501 /. Bowman — A Buried Wall at Cuzco. 



disposed in the form of terminal moraines or coarse oiitwash 

 from moraines, and therefore belongs to the glacial series. 

 The alluvium consists of a finer series below and a coarser 

 series above. When studied in detail the deposits indicate two 

 periods of glaciation. The alluvium of whatever kind has been 

 eroded periodically, giving rise to a series of terraces along the 

 streams. These vary in number according to the headwater 

 relations of the draining streams ; they also vary in width and 

 breadth and somewhat in their position with reference to the 

 valley mouths. In this connection it is sufficient to note that 

 the alluvial fringe in which the buried wall occurs belongs to 

 the last series of deposits which represent one of the last phases 

 of broad climatic changes which culminated in the glacial 

 period and have since declined, and is intermediate time 

 between the climax of the glacial period and to-day. It was 

 produced during one of the later and feebler climatic pulsa- 

 tions which did not lead to glaciation though they tended in 

 that direction. If the last glacial invasion occurred 50,000 

 years ago, the lower terraces might be said to be formed at 

 intervals about 30,000 and 10,000 years ago. If we accept 

 25,000 years as the length of the post-glacial interval, then 

 15,000 and 5,000 years would represent the age of the upper 

 and lower terraces respectively, as very roughly approximated 

 from the thickness of the deposits and the depth of erosion. 



The lack of correspondence between the grade of the allu- 

 vium and the grade of the wall forms one of the strongest rea- 

 sons for concluding that the wall has not been sunk into the 

 alluvium, but has been covered by it. Fig. 2 indicates but one 

 element of the alluvial slope; the second element is a south- 

 ward descent in the direction of the wall, though with a steeper 

 gradient. If a trench had been dug and the wall sunk into 

 or built into it, it is clear that the same relations might prevail 

 though it is unreasonable that they should prevail, unless the 

 trench were dug in the bottom of a natural water channel 

 eroded in alluvial material in process of dissection. The dis- 

 section would dispose the material on a flatter gradient, that 

 is, it would decrease the gradient of the channel as compared 

 with the gradient of the original alluvial slope. A Wall then 

 built into the channel would likewise have a flatter gradient 

 than the alluvial slope above. The channel can not be natural 

 however since it runs not down a slope but along it. 



On the other hand, our reasoning is entirely in accord with 

 the facts if we suppose the wall to have been built before the 

 last period of alluviation, and with a gradient conforming to 

 the slope of the surface. A later climatic change would then 

 account for the beginning of aggradation and the increased 



