of the Cuzco Valley, Peru. 17 



are grouped on the southwest flank of the Pachatucsa and 

 Atasccasa mountains and are best developed about the head 

 of Rio Huaccoto. In this region are ten or more lakes occupy- 

 ing kettle holes or perched on the inner slopes of abandoned 

 moraines. All of them are small ; the largest water body seen 

 covers less than 2 acres and the average area is but little more 

 than half an acre. The depth of water in the lakes examined 

 did not exceed 6 feet, but water marks at heights of 4 to 10 

 feet above the surface indicate considerable fluctuation in 

 response to rainfall. The number of water bodies which have 

 been converted into swamps and are now occupied by Yureta, 

 the chief peat-maker of southern Peru, exceeds the number of 

 existing lakes. 



Solution depressions 25 to 200 feet in diameter dot the 

 surface of the Yucay, Kkallachaca, and adjoining' pampas. A 

 few contain water throughout the year; many remain as bogs 

 through the dry season, and high-water marks indicate a large 

 number of water bodies whose life is limited to the season of 

 summer rains. 



II. PHYSIOGRAPHY. 



Regional Relations. 



From a physiographic standpoint Peru is essentially a 

 plateau included between a narrow coastal belt along the Pacific 

 Ocean and the low-lying Amazon Plains. As shown by the 

 regional profile (fig. 3), the surface of the plateau stands 

 between elevations of 13,000 and 15,000 feet, and its borders 

 are sharply marked. Along the line of the railway from 

 Mollendo to La Paz the Peruvian plateau is broken into two 

 segments by the great interior basin holding the waters of 

 Lake Titicaca. The two highlands thus formed in southern 

 Peru may be appropriately termed the eastern plateau and the 

 western plateau, occupying respectively about r /z and 2 /z of 

 the area included in the Andes Mountains. In northern Peru 

 the highland area narrows and the plateaus are merged; like- 

 wise in the latitude of Cuzco the Peruvian plateau is essentially 

 a physiographic unit. As indicated on the diagram (fig. 9) 

 the southwest flank of the plateau is drained by numerous 

 short streams directly tributary to the Pacific Ocean. Head- 

 ing in valleys of gentle gradient, these watercourses descend 

 rapidly to the coastal plain, where slopes of small inclination 

 are again encountered. Although these streams have cut deep 



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

 9, 



