of the Cuzco Valley, Peru. 81 



XL Section of San Sebastian formation half a mile south 

 of San Geronimo railroad station. 



Dip to 2° North. 



Feet. 



1. Sand, fine, and scattered cross-bedded lenses of gravel 



overlain by gravel and bowlders forming the surface 



of a fan 40 



2. Limestone, white, very thin-bedded, containing abundant 



shells ; also impressions of plants in one adobe-like 

 band; bed persistent and cut by a number of que- 

 bradas in this vicinity 2 



3. Gravel lens; pebbles of sandstone, subangular, as much 



as 2 inches in diameter ; partly cemented 1 



4. Sand, fine, and adobe in alternating very thin beds .... 55 



5. Clay, drab, highly calcareous, containing plant roots, 



and shells 0.3 



6. Sand, fine, compact, quartz ; beds a fraction of an inch 



thick, with rare lenses of coarse sand and beds of 

 hard calcareous adobe 4 inches to 1 foot in thickness. 

 Beds of adobe project as shelves on quebrada walls 70 



168.3 



The area of sands, limestone, and adobe about half a square 

 mile in extent, forming the flats south of San Geronimo at 

 elevations between 10,650 and 11,000 feet, may be considered 

 part of a slope included in the coast and under-water shore of 

 ancient Lake Morkill, now much dissected. The slope of the 

 rock-surface valley side on which the materials were deposited 

 is about 20°. 



ITear the west end of ancient Lake Morkill, where the gentle 

 shore slopes were alternately flooded and exposed in response 

 to fluctuating volume of lake water, a number of sections were 

 measured. A characteristic exposure is shown in fig. 39, 

 and the following section is considered typical for this locality : 



XII. Section of San Sebastian formation in Chunchullu- 

 mayo quebrada, near junction with Rio Huatanay. 



Dip about %° East. 



Feet. 



1. Soil of adobe, dark red 2 



2. Limestone, white, thin-bedded; shells abundant 2 



3. Gravel, coarse and fine, well stratified; pebbles 2 to 3 



inches in diameter, rarely larger 5 



Unconformity. 



Am. Jour. Sci. — Fourth Series, Vol. XLT, No. 241. — January, 1916. 

 6 



