Ayusbamba (Peru) Fossil Beds. 133 



The Ayusbamba Lake Area. 



General relations. — The Ayusbamba lake deposits cover 

 an area of about one-eighth of a square mile (see map, fig. 8), 

 situated in an unusual position, at the top of the pass traversed 

 by the trail from Ccoypa in the Mollemolle valley to Bilcokyma, 

 a tiny pueblo perched on a terrace within the Apurimac canyon. 

 The depression containing the fossil beds is walled in on the 

 east by sandstone ridges rising 800' above the center; the 

 ledges forming the west border are deeply buried in superficial 

 debris. At the north the lacustrine deposits are abruptly ter- 



Fig. 6. 















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Fig. 6. View of Ayusbamba fossil beds, looking south- southwest, toward 

 the pass into the Apurimac valley. Note the attitude of the strata. 



minated by a steep slope developed by vigorous headward 

 erosion of a tributary to the Chipura. Fringing the rock bor- 

 ders on the west, south, and east are slopes of surface wash and 

 fans more or less dissected, which in turn overlap the clays of 

 the central part of the depression. 



The surface of the lacustrine deposits (fig. 6 and map, fig. 8) 

 slopes northwestward, and the streams which drain the area 

 unite to form an outlet channel which, after passing through a 

 narrow, vertically walled gorge 30 feet deep, makes an abrupt 

 descent by a series of rapids and waterfalls to join the Chipura 

 and finally the Apurimac. A much shorter course with much 



