378 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



rises to the eastern alluvial slope of the valley, and Anally descends to 

 the Bonneville channel at a poinl beyond the lava tables. 



Where the Portneuf skirts the lava bed its channel is obstructed by 

 a series of low dams of travertine, which seems to be rapidly deposited 

 by the water of the river. 



At its southern end Marsh valley joins the northern end of Cache 

 valley, being separated by a low divide known as Red Rock pass. 

 Through this pass Lake Bonneville discharged its surplus water, and 

 it is here that the train passes from the basin of the Columbia river to 

 the basin of Great Salt lake. A halt will be made for the purpose of 

 examining the channel of outflow (Plate v). 



At this point a low ridge of Carboniferous limestone lies athwart the 

 valley trough, its crest projecting above the alluvium in two buttes, 

 whose iron-stained dill's give name to tine pass. Near them are Plio- 

 cene lakeheds, upturned at a high angle, but these arc seen only where 

 the alluvium has been washed away. The alluvium derived from the 

 mountain ranges rests in great conical heaps against them, the cones 

 joining along the middle of the valley. At the pass is an except ionally 

 large alluvial cone, built by Marsh creek, which issues from a canyon 

 at the east (left). Before Lake Bonneville existed this formed the 

 summit of the pass, and when the lake broke over its edge the alluvium 

 was washed away with speed, letting the imprisoned waters escape to 

 Marsh valley in a debacle of tremendous power. Marsh creek then 

 cut a channel through the cone it had previously built, and in tliis 

 channel it still Hows. It has built a small cone in the abandoned river 

 channel, which it is feebly laboring to (ill. A little farther south other 

 creeks have built alluvial cones in the Bonneville channel, partitioning 

 it into little basins occupied by swamps and ponds. In time of Hood 

 Marsh creek turns northward and follows the old channel to the Port 

 neuf, hut it ordinarily sinks in its own alluvium near the pass. 



The Mormon town of Oxford lies just beyond Bed Pock pass in the 

 northern end of Cache valley. Cache valley also lies between mountain 

 ranges of the Wasatch system, trending north and south. It is ha 

 versed by the Bear river, the largest tributary of Great Salt, lake, 

 which enters the valley through a canyon at the northeast and, after 

 traversing half its length, escapes to the west through a narrow gorge 

 in Paleozoic limestone known as "The Gates." It is joined by many 

 smaller streams issuing from the eastern range. The mountains on 

 either side are constituted chiefly of Paleozoic strata, but fresh-water 

 beds referred to the Pliocene (Humboldt beds) vest against their 

 flanks at several points and have shared in the later displacements. 

 Bonneville shore lines encircle the valley, and marl deposits of the 

 same date occupy its lower levels. Associated With these are great 

 delta deposits accumulated chiefly at the date of the Provo shore line. 



