I. Boioman — A Biwied Wall at Cuzco. 



503 



and (2) by the immediate change from aggradation to degrada- 

 tion as soon as the floods cease. Now in the ravine in point 

 the gravels are clearly coarse below, but they become finer 

 above and the top of the section in some places exhibits finely 

 laminated sand. Moreover, the mass of material with which 

 the alluvium of the ravine is correlated is so great, from a few 

 feet to 50 feet thick in places, and so widely distributed in the 

 Cuzco basin, as to give it a much greater importance than that 

 associated with occasional floods. 



Fig. 6. 





Fig. 6. Relation of the buried wall to the gravelly alluvium. A, the 

 undisturbed wall ; B, the gravel above the wall, to the right undisturbed, to 

 the left removed since the gravel here faced the wall in addition to lying 

 above and behind it ; C, the outer layer of faced stone removed to show 

 filling of rubble ; D, the inner layer of faced stone removed and undisturbed 

 stratified alluvium showing behind it ; E, the inner face set up roughly stone 

 by stone. The bottom of the wall lies three feet below the present channel 

 floor in the shaded foreground of the photograph. 



All about the Cuzco basin the lower slopes are composed of 

 land waste, a great alluvial fringe or piedmont belt of alluvium 

 which extends up the tributary valleys. In some cases the 



with glacial material 



older alluvium interlocks or interfin°;er 



