io8 



PHYSICAL GEOLOGY 



the two republics. Since no map was available when the boundary 

 was first fixed, this was stated as following the crest of the mountains, 

 as it was believed that this was permanent and was identical with 

 the divides between the Atlantic and Pacific rivers. Later a dispute 

 arose as to the exact boundary, and the survey made to settle the ques- 

 tion showed how inaccurate this belief was. It was found that the 

 Chilean rivers, with their short, steep routes to the Pacific Ocean, 

 had captured the upper courses of nearly all of the Argentine rivers, 

 obliging them to make sudden turns and to flow through the deep 

 gorges which lead to the Chilean coast. Another example of stream 



Fig. 93. — In this (third) diagram some of the subsequent streams are seen to 

 have cut back until they have captured part of the drainage of the parallel, consequent 

 streams, leaving wind gaps. When this region is reduced to base level it may again 

 have the appearance shown in Fig. 91. (Modified after Davis.) 



piracy is to be seen in the Kaaterskill Creek of New York, which, 

 because of its shorter course to the Hudson, has captured the lakes 

 at the headwaters of the Schoharie Creek which flows on a gentle 

 gradient by a circuitous route to the Mohawk River. Many of 

 the " wind gaps " (passes without streams flowing through them) 

 of the Blue Ridge in Virginia were eroded by streams flowing to 

 the sea, whose headwaters were captured by other streams which, 

 although following longer courses, were able, because of their greater 

 volume of water, to deepen their valleys more rapidly than those 

 flowing through these gaps. The Cumberland Gap, through which 

 passed many thousands of the early immigrants to Kentucky, has 

 such a history. 



( renditions favorable for river capture occur in regions of tilted beds 

 (Figs. 91 93) in which there is a marked difference in the strength of 

 the rocks. 'I lie larger branches follow the outcrops of the weaker beds, 



