CHAPTER XI 



THE STORY OF THE ISLAND RIVERS ; AND HOW 

 THE ISLE OF WIGHT BECAME AN ISLAND 



WE must now consider the history of the river system of 

 the Isle of Wight, to which our study of the gravels has 

 brought us. For rivers have a history, sometimes a most 

 interesting one, which carries us back far into the past. 

 Even the little rivers of the Isle of Wight may be truly 

 called ancient rivers. For though recent in comparison 

 with the ages of geological time, they are of a vast an- 

 tiquity compared with the historical periods of human 

 history. 



To understand our river systems we must go back to 

 the time when strata formed by deposit of sediment in 

 the sea were upheaved above the sea level. To take the 

 simplest case, that of a single anticlinal axis fading off 

 gradually at each end, we shall have a sort of turtle back 

 of land emerged from the sea, as in figure 6, aa being 

 the anticlinal axis. From this ridge streams will run 

 down on either side in the direction of the dip, their course 

 being determined by some minor folds of the strata, or 

 difference of hardness in the surface, or cracks formed 

 during elevation. On each side of the dip-streams smaller 

 ones will flow, more or less in the direction of the strike, 

 and run into the main streams. Various irregularities, 

 such as started the flow of the streams, will favour one or 

 another. Consider three streams, a, b, c, and let us sup- 

 pose the middle one the strongest, with greatest flow of 

 water, and cutting down its bed most rapidly. Its side 

 streams will become steeper and have more erosive force, 



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