468 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



the Bozeman Beds may be eventually traced southward so far as 

 the Arizona line. 



The gravels of the Pocatello valley are sometimes regarded as 

 having been deposited by the outlet of ancient Lake Bonneville. 

 These coarse, fluviatile beds are well displayed near the Red Rock 

 Pass, which, by the way, chances to be the present low-rim-point of 

 the Bonneville basin. Singularly enough the Red Rock Pass pre- 

 sents none of the earmarks of a lacustral outlet. The manifest 

 movement of the gravels and sands is directly opposite to that of 

 the old lake's hypothetical discharge. To all appearances it is a 

 point of stream entrance rather than of exit. As lately summed up 

 the history of the ancient lake is as follows : The great body of 

 water of which Great Salt Lake is a last vestige is not an anomaly 

 among desert features, as is often regarded, but it merely represents 

 a special phase of a through-flowing stream that was not quite large 

 enough to master the orographic barriers which happened to arise 

 athwart its path, while its nearest neighbor and parallel stream, the 

 Grand river, reinforced by the Grand, and other eastern tributaries, 

 proved sufficiently powerful to hold its own against all vicissitudes 

 and to carve through the rapidly bulging Colorado dome a Titan 

 among chasms. Blocked by such a formidable rampart as this great 

 uplift the old Virgen river spread out behind the dam far and wide 

 over the adjoining intermontane plains, until finally, after the 

 stream's headwaters were diverted, it could no longer furnish the 

 lake with its former supplies. 



The main cause for the decline of ancient Lake Bonneville ap- 

 pears to have been the cutting off of the headwaters of the old Vir- 

 gen river of which the lake was an inflated part. This depletion of 

 water supplies seems to have taken place in two ways. The cap- 

 ture of its large gathering area by the Missouri river, by the Yel- 

 lowstone river, by Clarke's fork of the Columbia river, and by the 

 Salmon fork of the Columbia, a tract larger than the state of Massa- 

 chusetts, displaced the larger part of the water supply. Choking of 

 the old river channel at Pocatello by basalt flows completely elim- 

 inated the supply from this direction and turned the volume of the 

 headwaters out over the Idaho lava plains where it finally gathered 

 along the lowest line as the Snake river of today. 



Now, the complex of headstreams which at the present time 

 gather together to make up the Snake river of Idaho presents strong 

 evidences of being a newly adjusted system. Traces of relatively 

 recent diversions are indicated in many places. One of the prin- 

 cipal tributaries, the Port Neuf river, after flowing due south for 



