THE STORY OF THE ISLAND RIVERS 93 



Sandown or Brading. They rather look as if they would 

 flow along the marshy flat by Blackwater into the Medina. 

 But the Yar cuts right across their course, and carries 

 them off eastward to Sandown. When we look, we find 

 a line of river valley with a strip of alluvium running up 

 from the Medina at Blackwater in the direction of these 

 two streams a valley which the railway up the Yar 

 valley from Sandown makes use of to get to Newport. 

 There can be little doubt that these streams from Niton 

 and Wroxall originally ran along this line into the 

 Medina ; but the Yar, cutting its course backward, has 

 captured them, and diverted their course. They probably 

 represent the main branches of the Medina in earlier 

 times, the direction of flow from south-east to north-west 

 instead of south to north being possibly due to the over- 

 lapping in the neighbourhood of Newport of the ends of 

 the Brook and Sandown anticlines. The sheet of gravel 

 on Blake Down belongs to this period of the river's history. 

 The river must have diverted between the deposition of 

 the Plateau Gravels and that of the Valley Gravels of the 

 Yar. For the former follow the original valley, the 

 latter the new course of the river. 



We must now take a wider outlook, and see what became 

 of our rivers after they had flowed across what is now the 

 Isle of Wight from south to north. We have been speak- 

 ing of times when the Island was of much greater extent 

 than at present. Standing on the down above the Needles, 

 and looking westward, we see on a clear day the Isle of 

 Purbeck lying opposite, and we can see that the headland 

 there is formed by white chalk cliffs like those beneath us. 

 In front of them stand the Old Harry Rocks, answering to 

 the Needles, both relics of a former extension of the land. 

 In fact Purbeck is just like a continuation of the Isle of 

 Wight. South of the Chalk lie Greensand and Wealden 

 strata in Swanage Bay, and north towards Poole are 

 Tertiaries. Clearly these strata were once continuous 



