172 THE BOOK OF NATURE STUDY 



the scarp, and causing it to retreat towards the south and east, 

 thus shortening the length of these tributaries by cutting back 

 the ground on which they rise, and themselves growing in size 

 and importance with the ground and water they are capturing. 

 The Yorkshire Ouse has captured numerous transverse streams 

 flowing down from the Pennine dip-slopes, and is carrying them 

 along a longitudinal path to the Humber. The Trent and the 

 Mersey are doing similar work. 



Instances to illustrate this part of the subject can be found 

 in any river system, but the finest one described as a connected 

 whole, and still one of the best for study of the principles, is that 

 of the south-eastern part of England. This area comprises part- 

 of Kent, Surrey, and Sussex, and is known as the Weald. 



The topographical features may be summarised as follows 

 (Fig. 64) : i. There is a central knot of hills about Ashdown Forest 

 and Tunbridge Wells, culminating at over 700 feet on Crowborough 

 Beacon, and reaching the sea at Hastings ; these are the Wealden 

 Heights. 2. A horse-shoe shaped valley, beginning at Romney, 

 passing inland to beyond Horsham, and back again to the sea at 

 Pevensey; this is the Wealden Valley. 3. The Ragstone Range, 

 again horse-shoe shaped, coming out of the sea at Folkestone, over- 

 topping 950 feet at Leith Hill, sweeping round at Hindhead and 

 Blackdown, and dying down towards the sea about Hailsham. 



4. The Gault Valley running parallel to the last from north of 

 Folkestone to Farnham, and back to the sea at Eastbourne. 



5. The Chalk Downs from Dover to Box Hill and Guildford, round 

 by Alresford, and so to the sea again at Brighton and Beachy 

 Head, attaining a maximum height of 880 feet near Oxted. The 

 steeper faces of these hill ranges all look towards the inside of 

 the horse-shoe. The two concentric valleys mentioned are not 

 occupied by single streams, but by innumerable longitudinal 

 streams, nearly all draining to eight principal transverse streams, 

 which rise in the central tract, and cut gorges through both 

 Ragstone Range and Downs, and so escape direct to the sea or 

 to the Thames. These streams are the Stour, Medway, Mole, and 

 Wey, cutting through the North Downs, and the Arun, Adur, 

 Ouse, and Cuckmere through the South Downs. All of these, 

 except the Wey; have some head-waters coming from the Wealden 



