PHYSICAL GEOGRAPHT. 249 



Tlie physical geography of the Weald has been too fully 

 described* to need more than an allusion here. The rivers of that 

 area rise in what is now the area of least elevation, and make 

 their way to the north and south through gaps cut in the bold 

 escarpments of the North and South Downs. The watershed, 

 however, though now low, follows the axis of the anticline, that 

 is the line of greatest upheaval in past times, and in this fact is 

 provided the key to the history of the rivers not only of the 

 Weald and Isle of Wight, but of all the part of England affected 

 by the synclinal and anticlinal folds described above. 



For we find that without exception the main lines of drainage 

 follow the synclinal axes, while the tributaries flow at right angles 

 to, and off the anticlinal axes. The first land to emerge from 

 beneath the sea was that formed by the crests of the anticlinal 

 folds, and each of these thereupon became a watershed, and has so 

 remained. The last land to emerge was the deepest line of each 

 synclinal fold, and along this was collected the drainage from the 

 anticline to the north and south of it. The lines of drainage and 

 watershed, thus initiated, have been maintained, though the form 

 of surface due to the original movements has been lost. It thus 

 happens that the watersheds have little relation to the hill-ranges 

 of the present day. 



The two leading examples of rivers following the synclinal 

 troughs are the Thames and the Frome. Part of the Thames, with 

 Its tributary the Kennet, form a line of drainage running the whole 

 length of the London syncline. On the north side it collects 

 the rivers which run down the back of the Chalk escarpment of 

 Berkshire, Buckinghamshire, and Hertfordshire ; on the south side 

 it gathers the streams which descend from the anticlinal axis of 

 the Weald and its continuation on the north side of Salisbury 

 Plain. 



Similarly, in the Hampshire Basin, we find the Frome followinf 

 the synclinal axis, and forming an exact counterpart of the 

 Kennet. On its north bank it receives rivers which flow down 

 the south side of the anticline named above, and on its south side 

 it must have received the drainage of the Isle of Wight anticline 

 until the Hampshire Basin was invaded by the sea. 



In the alterations brought about by the encroachment of the 

 sea, lies the principal diflerence between the rivers of the London 

 and the Hampshire Basins. The Frome now enters the sea near 

 Poole, but it is clear that, before the sea made the great breach in 

 the Chalk escarpment which separates Dorset and the Isle of 

 Wight, this river must have followed the syncline eastwards. For 

 this breach, though probably commenced as a river valley, can 

 hardly have been the course followed by the Frome, for in such a 

 case the river must have turned from following a syncline to cut 

 directly across an anticline. On the other hand, we have in the 

 Solent, and the arm of the sea at Spithead, an old valley and 



* W. Topley. Geology of the Weald (Memoirs of the Geological Survey), 

 chapter 16. 



