of the Cuzco Valley, Peru. 



23 



of features indicative of mature and youthful dissection. 

 Speaking broadly the mountainous masses of the Cuzco Valley 

 drainage system may be said to have attained maturity. For 

 large areas regional slopes range between 15° and 20°, and 

 slopes of 30° are not uncommon. ISTo mesas exist among the 

 mountains, and undrained interstream spaces are few and 

 small in area. 



The unrestricted run-off involved in maturity of mountain 

 form, when expressed in terms of human environment, means 



Fig. 11. 



Fig. 11. Plateau surface two miles south of Oropesa looking south- 

 east. A. H. Bumstead, photo. 



unstable soil. The Incas overcame this difficulty in the more 

 favored localities by terracing the slopes and building retaining 

 walls at an enormous expenditure of time and labor. So skil- 

 fully were these masonry structures made that many of them 

 stand to-day in series of ten or twenty, in places over fifty, 

 perched on slopes with inclinations of 10° to 30°, and extend- 

 ing to elevations 800 to 1,000 feet above the valley floors. 

 The present inhabitants find it impracticable to maintain the 

 steeper slopes in tilth, and the agents of erosion have been given 

 new life. 



