THE STORY OF THE ISLAND RIVERS 87 



and so will eat back their courses most rapidly until they 

 strike the line of the streams on either side. Their steeper 

 channels will then offer the best way for the upper waters 

 of the streams they have cut to reach the sea ; and these 

 streams will consequently be tapped, and their head 

 waters cut off to flow to the channel of the centre stream. 

 We shall thus have for a second stage in the history a 

 system such as is shown in fig. 7. The same process will 

 continue till one river has tapped several others ; and 

 there will result the usual figure of a river and its 

 tributaries, to which we are accustomed on our maps. 

 We shall observe that tributaries do not as a rule gradually 

 approach the central stream, but suddenly turn off at 

 nearly a right angle from the direction in which they are 

 flowing, and, after a longer or shorter course, join at 

 another sharp angle a river flowing more or less parallel 

 to their original direction. 



The Chalk and overlying Tertiary strata were uplifted 

 from the sea in great folds forming a series of such turtle- 

 backs as we have been considering. The line of upheaval 

 was not south-west and north-east, as that which raised 

 the older formations in bands across England, but took 

 place in an east and west direction. The main upheaval 

 was that of the great Wealden anticline. Other folds 

 produced the Sandown and Brook anticlines, and that 

 of the Portsdown Hills. The upheaval seemed to have 

 been caused by pressure acting from the south, for the 

 steeper slope of each fold is on the northern side. Our 

 latest Oligocene strata are tilted with the chalk, showing 

 that the upheaval took place after Oligocene times. But 

 the great movement was in the main earlier than the 

 Pliocene. For on the North Downs near Lenham is a 

 patch of Lower Pliocene deposit resting directly on the 

 Chalk, the older Tertiary strata having been removed by 

 denudation, clearly due to the uplift of the Wealden 

 anticline. The raising of the Pliocene deposit to its 



