396 H. W. Moncldon — Denudation and Elevation of the Wealden. 



clearly the case during the period from the beginning of the Purbeck 

 to the end of the Lower Greensand. 



Since that time a change in the earth movement of this area has 

 taken place ; for the line, which was then a line of excess of depres- 

 sion, has since become a line of excess of elevation forming the great 

 anticlinal of the Weald ; and it is clear that when the excess of 

 depression on this line ceased, deposition would tend to become more 

 uniform over the whole area ; and when the line began to rise, depo- 

 sition would be less over it than over the adjoining areas which 

 were stationary or rising more slowly. 



The great regularity in the thickness of the Gault on the north 

 and south of the Weald leads me to suspect that the excess of 

 depression of the central area ceased dux-ing the Gault period ; and it 

 is quite possible that if that area was beginning to rise during the 

 Chalk and Tertiary periods, little or no Chalk or Tertiary beds were 

 ever deposited there ; and I look with suspicion on any estimate of 

 the original height of the Wealden anticlinal founded on the assumed 

 regularity of deposition over the whole area. 



I thus reach the period when the elevation of the south-east of 

 England began, and proceeded at a rate varying greatly in different 

 places, thus forming the anticlinal of the Weald. At this point 

 I am in conflict with the views of previous writers as to the 

 formation of a plain of marine denudation. As I understand them, 

 they contend that the plain was formed by the advance of the sea 

 across the area " so as to shave it across." I doubt this. I believe 

 the plain was formed during the retreat of the sea, that is to say, 

 during the struggle of the land and water for mastery. 



Jf I am right, the central portion of the area which was being 

 elevated the more rapidly first reached the surface of the sea, and 

 on reaching the surface was at once destroyed by the waves ; but as 

 time went on, and the area of elevation gradually extended, a larger 

 and larger surface of the sea-bottom was brought in conflict with 

 the waves, and it is not hard to picture to oneself the progress of 

 the conflict and the final victory of the earth over the water. When 

 the Wealden area, therefore, first rose above the sea, it presented, 

 according to my theory, an extensive sandy flat ; and as it emerged, 

 subaerial denudation set in, which has gradually produced the pre- 

 sent surface of the country. 



In a well-known text-book the general effect of a partial sub- 

 mergence of the Weald is represented by a straight line parallel to 

 sea-level : but, if I am correct, this is probably inaccurate : for if the 

 area rises and falls at a rate varying in diffei-ent parts, the effect of 

 submergence would be represented by a curved line. 



It is quite true that large areas may be elevated or depressed 

 without losing their horizontality. According to Sir C. Lyell, the 

 primary rocks remain horizontal over thousands of square leagues 

 in America and Russia, in spite of great oscillations of level (Antiq. 

 of Man, 1873, p. 396). But any given area must be judged by the 

 evidence applicable to itself, and there is plenty of evidence that the 

 South of England has been subject to differential earth movements 



