140 H. E. Gregory — Ayushamba Fossil Beds. 



Origin and character of Lake Ayusljamha. — From the sec- 

 tions and descriptions given above it is evident that we are 

 dealing- with deposits which in part are truly lacustrine, — 

 material laid down in a body of quiet water which existed for 

 a relatively long period of time. On the assumption that each 

 layer of sand and of clay represents the amount deposited 

 during a single rainy season, approximately 100,000 years 

 would be required for the accumulation of the materials ex- 

 posed in the present fragmentary sections. It is probable that 

 the lake beds had greater thickness. That they formerly 

 extended much farther northward is shown by an unprotected, 

 truncated section including 185 feet of strata perched high on 

 the valley side overlooking the Chipura river. The containing 

 wall of rock is complete except on the north side, where it has 

 been entirely removed. Unlike the remaining portions of the 

 rim, the north wall probably consisted of fluvial and glacial 

 debris washed from the highlands. Moraines extend to the 

 edge of the present lake deposits and may have formed the 

 original barrier. During the life of the lake, fans from the 

 high ridge at the south encroached upon its waters and sepa- 

 rated the original sheet into more or less detached bogs. This 

 process, combined with the development of a channel through 

 the unconsolidated northern barrier, led to the extinction of 

 the water body. Judging from the physical data at hand, this 

 mountain tarn may have beautified the landscape of the late 

 Pliocene or any portion of the early Pleistocene epoch, — a con- 

 clusion which is in harmony with the paleontological evidence.* 

 Ancient Lake Ayusbamba is not an isolated case of extinct 

 water bodies in Peru. Duenasf speaks of similar deposits near 

 the pueblo of Paruro, and the unpublished reports of engi- 

 neers and travelers indicate the existence of unexplored Tertiary 

 and Pleistocene deposits scattered over the Andean highlands. 



* See the following article by Dr. George F. Eaton, pp. 141 to 154. 

 f Loc. cit. 



