34 H. E. Gregory — A Geologic Reconnaissance 



with weak cement, but tlieir influence in guiding streams has 

 in large part been offset by the steep inclination and complicated 

 folding of the strata, and no dominant control of jointing in val- 

 ley development is obvious. In a minor way, however, the influ- 

 ence of joints is evident. In three rock canyons where joint 

 direction and spacing were studied the streams were observed 

 to zigzag along the narrow floor, following the larger joints, 

 particularly at waterfalls. In several places, however, a stream 

 has gnawed a groove across a ledge, disregarding the joints 

 near at hand. In spite of the abundance and weakness of 

 joints in the. rocks of the Cuzco Valley, their influence on 

 stream alignment is less than in most other regions which have 

 come under my observation, and it appears that control of 

 streams by joints is minimized (1) in steeply inclined strata; 



(2) in thin-bedded strata of markedly different hardness, in 

 which erosion of weaker beds is easier than widening of joints ; 



(3) in narrow, youthful canyons of steep gradient, where 

 streams once established maintain their courses regardless of 

 structure. 



Valleys in unconsolidated deposits. — One of the prominent 

 features of the Cuzco Valley is the large number of canyons 

 and quebradas, cut in gravel and stratified sands. Tributary 

 to the Huatanay at Cuzco are four canyons each exceeding 

 150 feet in depth, more than a quarter of a mile in length, 

 and not over 1,000 feet in width. They are cut entirely in 

 gravel and have almost vertical walls (fig. 24). On the edge 

 of the limestone plateau at San Sebastian and traversing the 

 fans bordering the valley are many similar examples. That 

 these valleys should have retained their extremely youthful 

 form since Pleistocene time indicates rather exceptional 

 ground-water conditions and also alluvial deposits of unusual 

 compactness. (See pp. 46-51.) 



Lake Morkill. 



The lacustrine strata constituting the San Sebastian forma- 

 tion 1 2a marks the position of a lake which formerly occupied 

 the Cuzco Basin — an ancient water-body whose surface lay 

 well above the present site of San Sebastian and San Geronimo. 

 This vanished sheet of water, for which the name Lake Mor- 

 kill 13 is proposed, is believed to have reached its maximum 



12 a See p. 78. 



1S Named in honor of W. L. Morkill, a loyal friend of scientific research 

 in Peru. As the representative of the Peruvian Corporation Mr. Morkill 

 has substantially assisted the work of our several expeditions. 



