174 THE BOOK OF NATURE STUDY 



Heights ; and all have important longitudinal tributaries flowing 

 along either the Gault or the Wealden Valley, or along both. The 

 Medway, Arun, and Wey have especially important longitudinal 

 tributaries. 



The general geological map of the Weald is equally simple 

 (Fig. 65). The Wealden Heights correspond with the outcrop of 

 a set of hard sands called the Wealden Sands ; the Wealden 

 Valley is the outcrop of a thick seam of clay the Weald Clay ; the 

 Ragstone Range is the outcrop of hard sandstones and chert beds 

 of the Lower Greensand ; the Gault Valley the outcrop of a clay 

 seam called the Gault ; and the Downs are the outcrop of the Chalk, 

 a thick seam of white, pure, absorbent limestone. All the rocks 

 dip outwards from the central area. The gentle outer face of the 

 hills are dip-slopes, the steep, inward-looking slopes are escarp- 



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FIG. 66. Geological Section across the Weald. 



ments. A geological section across the area (Fig. 66) shows that 

 the oldest rocks are the Wealden Sands, and the newest the Chalk ; 

 that the rocks are bent into an arch, or rather into an elongated 

 dome, like half an egg, the long axis of which runs from a little 

 north of west to a little south of east. The centre of the dome, 

 probably to some extent planed by the sea in the way suggested 

 earlier in this chapter, formed the primitive high ground from 

 which the transverse streams radiated. There may have been 

 dozens of these, each with its own set of tributaries ; but, by a 

 process of capture, the eight strongest ones have survived. 

 Amongst the last to be captured were the head-waters of the 

 Darent, and the streams which flowed through Smitham Bottom 

 and the Caterham Valley to Croydon (Fig. 67). These at present 

 cut only the Chalk escarpment, but not the Greensand behind, 



