POST-CRETACEOUS HISTORY OF WESTERN WYOMING 329 



which have since been cut to pieces by the growth of canyons. 

 The canyons have been excavated, not only through the drift sheet 

 itself, but 200-1,000 feet in the underlying rock. Thus the deep 

 valley of the north fork of Teton River has been cut almost entirely 

 since the deposition of the Buffalo drift, for the river now has a 

 V-shaped gorge devoid of glacial features in the horizontal flows 

 of rhyolite, and the old drift caps a flat-topped divide on either side 

 (Fig. 45). The canyon of Buffalo Fork, about fifteen miles above 

 its junction with Snake River, is a similar trench sunk about i ,000 

 feet into the Cretaceous (?) shales which underlie the extensive 

 deposits of old drift. 



As should be expected, the ancient moraines have been much 

 more extensively destroyed where they rest upon weak strata in 

 proximity to an actively degrading stream. For example, in the 



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Fig. 45. — Diagram of the canyon of Teton River southeast of Ash ton, Idaho, 

 showing the present relations of the oldest (Buffalo) drift. 



valley of the Gros Ventre River about seventeen miles above its 

 mouth, there is a remnant of till identified by striated subangular 

 bowlders exposed in a new roadway. The granitic and other dis- 

 tinctive rocks entering into the composition of this material indicate 

 that the ice-tongue came originally from the Wind River Range, 

 and probably from the canyon of Green River. Yet no trace has 

 thus far been found of the old lateral moraines. 



Uncorrelated drift deposits greatly reduced by erosion were 

 observed on the high divide between Leigh and Teton creeks, on 

 the west slope of the Teton Range. There the glacial origin of two 

 or three little bodies of bowlder-clay is proven by the presence of 

 striated stones, and other characteristics of till, but their age is 

 not clear. One of the remnants occupies a saddle in an undulating 

 ridge which separates two deep canyons. Another remains on the 

 shoulder of a spur dividing the forks of a canyon 800 feet deep. 



