32 



II. E. Gregory — A Geologic Beconnaissance 



miles on the north by a single stratum of coarse sandstone of 

 high dip, from which the stream has stripped the less resistant 

 rocks to form a deep gorge less than 100 feet in width. The 

 selection of weaker rock for channels is strikingly shown at 

 the headwaters of this stream, where vertical strata of con- 

 glomeratic sandstone stand like teeth in a comb ; between the 



Fig. 22. 



Fig. 22. Valley of Rio Chchocco at its junction with the Huancaro. 



teeth are valleys, 4, 6, and 20 feet wide and 10 to 40 feet deep, 

 worn from the less durable beds of shaly calcareous sandstone. 

 The east branch of Rio Kkaira, 2 miles long, draining the 

 back slopes of Sencco-Orcco and Maho-Pinto, also occupies a 

 subsequent valley and when viewed from a favorable point 

 presents an unusually fine example of a valley developed by 

 stripping the back slopes of a hogback (fig. 23). The Lkalla- 

 Chaca has found a favorable location at the contact between 



