396 I. Bowman — Physiography of the Central Andes. 



matically developed to a region where the extraordinarily rapid 

 normal dissection had produced equivalent discordances, equiva- 

 lent but not similar. The distinctive features of the hanging 

 valleys of glaciated regions are as unlike those due to normal 

 but super-vigorous erosion as could possibly be imagined, save 

 for this one quality of discordant junction of tributary and 

 master stream. 



§§ :v The distribution of these hanging tributaries, indeed in large 

 measure their very existence, is controlled by geologic struc- 

 ture. Stream JB, fig. 27, is a hanging lateral in the Juntas 



Fig. 27. 



Fig. 27. Sketch of hanging valley relationships in the Juntas valley 

 Eastern Bolivia. 



valley twenty miles within the eastern border of the Andes. 

 Its small tributaries above X are cut in soft shales. After 

 running two or three miles or more in the shale belt, they turn 

 through slate and quartzite of superior hardness which dips 

 often as steeply as 50°, and occasionally 60°. A waterfall or 

 a series of waterfalls occurs down the dip of slate or quartzite. 

 The hanging part of the tributary valley upheld by the thick 

 layer of highly resistant schist is, in some cases, 1000 to 2000 

 ft. above the bottom of the main valley. One is scarcely ever 

 out of sight of one of these in a whole day's ride. On the 

 other side of the valley the rapid erosion of the master valley, 

 itself in a belt of shales, has under-cut the rock so rapidly that 

 the tributaries are cut off, as Eussell has described certain 

 similar valley tributaries.* Exceedingly complex structure, 

 strong dip, and sharp alternations of hard and soft rock com- 

 *Bull. Geol. Soc. Am., vol. xvi, pp. 75-90, 1905. 



