of the Cuzco Valley, Peru. 49 



is confined largely to transporting gravel from the head of the 

 fan to its lower margin, where deposition is progressing to such 

 an extent that many ancient fields have been buried. 



At Angostura Narrows the Ayahuaycco and Llampuhuayeco, 

 rising in the Bambanusa Pampa, aided by streams from 

 Sencco-Orcco, deposited gravels in the pregiacial gorge, com- 

 pletely blocking the channel. On the north side of the valley 

 much of this gravel remains as a dissected terrace whose front 

 is over 200 feet in height and whose top forms a table at an 

 elevation of 11,100 feet. 



The assignment of the piedmont gravels of the Cuzco Valley 

 to late Pleistocene time rests on the following observations : 



(1) The deposits were obviously not laid down under pres- 

 ent conditions, for they are now undergoing dissection. Their 

 interpretation demands the assumption of climatic and soil 

 conditions favorable for vigorous erosion of waste-cloaked upper 

 slopes and rapid aggradation of lower slopes and valley floors. 

 The inference that Pleistocene time provided these conditions 

 for most localities is supported by world-wide observations. 



(2) When traced up valleys in which the deposits have not 

 been obliterated by erosion, the gravels are found to interlock 

 with materials whose glacial origin is unquestioned. 



(3) As regards degree of freshness of material and amount 

 of dissection the Cuzco gravels differ in no essential from pied- 

 mont deposits of Switzerland, the Sierra Nevada, and the 

 Rocky Mountains, whose Pleistocene age has been demonstrated. 



(4) The close association of the piedmont gravels with well- 

 defined glacial remains, the shallow weathering of the deposits, 

 and the insignificant amount of more recent waste overlying 

 the fans and terraces support the belief that the gravels are 

 contemporaneous with the moraines and cirques of the highlands 

 and belong to the latest period of glaciation represented in the 

 Andes, an ice age which may tentatively be correlated with 

 the late Wisconsin epoch of North America. Patches of an 

 older till-like material beneath the moraines of the Bambanusa 

 Pampa and beds of poorly sorted gravel interstratified with 

 the lacustrine deposits in the Cuzco Basin suggest that sedi- 

 ments laid down during an earlier cycle of glaciation are pre- 

 served in the Cuzco region. The evidence for this assumption 

 is, however, inconclusive. 



Postglacial erosion of gravels. — With the retreat of the ice 

 from the Cuzco Valley the removal of glacial deposits began. 

 Streams which formerly united to build a fringe of gravel fans 



Am. Jour. Sci.— Fourth Series, Vol. XLI, No. 241.— January, 1916. 

 4 



