seale.] DESCRIPTIVE GEOLOGY SNAKE EIVER AREA. 565 



to consider thein as older — probably Pliocene. They are disturbed, dip- 

 ping to tlie northwest at an angle of from 25° to 30°. The terraces, 

 as Professor Bradley says, show no evidences of any local disturbances. 

 I am inclined to think that the gap in the Bannack Bange, which sep- 

 arates Marsh Creek Valley from the valleys to the westward, marks the 

 point where the great lake filling the valleys to the southward had its 

 outlet. One of the two anticlinals formed the barrier which separated 

 the southern lake from the lake that filled the Snake Elver plains. After 

 the barrier was worn away the canon was the bed of a great river whose 

 head was at Bed Bock Gap, the lake occupying j) i eek Valley 



having been completely drained. This river seems to have removed the 

 late deposits in the canon probably on account of its more rapid flow in 

 this portion of its course. This subject will be referred to again. 



Lower Valley of the Portneuf. — As the valley south of the canon prop- 

 erly belongs to the upper end of Marsh Creek Valley, I shall take it up 

 there. Below the canon it merges gradually into the Snake Biver plain. 

 Basalt here, as we have seen, appears to have spread from the Snake 

 Biver plain up the valley of the river. 



MARSH CREEK. 



This stream is the principal branch of the Portneuf, and the only one 

 that will receive a special consideration here. It is a small, sluggish 

 stream, flowing in a muddy channel with swampy banks, back of which 

 are broad meadows. The valley at its head is about a mile in width on 

 the immediate stream ; this increases to four or five miles as we descend, 

 and again decreases as its mouth is approached. Outside of this imme- 

 diate bottom, bounded by the terraces, the valley is from ten to twelve 

 miles in width above the mouth of the Portneuf Canon. Below this 

 point it decreases to about four miles between the bounding ranges. 

 This valley is underlaid by lake deposits covered with gravel and drift 

 from the surrounding mountains. The sources of Marsh Creek are 

 in the southern end of the Bannack Bange, in the northern part of the 

 Malade Bange, and in the southern end of the Portneuf Bange. The ex- 

 treme head in the latter is opposite a branch of the Portneuf that joins 

 the river in the canon. This branch of Marsh Creek flows south, cutting 

 through white friable sandstones, that are almost horizontal in position. 

 At Bed Bock Gap it turns abruptly westward, and flows to the northwest 

 through Bed Bock Gap. The latter is a pass between two masses of 

 limestone that outcrop from beneath the modern sandstones. The East 

 Eock is the most prominent and rises 280 feet above the creek. South 

 of the pass the divide between Marsh Creek and the waters of Bear 

 Biver is a swamp, in which it is difficult to tell which way the water goes. 

 From Bed Bock Gap, Marsh Creek flows northwestward in a broad, rather 

 marshy valley, bordered by terraces cut in the sandstones that fill Marsh 

 Creek Valley. There are three of these terraces — the lowest 50 feet high, 

 the second 150 feet, and the highest rising about 300 feet above the 

 creek. When the creek reaches the western side of the valley it turns 

 northward, bending somewhat toward the east and joining the Portneuf 

 about 24 miles north of the bend. In all this course the descent of the 

 stream is very slight. The descent from Bed Eock Gap to a point 26 

 miles farther north, as the creek is followed, averages only 1.07 feet per 

 mile. In the bordering terraces the sandstones underlying are seldom 

 exposed, being covered with alluvium and drift. The first thought on 

 seeing these terraces is, that once a large stream must have carved 



