192 THE ANDES OF SOUTHERN PERU 



formation of the slopes the Andes have been uplifted at least a 

 mile, or, to put it in another way, the Andes at the time of forma- 

 tion of the mature slopes were at least a mile lower than they are 

 at present. 



Further proof of recent and great uplift is afforded by the 

 deeply intrenched streams. After descending the long graded 

 slopes one comes upon the cliffed canyons with a feeling of con- 

 sternation. The effect of powerful erosion, incident upon uplift, 

 is heightened by the ungraded character of the river bed. Falls 

 and rapids abound, the river profiles suggest tumultuous descents, 

 and much time will elapse before the river beds have the regular 

 and moderate gradients of the streams draining the mature sur- 

 face before uplift as shown in the profiles by the dotted lines rep- 

 resenting the restored valley floors of the older cycle. Since the 

 smooth-contoured landscape was formed great changes have taken 

 place. The streams have changed from completely graded to al- 

 most completely ungraded profiles; in place of a subdued land- 

 scape we now have upland slopes intersected by mile-deep can- 

 yons; the high-level slopes could not have been formed under 

 existing conditions, for they are being dissected by the present 

 streams. 



Since the slopes of the land in general undergo progressive 

 changes in the direction of flatter gradients during a given geo- 

 graphical cycle, it follows that with the termination of one cycle 

 and the beginning of another, two sets of slopes will exist and that 

 the gradients of the two will be unlike. The result is a break in 

 the descent of the slopes from high to low levels to which the name 

 "topographic unconformity" is now applied. It will be a promi- 

 nent feature of the landscape if the higher, older, and flatter gradi- 

 ents have but little declivity, and the gradients of the lower 

 younger slopes are very steep. In those places where the relief 

 of the first cycle was still great at the time of uplift, the erosion 

 forms of the second cycle may not be differentiated from those of 

 the first, since both are marked by steep gradients. In the Cen- 

 tral Andes the change in gradient between the higher and lower 

 slopes is generally well marked. It occurs at variable heights 



