248 GEOLOGY OF THE ISLE OF WIGHT. 



CHAPTER XV. 

 PHYSICAL GEOGRAPHY. 



Having already noted the leading physical features of the Isle 

 of Wight, we will now proceed to show that they have been 

 produced by denudation acting along lines of drainage which were 

 determined by the formation of the anticlines and synclinea 

 described in the previous Chapter. Though the modern rivers 

 still follow the courses so determined, the actual surface-features 

 produced by the movements of the strata have long since dis' 

 appeared ; and, as in the case of the Weald, the anticlinal areas 

 of the Isle of Wight show that the regions of greatest upheaval 

 in past times are often those of least elevation at the present. In 

 studying the physical features of the Island, one of the most 

 prominent facts that strikes us is the comparative insignificance of 

 the central chalk ridge or back-bone as a watershed. Both in past 

 times and at the present day, the principal rivers of the Island cut 

 right across it, ignoring, as it were, the easier passage which seems 

 to exist for them along it either to the east or west. The ex- 

 planation has been already found in the case of the Weald, and it 

 will be sufficient here to point out the similarity between the 

 two districts. 



The existing watersheds of the Isle of Wight are complicated 

 by the fact that there are so many small streams having a sepa- 

 rate existence. le-norlno; the minor watersheds between these, we 

 will trace that which separates the water draining south into the 

 Channel, from the water which runs north into the Solent. This 

 line runs from the cliff near Sandown over Shanklin and Boniface 

 Downs to the cliff above Ventnor, and thence over ^ew Dowia. 

 Westwards it keeps close to the cliff edge as far as St. Catherine's 

 Down, along which it runs, turning down south of Chale Green to 

 Kingston, and thence along the southern brow of the Downs to 

 the Needles. 



It has been shown that the streams which run into the sea in 

 Brixton Bay were within a geologically recent period tributaries 

 of the western Yar, and that similarly the streams of Shanklin 

 Chine and Luccombe Chine were tributaries of the eastern Yar. 

 The separation of these streams from their original drainage- 

 basins has been due to the encroachment of the sea, and if we 

 trace the watershed as it existed before the separation took 

 place, we find that it must have run south of the whole Island, 

 excepting only a small portion of Week, Rew, and Boniface 

 Downs. That is to say that the whole drainage of the Island, 

 excepting the short and steep heads of valleys in the south side of 

 these downs, must have made its way northwards, the water from 

 the area now occupied by the Lower Greensand all escaping in this 

 direction across the high central ridge of chalk. 



