OVERTHRUST FAULT IN NEARLY FLAT STRATA* 
G. SHERBURNE ROGERS 
During the past summer the writer in the course of an examina- 
tion of a part of the ceded lands of the Crow Indian Reservation, 
Montana, for the United States Geological Survey, discovered a 
small overthrust fault in nearly flat strata. The following brief 
description has been written, not only because of this peculiar 
occurrence of the fault, but because of the extraordinarily clear and 
almost diagrammatic condition of its outcrop, giving the photograph 
a possible value for the purposes of textbook or other illustration. 
The fault is located in the N.E. ¢ S.W. { sec. 3, T. 2 N., R. 35 E., 
P.M., Montana, a point about ro miles east of the Bighorn River 
and about 15 miles south of its junction with the Yellowstone. 
The district is on the remote edge of the Bighorn uplift, the Bighorn 
- Mountains lying about 70 miles away, somewhat to the west of 
south. About 15 miles southwest of the fault in T. 1 N., Rs. 33 
and 34 E., the strata dip in a general northeasterly direction at 
angles ranging up to 35 degrees. This dip, however, decreases in a 
short distance, and within a radius of about to miles around the 
fault the rocks commonly lie at angles less than one degree, except 
for 500 feet immediately around it where the dips locally reach five 
or six degrees. About 20 miles to the north the strata dip uni- 
formly from three to five degrees in a southerly direction so that the 
greater part of the area examined occupies the hollow of a broad, 
flat asymmetrical syncline, the fault being located nearer the 
southern and more steeply dipping limb. Faults are not uncommon 
in this area, but all of the other faults appear to be normal and none 
was observed within 4 miles of the small overthrust in question. 
The general relations of the fault, which outcrops in the north 
bank of the coulee, are shown in Fig. 1. The heavy black stratum 
is a coal bed, the dislocation of which furnishes the key to the 
amount of displacement and the position of the fault zone. This 
t Published by permission of the Director, U.S. Geological Survey. 
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