566 REPORT UNITED STATES GEOLOGICAL SURVEY. 



tliein. This was recognized by Professor Bradley in 1872.. In; his re- 

 port for that year he says (p. 203) : 



These terraces are very strongly marked througli the whole length of this valley ; 

 and an upj)er one is readily identified, though not so j)rominent, at the level of about 

 1,000 feet above the stream. They are on too large a scale, and the valley is too wide 

 to have resulted from merely the drainage of the small area of mountains about the 

 head of the stream, and I am strongly of the opinion that this must have been at one 

 time the channel for a large outflow from the Great Basin. 



Mr. Gilbert has in several places stated that this pass is the ancient 

 outlet of Lake Bonneville. 



In an article in the American Journal of Science and Arts (vol. xv, 

 June, 1878, p. 439) I stated my belief that it was the outlet of a lake 

 having a lower level than Lake Bonneville, as the shore-hne of Lake 

 Bonneville as indicated by the Bonneville Beach is about 100 feet higher 

 than the top of the pass. The level of the Provo Beach indicates it to 

 have been the lalte drained by the river which cut the terraces in the 

 deposits of Marsh Creek Valley. I shall, however, have to defer the fur- 

 ther consideration of this to a subsequent chapter. 



Several terrace lines can be traced as continuous from Cache YaUey 

 into Marsh Creek Valley south of the pass. One of these is very high, 

 and on the east side of Marsh Creek Valley, south of the exit of the 

 Portneuf, this high terrace is quite distinct. Its level is above the general 

 level of the valley. This is probably the same indicated by Professor 

 Bradley in the extract quoted above. It marks in its outcrops the anti- 

 clinal that is seen north of the bend of the Portneuf. Evidences of it are 

 seen also at Eed Eock Gap. On the west side white quartzites out- 

 crop dipping west, and on the east side above these quartzites are dark- 

 blue limestones (probably of Quebec age). 



The general elevation of Marsh Valley ranges from 5,000 in the middle 

 to about 5,140 feet at the sides. The southern edge of the basalt is not 

 reached untd we get within about four or five miles of the exit of the 

 Portneuf from the canon. Here it appears to have its least thickness. 

 It increases as we descend, showing that, previous to its outpouring, the 

 valley sloped to the northward. This is indicated also by the shape of 

 its surface. The top of the basalt is lower in level than the top of the 

 lake deposits on the west, showing that they were eroded partially, at 

 least, before the basalt was poured out. No basalt is found on the west 

 side of Marsh Creek. It seems, therefore, that the pouring out of the 

 basalt crowded the large stream which then occupied the valley to the 

 west side and forced it to excavate its valley from the soft beds of the 

 west side along the west edge of the basalt. Marsh Creek and the Portneuf 

 probably once united at a point just west of the exit of the latter. When 

 the basalt was poured down the canon it naturally took the channel 

 formed by these two streams, as it was the lowest portion of the valley. 

 The river occupying Marsh Creek Valley was probably too large to be 

 entirely obstructed by the basalt which must have been somewhat 

 cooled at this distance from its source. That it was partially obstructed 

 is probable from the fact that a large bay-hke valley is now found just 

 above the southern limit of the basalt. No beds are exposed below the 

 basalt, which seems to imply that little erosion has taken place since its 

 pouring out. This could certainly be predicated did we know that the 

 bottom of the basaltic bluff is the lowest part of the basalt. The basalt, 

 however, was certainly poured out after the draining of the lake that oc- 

 cupied Marsh VaUey and before the complete draining of the lake whose 

 barrier was at Eed Eock Gap. There is plenty of room for more study 

 in this reaion. 



