78 H. E. Gregory — A Geologic Reconnaissance 



shale. The beds, broken by faults and intruded by basic 

 rock, strike west to J$. 70° W. and dip to the south at angles 

 of 30° to 40°. The bedding planes of both shale and sandstone 

 present firm, glistening surfaces traversed by mud cracks, 

 ripple marks, and worm trails ( ?) and are dotted with irregu- 

 larly shaped fragments of mud shale. For a distance of about 

 a mile south of Huambutio station, partly decomposed strata 

 of limestone and gypsum in approximately horizonal attitude 

 replace sandstone and shale. The limestone is blue-gray or 

 banded with brown, and both massive and laminated beds are 

 present. The gypsum, which is dark blue and white in color, 

 is displayed as lenses and furnishes elaborately carved oblong 

 and cubical blocks of the talus. A large part at least of the 

 gypsum is secondary in origin and occupies joints in the 

 limestone. 



The canyon of the Urubamba along the railroad between 

 Sierra-Bella and Cay-cay is cut in brown sandstone interbedded 

 with shale and lenses of conglomerate composed of imperfectly 

 rounded pebbles of quartz, sandstone, and bright-red shale. 



San Sebastian Formation. 



The Cuzco Basin northward from Angostura Narrows is 

 inclosed by a low interrupted wall of poorly consolidated 

 sediments. These beds are well displayed both north and south 

 of the village of San Sebastian, from which the name of the 

 formation is derived. On the south side of the valley eastward 

 from the mouth of the Huancaro the strata of the San Sebas- 

 tian formation are practically horizontal and form conspicuous 

 white banded cliffs along Bio Huatanay (fig. 32) ; along the 

 northern rim, in the vicinity of San Sebastian, the strata dip 

 gently to the south and merge with the alluvium of the main 

 valley floor. The sediments include sand, limestone, peat, and 

 adobe, with small amounts of gravel, and were deposited within 

 and on the immediate shores of ancient Lake Morkill. 26 The 

 nature of these lacustrine deposits is shown in the following 

 typical sections : 



IX. Section of San Sebastian formation on Rio Huancaro 

 near its junction with the Huatana3 r . 



20 See p. 34. 



