NOTES ON THE GEOLOGY OF THE SUN RIVER 

 DISTRICT, MONTANA 



SIDNEY POWERS and HERVEY W. SHIMER 



Massachusetts Institute of Technology, Boston 



While making a geological examination last fall for the location 

 of an irrigation dam on the North Fork of the Sun River, Montana, 

 Professor W. O. Crosby made a careful collection of some fossils. 

 The latter have been identified by the writers and they form the 

 basis of these notes. 



The Sun River has its origin in the Livingston Range and flows 

 eastward to Great Falls. The district in which the fossils were 

 collected, lies in the southern half of the Saypo atlas sheet, about 

 65 miles west of Great Falls. Owing to the paucity of names in 

 this region, the Sun River is divided into a North Fork and a South 

 Fork. These forks are in turn subdivided into North and South 

 Forks. As only the North Fork of the Sun River is shown on the 

 Saypo sheet, references will be made directly to the Sun River, 

 implying the North Fork of this stream. 



In the region near Milk River, about 70 miles northwest of the 

 Sun River, Willis found one large overthrust of the Algonkian upon 

 the Cretaceous.' It is probable that this overthrust divides into 

 several smaller thrusts before reaching the Sun River. 



There are nine N-S. "reefs" or ridges from the junction of the 

 Sun River with its South Fork east to the Cretaceous plains. The 

 indications are that each of these parallel ridges represents an 

 overthrust to the east. Besides the held evidence of this structure 

 is the evidence furnished by the stratigraphic position of the fossils. 

 These were collected at too few points to show the existence of 

 more than five faults. The fossils indicate a fault both east and 

 west of the Dam Site. There is evidence also of a fault to the east 

 of Arsenic Reef and of another separating the two ridges of Arsenic 



' G. S. A. Bull. 13, 1902, p. 305-52. 



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