PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 251 



coarse and fine sandstone beds. Such alternations of character in 

 sedimentary rocks are commonly marked by alternating shales 

 and sandstones, but in this locality shales are practically absent. 

 Toward the top of the section gypsum deposits again appear first 

 as beds and later, as in the case of the hill-slope on the southern 

 shore of Lake Huaipo, as veins and irregular masses of gypsum. 

 The top of the deformed Cretaceous (?) is eroded and again cov- 

 ered unconformably by practically flat-lying Tertiary deposits. 



TERTIABY 



The Tertiary deposits of the region under discussion are 

 limited to three regions: (1) the extreme eastern border of the 

 main Cordillera, (2) intermontane basins, the largest and most 

 important of which are (a) the Cuzco basin and (b) the Titicaca- 

 Poopo basin on the Peruvian-Bolivian frontier, and (3) in the 

 west-coast desert and in places upon the huge terraces that form 

 a striking feature of the topography of the coast of Peru. 



It has already been pointed out that the eastern border of the 

 Cordillera is marked by a fault of great but undetermined throw, 

 whose topographic importance may be estimated from the fact 

 that even after prolonged erosion it stands nearly four thousand 

 feet high. Cross-bedded and ripple-marked features and small 

 lenses of conglomerate are common. The beds now dip at an 

 angle approximately 20° to 50° northward at the base of the scarp, 

 but have decreasing dip as they extend farther north and east. 

 It is noteworthy that the deposits become distinctly conglomeratic 

 as flatter dips are attained, and that there seems to have been a 

 steady accumulation of detrital material from the mountains for 

 a long period, since the deposits pass in unbroken succession from 

 the highly indurated and massive beds of the mountain base to 

 loose conglomerates that now weather down much like an ordi- 

 nary gravel bank. In a few places just below the mouth of the 

 Ticumpinea, logs about six inches in diameter were observed 

 embeded in the deposits, but these belong distinctly to the upper 

 horizons. 



The border deposits, though they vary in dip from nearly flat 



