of the Cuzco Valley, Peru. 71 



Origin and Age of the Huayllabamba Formation. 



The source of the materials composing the strata of the 

 Huayllabamba formation is unknown. The terranes whose 

 disintegration gave rise to the sediments were doubtless pre- 

 vailingly siliceous and must have included metamorphic rocks 

 and a variety of igneous intrusives. Many features of the 

 sediments indicate deposition by streams of fluctuating flow. 

 The prevailing red color of the beds and the presence of slightly 

 altered feldspars suggest an arid climate. On the evidence 

 derived from field observation a large part at least of the 

 Huayllabamba formation is believed to be of continental rather 

 than marine origin. It is interesting to note that in general 

 appearance as well as in several detailed features of composition 

 and structure the Huayllabamba strata are not unlike the 

 Newark sediments of Connecticut and- New Jersey. No fossils 

 other than impressions of plant fibers and doubtful footprints 

 of a small animal of the dinosaur type were found in the 

 Huayllabamba beds. The age of the strata is therefore 

 unknown. They underlie the Yucay limestone (Upper (?) Cre- 

 taceous) and are unlike the richly fossiliferous Carboniferous 

 (Pennsylvanian) strata of the Titicaca region. 25 The forma- 

 tion may be Permian, Triassic, or Jurassic, or it may include 

 strata from more than one Mesozoic system. In the present 

 report the Huayllabamba formation, including the Molle-Orcco 

 conglomerate, is assigned tentatively to the Jura-Trias. 



Yucay Fobmation. 



The plateau surface north of the city of Cuzco and including 

 the Yucay, Pucro, Kkallachaca, and Era pampas is developed 

 on limestone. The plateau is sharply bounded on the south 

 by steep cliffs facing Rio Huatanay; on the northwest, north, 

 and northeast the sandstone rim of Seneca continued through 

 Fortaleza and Seqqueray forms the bordering wall. The topog- 

 raphy of the pampa surface is subdued ; rounded knobs 

 separated by basin-like valleys are typical features (fig. .13). 

 Sink-holes are common, and at one point part of the roof of 

 an underground channel remains as a natural bridge. The 

 rock forming the plateau is typically developed on the Yucay 

 Pampa, and Yucay appears therefore to be a suitable name 

 for the calcareous beds of this area. 



"Gregory: Geologic sketch of Titicaca Island and adjoining areas, 

 lhis Journal, xxxvi, pp. 187-213, 1913. 



