Weald valley was de- 

 nuded — Why no alluvium, or wreck of tlie chalk, in the central district of the 

 Weald — Land has most prevailed where denudation has been greatest — Ele- 

 phant bed, Brighton — Sangatte Cliff — Conclusion. 



All the fossiliferous formations may be studied by the geologist in two 

 distinct points of view ; first, in reference to their position in the series, 

 their mineral character and fossils; and, secondly, in regard to their 

 physical geography, or the manner in which they now enter, as mineral 

 masses, into the external structure of the earth ; forming the bed of lakes 

 and seas, or the surface or foundation of hills and valleys, plains and 

 table-lands. Some account has already been given on the first head of 

 the Tertiary, the Cretaceous, and the Wealden strata ; and we may now 

 proceed to consider certain features in the physical geography of these 

 groups as they occur in parts of England and France. 



The hills composed of white chalk in the S. E. of England have a 

 smooth rounded outline, and being usually in the state of sheep pastures, 

 are free from trees or hedgerows ; so that we have an opportunity of ob- 

 serving how the valleys by which they are drained ramify in all directions, 

 and become wider and deeper as they descend. Although these valleys 

 are now for the most part dry, except during heavy rains and the melting 

 of snow, they may have been due to aqueous denudation, as explained in 

 the sixth chapter ; having been excavated when the chalk emerged gradu- 

 ally from the sea. This opinion is confirmed by the occasional occurrence 

 of what appeared to be long lines of inland cliffs, in which the strata are 

 cut off abruptly in steep and often vertical precipices. The true nature of 

 such escarpments is nowhere more obvious than in parts of ISTormandy, 

 where the river Seine and its tributaries flow through deep winding val- 

 leys, hollowed out of chalk horizontally stratified. Thus, for example, 

 if we follow the Seine for a distance of about 30 miles from Andelys to 

 Elboeuf, we find the valley flanked on both sides by a steep slope of 

 chalk, with numerous beds of flint, the formation being laid open for a 

 thickness of about 250 and 300 feet. Above the chalk is an overlying 

 mass of sand, gravel, and clay, from 30 to 100 feet thick. The two 

 opposite slopes of the hills a and h (fig. 313), where the chalk appeal's at 



