academy of sciences) jj g GEO LOGICAL SURVEY 127 



in 1872 suggested that it might be, at Red Rock Pass on the flat floor of Cache Valley, adjoining 

 the Port Neuf Range of southern Idaho. The floor of the pass was reached on August 16 and 

 was described in a notebook as "so flat that it is a marsh with a growth of wire grass and sedge" 

 for 7 miles. 



The red rock is only one of a number that are here exceptionally bared, in testimony to the stream that 

 washed them in degrading this pass. Above the beach level the rock exposures are inconspicuous. If there 

 was a current through here when the lake was at its full height, it must have continued until the outlet was 

 deepened several hundred feet. There is no evidence yet of the direction of flow but there will be evidence if the 

 beach is found not to continue to the north. 



Later in the day, after an advance to a point which commanded a broad northward view, 

 a cautious note was added : 



I can neither affirm nor deny beaches, but I think they are absent. The valley northward should exhibit 

 them as well as that at the south if it ever had them. . . . On the whole it is extremely probable that an outlet 

 (and the last outlet) of Lake Bonneville was here. 



In explanation of the outlet, as discussed in the field notes, the hypothesis was at first 

 entertained that the highest or Bonneville beaches as they came to be called, were related to 

 an outlet at some other point, from which it was transferred to Red Rock Pass by a crustal 

 tilting; but a second hypothesis was framed the same day, according to which the lingering of 

 the lake at its highest level, as indicated by the great Bonneville beaches, was explained by a 

 small excess of the gradually increasing water supply over evaporation, so that the resulting 

 volume of discharge would be so small that the outlet must be for a time very slowly degraded; 

 but as the water supply increased more definitely and the outflow gained in volume, its channel 

 would be rapidly degraded to the present level of the pass; then a decrease of supply, while 

 still permitting a small volume of overflow, would practically be unable to accomplish further 

 degradation; thus the lake would be held for a considerable period at the pass level and per- 

 mitted to form the strong Provo beaches. A different interpretation was given later, when more 

 value was attached to the resistance of the rocks encountered in the bed of the outflow channel 

 or pass, as a cause for the Provo level being so long maintained. 



During the following year, while Gilbert was working on irrigation problems he gave atten- 

 tion also to the ancient shore lines, and resolved the doubts felt at the beginning of the previous 

 season concerning the Red Rock outlet; he wrote to Powell from Ogden, Utah, under date of 

 October 9, 1877: 



I should like to go by rail to Humboldt Wells and thence by horse down Steptoe Valley, to settle the last 

 question about the outlet of Lake Bonneville. I have this year seen the whole northern border of the lake and 

 have made sure that the only northern outlet was through Cache Valley. That outlet I have revisited and 

 studied with more care than before, and I now think its phenomena all consistent with the hypothesis that there 

 was no other outlet. Still I should like to go to Steptoe and make sure. 



But this desired excursion was not made until two or three years later. 



The establishment of Red Rock Pass as the outlet of Lake Bonneville was announced by 

 Gilbert in the spring of 1878 in a short article ' which led him into more of a controversy than 

 he engaged in at any other time in his life. His claim of discovery was after the fashion of the 

 time disputed by Peale, of the Hayden survey, who asserted in an article 3 bearing the same 

 title as Gilbert's that the real outlet lay in a more open valley about 45 miles farther north, 

 where the original level of overflow was higher than Red Rock Pass, and higher indeed than the 

 highest of the Bonneville shore lines. Gilbert did not reply until two years later, after he had 

 revisited the localities and satisfied himself on three essential points : The error of Peale's deter- 

 mination of certain stream terraces as lake shore terraces; the vastly greater age of the more 

 northern open valley, which Peale had taken to be the outlet, than of the comparatively narrow 

 and recent incision at Red Rock; and the absence of shore lines in the intermediate basin. The 

 controversy went no further; there was no place left for altercation. A characteristic passage 

 may be quoted from Gilbert's closing article: 



1 The Ancient Outlet of Great Salt Lake. Amer. Jour. Sci., XV, 1878, 250-259. 

 ' Amer. Jour. Sci., XV, 1878, 439-444. 



