SOUTHWARD EXTENSION OF THE BOZEMAN TER- 

 TIARIES INTO UTAH 



CHARLES KEYES 

 (ABSTRACT) 



Bozeman Beds is a term which is apphed to certain gravel de- 

 posits that saddle the continental divide in Montana and Idaho. 

 These beds are true epirotic accumulations. They appear to be 

 mainly fluviatile in origin; although they manifestly are partly eolic 

 in nature. Consisting of gravels, sands and silts they chiefly lie in 

 old imterrn'ontane valleys. Their age is pre-Glacial. 



Originally described and mapped in southwestern Montana and 

 around the headwaters of the Missouri river they are traceable far 

 beyond the confines of this basin. Inasmuch as the formations cover 

 the flanks and crest of the Rocky Mountains it is inferred that they 

 were laid down before that range was upraised. The peneplain 

 surface so conspicuous in the Yellowstone Park may be the de- 

 structive contemporary of the constructive Bozeman deposits. 



Since the time when the Bozeman Beds accumulated some curious 

 physiographic changes appear to have taken place in the Missouri 

 headwaters region. The Atlantic-Pacific drainage divide has shifted 

 more than 150 miles to the westward. By headwater erosion the 

 tributaries ot the Missouri river have extended their valleys from 

 Great Falls to the crest of the Bitterroot range. Thus the Mis- 

 souri river has captured a large portion of the former catchment 

 basin of the present Snake river. The latter was presumably the 

 headwaters 'area of the old but now vanquished Virgen river, which 

 formerly appeared as the twin branch of the Green river. As such 

 it flowed through the Bonneville basin of eastern Utah, and united 

 with Green river at the great bend in Arizona to form the Colorado 

 river. 



The extension of the Bozeman Beds of Montana over the Rocky 

 Mountains into the valleys of the Salmon fork of the Columbia and 

 the Snake river explains the presence of similar or identical deposits 

 in the last named valley at Pocatello, Idaho. In the broad north 

 and south valley south of that place the gravels and sands continue 

 far into the Great Salt Lake basin. With this clue it is possible that 



