556 REPORT UNITED STATES GEOLOGICAL SURVEY. 



apparent. Toward the north there is considerable complication. The 

 cause of some of this complication is, perhaps, to be found in Mr. St. 

 John's district in the uplift of Caribou Mountain. 



After the elevation and folding of the beds, a lake filled the lower 

 valley of Salt Eiver. Since the draining of this lake its deposits have 

 nearly all been removed. As to its age, we have no certain clue. The 

 probability is that it was Post Pliocene. 



As to the age of the conglomerates, red sandstones, and gray and 

 greenish shales which enter into the structure of the hills on Crow, 

 Beaver, and Smoking Creeks, we are in doubt; but the probability is 

 that they represent the Cretaceous and Post Cretaceous. Near Station 

 41 Laramie fossils were obtained; and Mr. St. John obtained a leaf of 

 Cretaceous facies farther north from the similar beds. 



BLACKPOOT BASIN. 



Blackfoot Eiver is a large stream with a most eccentric course. The 

 main stream heads opposite the heads of Smoking Creek. Curving around 

 the dividing ridge which separates them, it flows south into the northern 

 portion of the valley of the East Fork of the Blackfoot. When the lat- 

 ter stream joins it, it turns abruptly and forces its way in a canon across 

 the ridge that forms the western boundary of the valley, flowing south- 

 west until the South Fork comes in, when it turns to the northwest, 

 keeping that course until it is joined by a branch irom the north, when 

 it flows westward into the broad plain-like valley which has been called 

 the Hollow Hand. Through this it flows sluggishly with northerly and 

 northwesterly courses to the north hue of our district. Here it turns 

 once more, flowing south of west, and finally leaving our district with a 

 northwest course among the spurs of the Blackfoot Eange. The most 

 noticeable feature of the Blackfoot region is the basin-like character of 

 the country. Hitherto we have crossed a succession of mountain ranges 

 alternating with broad valleys whose directions are parallel to the axes 

 of the ranges. Now, however, the valleys assume different directions. 

 The only regularity or system is seen in the southern or southeastern 

 branches, which rise in the spurs of the Preuss Eange. I shall take up 

 these in order. 



East Fork of the Blackfoot. — This creek is, in reality, the head of the 

 river, although its course is at right angles to the course taken below its 

 mouth, while the stream from the north does not make so abrupt an 

 angle. 



The direction of the valley is northwest. Yiewed from a high point, 

 it is seen to be the same as the direction of the valley of Smoking Creek 

 and that of the South Fork of the Blackfoot. It is about a mile in 

 width in the widest portion of the upper valley. This narrows a little 

 -as we go down, but widens again, until in the lower valley it is between 

 two and three miles. Here there is considerable swamp and marshy 

 ground, and the streams are divided, enclosing islands. On the north 

 side of the Blackfoot, near the mouth of the East Fork, there is a table 

 of basalt, which extends clown the river as far as the head of the canon. 

 The valley of the East Fork was not followed, but, as seen from the 

 surrounding ridges and hills, its beds must be concealed. On the east 

 side is the ridge forming the divide on the west of Smoking Creek. ' Eed 

 sandstones, probably Triassic?, outcrop at the top of the somewhat 

 bluffy face fronting East Fork. Carboniferous limestones, probably, 

 outcrop in the lower part of the bluff. The dip of the beds is to the 

 ■eastward, and the same dip is noted in the ridge on the west, so the 



