462 Wealden Gorges dice to two Opposite Series of Flexures. 



! rain, the brooks would travel with thrice the velocity they have at present, and 

 if glaciers twenty-seven times higher would slide with a threefold velocity, 

 then we have a cause which can sufficiently explain the erosions or lowerings 

 by denudation during the glacial and pluvial periods. 



The rounded London Clay hills in many places are curved like Fig. 

 19. The deposits of the large brick earth and gravel beds are conspicuous 

 in the valleys of the rivers of the London basin, and the tracks of ancient 

 rivers (as at Crayford) in the eroded surface of the chalk, now filled up with 

 gravel and brick earth, may be seen in the excavations for brick earth. There 

 are no beds of this kind now forming, and this shows that the present is not a 

 guide to the past in the science of geology in many cases. 



It is from these superficial accumulations of the pluvial period that the 

 brick earth is derived, from which the building materials of this city are almost 

 entirely derived, and the quantity of brick earth is a kind of measure of the 

 intensity of the ancient rains. 



Philosophers and writers of books on philosophical geology are always spe- 

 culating about what are called remarkable phenomena, and very much neglect 

 what occurs every day, and can be seen every hour. We ought to look at the 

 low ground, as well as at the mountains. Being, as Mr. Pattison observed 

 in a paper read at the Victoria Institute, March I, 1875, more of an observer 

 than a theorist, I would rather speak to you if I can, to-night, about what is 

 ■the action of rivers and springs in places you know well. 



The Fig. 25, page 457, is a type of the north and south flexures of the Weald. 

 The gradient being so rapid north and south, the series of beds, Weald clay, 

 Lower Greensand, gait, Upper Greensand, chalk, marl and chalk, crop out 

 within two or three miles or less in the case of Guildford. In walking north 

 and south you walk at right angles to the strike of the principal flexures. On 

 the other hand, the east and west flexures are slight and always along the 

 principal strike, so that you may walk along the out-crops of the same bed 

 hundreds of miles. That is, you may walk along the strike of the north and 

 south great Wealden flexures, which is of course E. and W., or in the direc- 

 tion of the dip in the small E. and W. flexures. The Wealden physical features 

 are so difficult to describe because there are these opposite sets of binomial 

 flexures, and what is strike to one set of flexures is dip to the other. There 

 were evidently forces acting at right angles to each other when the Wealden 

 was lifted up, each foot of surface having a simultaneous opposite flexure 

 impressed on it, causing a double curve, affecting the whole depth of strata. 



East West 



Fig. 35 represents the east and west flexures in the gait and Weald clay beds 

 in the north escarpment of the Weald, which facilitated the original formation 

 of the gorges through which the rivers run. This is explained in a note page 

 473. A river directed north against G would be repulsed by the clay, and have 

 to flow east or west to B 2 or B 3 to make a gorge. Clay is very stable here. 



There are a series of flexures east and west along the north escarpment of these 

 Surrey hills, about 20 ft. per mile from Hildenborough to Maidstone, and 10 ft. 

 per mile from Hildenborough to Dorking. The strata are only gently lifted and 

 depressed in E. and W. direction 20 ft. to 10 ft. in a mile, while the great north and 

 south flexures are near the gorges at gradients from 250 ft. to 1,000 ft. in a mile. It 

 is this system of double flexures, one set in the medial series at nearly right angles 



