MARSTERS, PHYSIOGRAPHY OF THE PERUVIAN ANDES 241 



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as the Eio Grande and the Ocoha have succeeded 

 in cutting through, not only the soft lava referred 

 to above and the underlying sediments, but also 

 have deeply incised themselves in the underlying 

 pre-Tertiary formations of the partly buried foot- 

 hills, or, more accurately stated, have cut through 

 the coastal plain formations until in the Valley 

 of Chorunga the line of erosion adjusted itself to 

 a pre-Tertiary topography. That is to say, the 

 Chorunga simply cleaned out the mass of sedi- 

 ments and sheet of lava which had been dumped 

 into it during Tertiary time. This is evidenced 

 by the patches of lava that still hang in the little 

 recesses on the southwest slope at various points, 

 between the Eio Grande and the Andaray cuesta.. 

 Ascending the Andaray cuesta, we go but a. 

 short distance up the slope before we again see 

 remnants of the familiar lava sheet of the CunO' 

 cuno plain, still nestling on the slopes and in the 

 protected valleys of the uncovered foothills lead- 

 ing up to the base of the Cora Puno domes. At 

 Andaray and Yanaquiqua, we can see the same 

 facts well illustrated. Andaray is located in a 

 depression in the foothills, the bottom of which 

 is still in part occupied by the light-colored lava. 

 From Andaray, we pass up another steep slope, 

 or series of slopes, composed in part of pre-Ter- 

 tiary rocks, only to find that we are again on a 

 somewhat dissected plain which extends away to 

 the east in the direction of Chuquibamba and 

 north in the direction of the picturesque Cora 

 Puno. Lithologically, this is the same type of 

 rock as that which we have already seen in the 

 Chorunga Valley, the Andaray cuesta and the 

 inner edge of the coastal plain. It is my belief 

 that the plains at the base of Cora Puno group 

 of domes are but parts of the same lava plain that 

 we have already recognized in a portion of the 

 Cuno cuno desert. Continuing our journey to 

 the north, we come to the somewhat rugged, 

 jagged edge of a still higher plain that is made 



