558 REPORT UNITED STATES GEOLOGICAL SURVEY. 



clinal with the -western side most prominent. The valley of Middle 

 Creek is therefore a monoclinal. About two miles south of the most 

 northern of the two creeks that cut through the ridge a station was 

 located. Gray sandstones dipping rather steeply to the northeast formed 

 the summit. Below these, near the base of the hill on the west side, was 

 an outcrop of limestones dipping in the same direction, and containing 

 the following fossils : 



Meelcoceras apla n atu m . 



Mcekoccras gratilitatis. 



Aviculqpecten altus. 



These prove the beds to be of probable Triassic age. The outcrops 

 are few and somewhat obscure, as the hills are rounded in their outline. 



The axis of the synclinal lies to the eastward of the line of the ridge. 

 To the southward the hills disappear and the synclinal cannot be defi- 

 nitely traced. There are traces of a synclinal west of station &, which 

 may be the same one, but its relations are obscure. 



JSouth Fork of the Blackfoot. — This is probably the most important 

 branch of the river. It is made up of two creeks, one rising in the angle 

 between the Preuss Range and Aspen Ridge, opposite the head of Twin 

 Creek, and the other draining the western slope of Aspen Ridge. The 

 two valleys are in general parallel, and are separated by a spur from 

 Aspen Ridge, which is probably an anticlinal axis. The western branch, 

 about eight miles below its head, turns to the eastward and cuts acros 

 this ridge to join the eastern branch. Directly north from the bend, and 

 continuing the line of the valley, is a small branch of the Blackfoot 

 separated from this stream by a low divide. 



The main branch has a length of some 15 miles. For the most part its 

 valley is broad and well grassed. Two of its branches, as we have already 

 noted, come from the eastward through the ridge that bounds the valley 

 on the east. On the west side of the valley below the mouth of the west- 

 ern branch the beds exposed in the bluff dip north of east, and in the 

 divide east of Station 89 the dip is south of west, so between the two 

 points there must be an anticlinal, which is probably the same as that in 

 the hills separating the two branches of the South Fork. The western 

 branch probably occupies the anticlinal axis just before it cuts across to 

 join the eastern branch. The rocks are probably all Jura-Triassic. In 

 the valley of the Blackfoot opposite the mouth of the South Fork there 

 are isolated patches of basalt showing on the north side of the river. I 

 could not determine the presence of basalt on the south side, and it pro- 

 bably does not occur on that side of the river. 



Aspen Ridge. — This ridge forms the divide between the South Fork of 

 the Blackfoot and Bear River. It is a spur from the Preuss Range. Its 

 trend at first is about northwest, but as it is followed it turns slightly to 

 the northward. It is about 24 miles in length. The southern end near 

 the Preuss Range is heavily timbered and the geological structure some- 

 what obscured. There are probably two anticlinal folds between the 

 Preuss Range and the valley of Bear River ; the western one is probably 

 considerably eroded. 



On Station 90 there are sandstones and limestones outcropping con- 

 taining Ostrea strigulecula and obscure Jurassic forms. These beds are 

 so covered that the clip cannot be determined, but on the hill west of the 

 station it is south of west, or west, and in the canon leading down from 

 the station to the Sulphur Springs the dip is, in general, west; Near the 

 foot of the canon, however, we cross limestones with a strike of north 

 7° east. Before reaching this point the beds are dipping west about 50°. 



