STRUCTUKE. 51 
side of the fault. From Mill Creek nearly to Rock Creek a syncline 
holding upper beds of the Montana extends parallel to this fault. 
The dips are 30° on both limbs. Next east is an anticline in Montana 
shales forming the west margin of the deep portion of the Montana 
basin which lies about the Cooper Lake region. Dips on the east side 
of this anticline range from 20° to 40° and on the west side from 5° to 
30°. The relations near the mountains in this belt are greatly 
obscured by glacial drift and talus, but local exposures mostly present 
moderately steep dips. The Montana beds dip from 20° to 40° E. 
along the west side of the Tertiary area in the Cooper Creek basin. 
ROCK RIVER ANTICLINE. 
The Rock River anticline begins in the red beds west of McGill. 
A short distance west of Chalk Bluffs it has the relations shown in 
section 2, Plate VIII, with steep dips (35° to 50°) on the northwest 
side and gentle dips on the southeast side. It causes the westward 
deflection of outcrops ranging in age from Chugwater to Montana, in 
which the Niobrara formation rises into the prominent exposure in 
Chalk Bluffs. The lower formations pitch down steeply 6 miles west 
of these bluffs, but the anticline continues far to the southwest past 
Rock River station, up the valley of Rock Creek. Along its axis the 
lower beds of the Montana extend up the wide valley of Rock Creek 
to the fault at Arlington. The relations in this valley are shown 
in section 3, Plate VIII. In the vicinity of Chalk Bluffs the beds on 
the northwest limb dip 20° and those on the other side incline to the 
southeast at angles of 10° to 14°. At the point where the rim of upper 
sandstone of the Benton passes beneath the surface 4 miles southeast 
of Rock Creek post-office the dip is to the west at an angle of 35°. Sand- 
stone and coal in the bank of Rock Creek 5 miles above Rock River 
station dip to the southeast at angles of 5° to 8°, but the sandstone 
in the bluff 2 miles farther north dips to the northwest, indicating 
that the axis of the anticline passes between these points. The Mon- 
tana sandstones outcrop on both sides of the valley, dipping to the 
northwest on the northw r est side and to the southeast on the south- 
east side. The dips are mostly low. On the south side of the valley 
near Rockdale the anticline appears to branch and the southern arm 
joins the flexure extending along the west side of Cooper Creek basin. 
Exposures in the valley are poor, owing to the heavy covering of 
alluvium. 
ROCK CREEK ANTICLINE. 
A short but prominent anticline passes just north of Rock Creek 
post-office. It develops in red beds near Boswell Spring, is exhibited 
in beds ranging from Morrison to Benton near Rock Crock, and dis- 
appears in the low r er beds of the Montana north of Rock River station. 
