of the Cuzco Valley, Peru. 85 



Origin and Age of the San Sebastian Formation. 



Strata of sand and of "tripolite" at various localities in 

 the Department of Cuzco, probably including San Sebastian, 

 were noted by Duenas, 27 who offers two hypotheses to account 

 for their presence — (1) that the ocean of Tertiary time 

 covered the Andes Mountains ; (2) that Lake Titicaca formerly 

 extended northwestward into the Apurimac and Urubamba 

 valleys — an assumption which appeared to Duehas as the more 

 probable. The San Sebastian beds are, however, local deposits, 

 and their composition and stratigraphic arrangement indicate 

 lacustrine origin. Quiet, fresh water is required for the 

 deposition of pure-white limestone containing abundant shells 

 of Physa, Sphserium, and Planorbis. Some of the clay beds 

 also demand quiet water for their accumulation, and the long- 

 root tubes in the compact silt and adobe, as well as impressions 

 of reeds and layers of earthy lignite, suggest shores or delta 

 flats. Lenses of gravel, channeled beds, and ancient soils indi- 

 cate that the lake was playa-like in character, or at least suf- 

 fered fluctuations in volume sufficient to expose and cover the 

 shore slopes alternately. Fossils occur in abundance, and it 

 is keenly disappointing to learn from Dr. Dall and Professor 

 Schuchert, who kindly examined my collections, that the shells 

 belong to species which range from the Jura to the present. 

 The plant fragments, also, are unfortunately not diagnostic. 

 In the absence of paleontologic evidence the age must be deter- 

 mined by the less direct methods of physiography. The prob- 

 lem is to interpret the relations existing between the lacustrine 

 deposits and the glacial gravels of the Cuzco region. On the 

 basis of such study the San Sebastian formation is assigned 

 to late Pliocene or early Pleistocene time. This conclusion is 

 fairly satisfactory from the physiographer's standpoint and 

 conflicts in no way with the paleontologic evidence. 



It may be noted that the age of the San Sebastian beds is 

 the same as that assigned to the deposits at Ayusbamba, where 

 vertebrate fossils were obtained. 28 



Pleistocene and Recent Formation's. 



Deposits of glacial origin, including moraines, gravel fans, 

 and lacustrine sediments, are widely distributed within and 

 on the borders of the Cuzco Valley. These are discussed under 



27 Op. eit., p. 24. 



28 Eaton, G. F., Vertebrate fossils from Ayusbamba, Peru, this Journal, 

 xxxvii, pp. 141-154, 1914. 



