of the Cuzco Valley, Peru. 61 



are assigned to- the same system (p. 71). Several thousand 

 feet of strata underlying the Pachatucsa beds, displayed in the 

 wall of the Urubamba canyon, have not been studied. 



HlTAYLLABAMBA FORMATION. 



Brown sandstone and chocolate-colored arenaceous shale are 

 by far the most common rocks in the Cuzco Valley. They 

 underlie the highlands south of the Huatanay and form the 

 north side of the valley of that stream from Tticapata eastward 

 to the Urubamba. The part of these widely extended arenace- 

 ous beds west of the Huancaro fault is included in the Qquilque 

 formation; 24 for the remaining much larger part the term 

 Huayllabamba formation is proposed. 



In addition to the typical strata of brown sandstone and 

 chocolate-colored shale which constitute about 90 per cent of 

 the Huayllabamba formation, it contains gray sandstone and 

 a few thin layers of black shale, gray shale and limestone. 

 Along the highlands forming the divide between the Huatanay 

 and the Apurimac drainage basins, gray and brown conglomer- 

 ates are so abundant as to justify separate grouping: the 

 Molle-Orcco conglomerate member. Throughout their extent 

 the strata of the Huayllabamba formation are disturbed; a 

 bewildering variety of normal and asymmetric folds are repre- 

 sented by truncated remnants and faults of small displacement 

 may be numbered by hundreds. The trends of the axes of 

 synclines and anticlines fall in general within the northwest 

 quadrant, with an average for the more prominent folds of 

 about 1ST. 50°-60° W. ; many fault planes strike in the same 

 direction. There are, however, numerous exceptions to the 

 general trend and a complete analysis of the stratigraphy 

 involves traverses separated by distances of a mile or less. 

 - In the time at my disposal I found it impossible to determine 

 the limits of the Huayllabamba formation, but typical sections 

 showing the character and arrangement of its strata were 

 measured at a number of places both north and south of the 

 Cuzco Valley axis. 



Huayllabamba Formation north of Rio Huatanay. 



III.. Section of the Huayllabamba formation: north side 

 of Angostura Narrows, beginning at the 13,000-foot contour 

 line and extending S. 50° W. down the slope. 



2i See p. 55. 



