Ch. xix.i inland chalk cliffs m normandy. 353 





CHAPTER XIX. 



DENUDATIOX 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 pillars 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 escarp- 

 ments — Rise and denudation of the strata gradual — Ridges formed by harder, 

 valleys by softer beds — At what periods the Weald Yalley was denuded — Why no 

 alluvium, or wreck of the chalk, in the central district of the Weald — Successive 

 periods of marine denudation — The latest of these posterior to the Upper Miocene 

 era — Elephant-bed, Brighton — Sangatte Cliff — The great escarpments and trans- 

 verse valleys of the chalk mainly due to the waves and tides of the sea — Paroxys- 

 mal causes unnecessary for explaining the external configuration of the Wealden. 



All the fossiliferous formations may be studied by the geologist in 

 two distinct points of view : 1st, in reference to their position in the 

 series, their mineral character and fossils ; and, 2dly, 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 now proceed to consider certain features in the physical geogra- 

 phy 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-pas- 

 tures, are free from trees or hedgerows ; so that we have an opportu- 

 nity of observing 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 denu- 

 dation, having been excavated when the chalk emerged gradually from 

 the sea. This opinion is confirmed by the occasional occurrence of 

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

 cut off abruptly in steep and often vertical precipices. The true na- 

 ture of such escarpments is nowhere more obvious than in parts of Nor- 

 mandy, where the river Seine and its tributaries flow through deep wind- 

 ing valleys, hollowed out of chalk horizontally stratified. Thus, for ex- 

 ample, if we follow the Seine for a distance of about 30 miles from An- 

 delvs 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 

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

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