502 



I. Bowman — A Buried Wall at Cttsco. 



water of the ravine to escape and erode the gravels, and by the 

 natural effects of continued cultivation and irrigation away from 

 the higher edge of a terrace. A wall not too deeply buried in 

 loose material with a pronounced surface slope is a defence 

 against excessive erosion of material above it ; it cannot pre- 

 vent the erosion of material below it. While tbe terraced 

 fields below the wall were gashed by the incipient ravines 

 of irrigation water, the slope above the wall remained practi- 



Fig. 5. 



Fig. 5. The buried wall is several feet behind the gravel bank on the 

 extreme left, looking south toward Cuzco. 



cally intact. The agreement of the gradients on opposite sides 

 of tbe wall as shown in fig. 1 is too close to be explained by 

 accident ; it can be explained only by assuming that both slopes 

 are part of a general surface built up during a period of aggra- 

 dation which has now given place to a period of stream dis- 

 section. 



It is now necessary to consider the work of individual floods 

 which may effect great changes in the minor slopes of the land 

 and in the position of loose surface material. A series of floods 

 in a succession of unusually wet years may lead to changes 

 which appear almost cataclysmic. But the changes may be 

 recognized as the work of sudden and unusual floods, (1) by 

 the coarseness of the material where coarse material is available 



