THE WORK OF STREAMS 



II 



courses in spite of differential elevations, as has been seen (p. 102), 

 are called antecedent streams. 



If a region underlain by tilted rocks which vary in composition, 

 some resisting erosion more than others, is reduced to base level and 

 then raised, the sub- 

 sequent erosion is ^*58&» 

 such as to give cer- I 



tain proof of its \ 



earlier history. An Ml \ 



interesting example, 

 in which, however, 

 the river encountered 

 granite (Fig. 100 A 

 and B) rather than 

 tilted rock, is found 

 in the history of the 

 Gunnison River in 

 Colorado. When the 

 Rocky Mountains 

 were being uplifted 

 to their present posi- 

 tion, the streams 

 which now drain 

 them began to cut 

 theirvalleys. Among 

 them the Gunnison 

 River followed along 

 the depression of the 

 plateau and began to 

 deepen its bed. Its 

 course happened to 

 lie over a great mass 

 of granite, buried be- 

 neath softer strata. 

 The river, having 

 a steep gradient, 

 rapidly cut its way through the soft surface rocks and finally 

 encountered the granite. Since its valley was already deep when 

 this occurred, it was unable to turn aside from the hard rock and 

 continued to cut its way through it until the picturesque Black 



nz 



a 



b 



B 



Fig. 100. — Two block diagrams showing the effect of 

 erosion upon resistant and weak rocks. The streams in A 

 have approximately the same slope and are deepening 

 their valleys in strata of the same kind. B shows that 

 the stream on the right encountered resistant granite 

 which was both eroded and weathered more slowly than 

 the weaker rock. As a consequence, the stream on the 

 right has cut a deep and steep-sided gorge, while that on 

 the left has cut a broad valley with gently sloping sides. 



