252 THE ANDES OF SOUTHERN PERU 



to 50°, are everywhere somewhat inclined and now lie np to sev- 

 eral hundred feet above the level of the Urubamba Eiver. Their 

 upper surface is moderately dissected, the degree of dissection be- 

 ing most pronounced where the dips are steepest and the height 

 greatest. In fact, the attitude of the deposits and their progres- 

 sive change in character point toward, if they do not actually 

 prove, the steady and progressive character of the beds first de- 

 posited and their erosion and redeposition in beds now higher in 

 the series. 



Upon the eroded upper surfaces of the inclined border de- 

 posits, gravel beds have been laid which, from evidence discussed 

 in a later paragraph, are without doubt referable to the Pleis- 

 tocene. These in turn are now dissected. They do not extend to 

 the highest summits of the deformed beds but are confined, so 

 far as observations have gone, to elevations about one hundred 

 feet above the river. From the evidence that the overlying hori- 

 zontal beds are Pleistocene, the thick, inclined beds are referred 

 to Tertiary age, though they are nowhere fossiliferous. 



Observations along the Urubamba River were extended as far 

 northward as the mouth of the Timpia, one of the larger tribu- 

 taries. Upon returning from this point by land a wide view of 

 the country was gained from the four-thousand-foot ridge of 

 vertical Carboniferous limestone, in which it appeared that low 

 and irregular strike ridges continue the features of the Tertiary 

 displayed along the mountain front far northward as well as east- 

 ward, to a point where the higher ridges and low mountains of 

 older rock again appear — the last outliers of the Andean system 

 in Peru. Unfortunately time enough was not available for an ex- 

 tension of the trip to these localities whose geologic characters 

 still remain entirely unknown. From the topographic aspects of 

 the country, it is, however, reasonably certain that the whole in- 

 tervening depression between these outlying ranges and the 

 border of the main Cordillera, is filled with inclined and now dis- 

 sected and partly covered Tertiary strata. The elevation of the 

 upper surface does not, however, remain the same; it appears to 

 decrease steadily and the youngest Tertiary strata disappear 



