366 C. R. KEYEf^ OROGRAPHIC ORIGIN OF LAKE BONNEVILLE 



beyond tlio roinotost iMoiistrino ntago. As is also well shown in other 

 parts of the Great basin, river channels sometimes flowed with water, 

 sometimes with molten rock almost as mobile. 



On the assumption that Lake Bonneville was formed in an ancient 

 river valley by the nnsnrmonntable uprising of the Colorado dome across 

 the path of the stream while the headwater tributaries continued to pour 

 into the basin their supplies with unchecked volume, the query comes up 

 whether there is not reason other than the climatic one which may fully 

 account for the disappearance of the great lake. In view of the more 

 recent better understanding of the workings of desert agencies of erosion, 

 it may be asked if it is really at all necessary to postulate the lowering 

 of Bonneville waters after the manner of Lake Erie by the cutting l)ack 

 of the Niagara gorge. 



All available evidence recently o1)tained indicates in no uncertain manner 

 that the insignificant channel extending from Eed Eock Pass to beyond 

 Pocatello and now occupied by Marsh and Port N"euf creeks is not the 

 remnant of an old outlet of the lake, but one very recently formed. The 

 old channel which appears to be the course of the main tributary of the 

 lake instead of the outlet is not one of the magnitude of the Niagara 

 Eiver, but of a considerably smaller stream. At and immediately south 

 of Pocatello the old channel is filled to overflowing with basalt — two 

 flows in fact. At one point the present Port Neuf Creek which is flowing 

 southward abruptly turns westward, crosses tlie lava stream in a narrow 

 gorge a couple of hundred yards wide, and continues back iiorthward 

 along the flank of the lava-filled channel. The slight northward slope 

 of the present lower Port Neuf A''alley is not necessarily proof that this 

 has long been the direction of local moving waters. The lavas manifestly 

 flowed in a southern direction, and presumably the river before them did 

 also, until it was directed out on to the Idaho plains by the choking of 

 its youthful course 1)y the basalt flood. 



The behavior of tlie Port Neuf Eiver is by no means unique. It has 

 its counterpart in other parts of the arid region. The Eio Mora of New 

 Mexico once experienced a lava flood." The Maxwell ash cone, situated 

 Ave or six miles distant from the ri\er, breached its crater wall and let 

 loose a basalt stream four miles wide. This lava stream floA\'ed oxer tlie 

 surrounding plain a distance of over 30 miles before it reached the ])rink 

 of the Mora Canyon, there nearly 1,000 feet deep. This it filled for a 

 distance of many miles to a depth of 400 feet, forcing the river to cut a 

 new canyon in the soft strata alongside of the basalt mass. 



» Proc. Iowa Acad. Sci., vol. xvii, 1911, p. 16.5. 



