THE STORY OF THE ISLAND RIVERS 89 



of this the three rivers of the Island, the two Yars and the 

 Medina, all flow north, and cut through the chalk escarp- 

 ment of the Central downs, as if an earthquake had made 

 rifts for them to pass, and so find their way into the 

 Solent. The explanation is the same as in the case of the 

 Weald. The rivers began to flow when the Chalk strata 

 were continuous over the centre of the Island ; and their 

 course was determined when the east and west anticlinal 

 axis rose above the sea. 



We shall notice, however, that the Island rivers start 

 from south of the anticlinal axis. The centre of the 

 Sandown anticline runs just north of Sandown, but the 

 various branches of the Yar and Medina flow from well 

 south of this. The explanation would appear to be that 

 the anticline is almost a monoclinal curve, that is to say, 

 one slope is steep, the other not far from horizontal. 

 Streams starting from the ridge would flow with much 

 greater force down the northern than the southern side, 

 and would cut back their course much more quickly. 

 Thus they would continually cut into the heads of the 

 southern streams, and turn the water supplying them 

 into their own channels. 



In its early history a river cuts out its bed, and carries 

 along pebbles, sand and mud to the sea. The head waters 

 are constantly cutting back, and the slope becoming less 

 steep, till a time comes when the stream in its gently 

 inclined lower course has no more power to excavate, 

 and the finer sediment, which is all that now reaches the 

 lower river, begins to fill up the old channel. And so the 

 alluvium is formed which fills the lower portions of our 

 river valleys. 



Beyond this, the great rush of waters from melting 

 snows and ice of the Glacial Period has come to an end. 

 The gentler and diminished streams of a drier age have 

 no power to roll flint stones along and form beds of gravel. 

 Gravel terraces border our river valleys at a higher 



