4o6 



THE NATURE BOOK 



the wonderful zigzag gorge of the Zam- 

 besi below the \^ictoria Falls, where the 

 river courses between two vertical walls 

 of rock over 400 feet in height. 



It is difficult to convince the casual 

 observer that this mighty 

 not been riven asunder by 

 v^ulsion of Nature, and yet 

 followed down stream for 

 the slopes begin to recede, 

 take the form of an ordinary stream 

 valley. This only means that the banks 

 of the lower part of the ri\'er have been 

 subject to erosive influences for a longer 

 time than those nearer the Falls. Hence 

 it is impossible that the whole can repre- 



chasm has 

 some con- 

 when it is 

 some miles 

 and finally 



rho(o_:^raf'h by ('•. /i/m^'/ty, //eatiz>ri,'/<y. 



"THE BROAD OPEN V OF THE LOWER PART ONCE HAD THE 

 NARROWER, STEEPER SECTION OF THE MIDDLE PORTION OF 

 THE VALLEY." 



Hish Cup Nick. 



sent a gigantic rupture such as is popu- 

 larly supposed to have taken place. 



Ordinary stream valleys present just 

 such features, although of differing magni- 

 tudes — i.e. they are narrow and gorge- 

 like near their origins, widen gradually 

 down stream, and finally merge into the 

 outlines of the plain. 



Thus in their typical forms valleys of 

 this nature are \' -shaped when viewed 

 from above, and it is only when special 

 conditions intervene that they depart 

 from the type. 



Again, if taken in vertical section the 

 same V-shaped outlines should predom- 

 inate. The broad open V of the lower part 



once had the nar- 

 rower, steeper sec- 

 tion we see in the 

 middle portion of 

 the valley, and 

 this in turn was, 

 in former times, a 

 gorge like that 

 existing near the 

 source. So we 

 can appropriately 

 speak of these 

 different parts as 

 y o u n g, mature, 

 and old. Some- 

 times the V is not 

 symmetrical, but 

 shows a long limb 

 and a short limb. 

 Such a form re- 

 sults when the 

 beds are inclined, 

 or dip away from 

 the hor i z ont al 

 The longer limb 

 usually follows a 

 hard band of rock 

 which the stream 

 h a s encountered 

 during excavation. 

 In our studies 

 of r i \' e r s \\' e 

 found that they 

 s e Id o m keep a 

 straight course 

 but meander in 

 curves, the sizes 

 of which bear a 

 definite relation to 

 the volume of 



