66 II. E. Gregory — A Geologic Reconnaissance 



Huayllabamba Formation south of Bin Huatanay. 



On the south side of the Cuzco Valley, beds assigned to the 

 Huavllabamba formation were studied at several localities. 

 The rocks forming the deeply carved slopes bordering the 

 Huatanay east of Rio Huancaro are brown sandstone and 

 chocolate-colored shales which differ in no essential particular 

 from the strata of the Huayllabamba formation previously 

 described. On the highlands south of Saylla few of the alter- 

 nating beds of arenaceous shale and fine-grained brown sand- 

 stone exceed 1 foot in thickness on the lower north slopes. 

 Above 11,500 feet the strata thicken, and the ragged serrate 

 crest of Aiaho-Pinta, at about 13,500 feet, consists of beds of 

 sandstone in nearly vertical position, 1 to 20 feet in thickness. 

 Between the sinuous rows of teeth are trenehlike passages 

 resulting from the erosion of the less resistant shale. One mile 

 farther south the tiny streams forming the headwaters of Rio 

 Kkaira follow the strike of shale beds and are inclosed between 

 unevenly spaced walls of brown and gray sandstone 20 to 80 

 feet in height. Approximately 80 per cent of the sandstone 

 between the Huatanay and Rio Uspha is brown in tone and 

 fine to medium in texture. Gray sandstone is irregularly dis- 

 tributed and in common with the more abundant brown rock 

 contains lenses of conglomerate consisting of fragments of 

 chocolate, red, rarely green clay shales. At a point on the 

 middle course of Rio Huanacauri several beds of gray and pink 

 limestone, of small extent but sufficiently thick for quarrying, 

 were noted among the prevailing arenaceous strata. At a num- 

 ber of localities the surfaces of shale and of thin sandstone 

 beds are sun-baked and cracked and exhibit other features 

 indicative of intermittent exposure to the atmosphere. 



Molle-Orcco conglomerate member.- — On the highlands form- 

 ing the divide between the Huatanay and the Apurimac 

 drainage basins the typical sandstones and shales of the Huayl- 

 labamba formation grade upward into a series of beds includ- 

 ing many strata of a coarse conglomerate. This phase of the 

 formation is well displayed at Molle-Orcco. The conglomerate 

 beds, involved in a series of intricate folds, form many high 

 ridges and have determined the courses of the smaller streams. 

 Where in vertical position they rise above the surface as 

 a series of parallel dikes, producing an extremely rough 

 topography (fig. 31). 



The proportion of conglomerate to sandstone in the Molle- 

 Orcco member is variable. At Ccochacolla a succession of 



