52 Cl DAKE 
This fault seems to pass wholly into Cody shales both to the 
north and south and cannot be traced more than 6 or 8 miles. It 
probably shows again, in Sec. 11, T. 52 N., R. 104 W., on the north 
side of the North Fork, where abundant Sundance fossils are 
found on Cody shale slopes, just at the base of the main Madison 
scarp. It has a horizontal displacement approximating 10 miles, 
and a vertical movement of about 3,000 feet, or nearly 10,000 feet 
for the two faults combined. 
Near the west line of T. 51 N., R. 103 W., the trace of this 
fault, which lies about S. 45 W., on both sides of the valley is 
suddenly lost in an area of intense brecciation, which is believed 
to mark the site of a transverse fault. The transverse fault seems 
to have shifted the trace of the thrust northwest about a mile, and 
west of the point of disturbance the thrust can be traced only on the 
south side of the valley. On the north side it is probably buried 
beneath Tertiary lavas. It has not been possible to prove the 
identity of the thrust planes east and west of the transverse area 
of disturbance, but the similarity of stratigraphic relationships — 
[Sundance on Cody and Fort Union( ?)| seems to indicate the possi- 
bility of the foregoing explanation. 
The thrust plane appears to have been sharply folded along an 
axis lying about northeast and southwest, parallel to the trend of the 
South Fork Valley. At one point where a deep gorge cuts the 
axis of a sharply overturned anticline, not far south of Ishowooa 
Post-Office, yellow sandstones are exposed in a very small area 
beneath typical Sundance beds. The sandstones carry no fossils, 
but are similar in appearance to the Fort Union(?), and if of 
Fort Union(?) age the exposure represents a ‘‘Window”’ or “ Fen- 
ster,’ such as has been described by several writers, in connection 
with major thrusts elsewhere. 
Beartooth fault zone.—Along the eastern edge of the Beartooth 
Plateau, from the Clark Fork to the Montana line, is a zone of thrust 
faults, probably related to the same forces producing the faults 
already described though not continuous with them. The southern 
extremity of this zone lies about 8 miles north and 4 miles east of 
the northernmost point to which the Hart Mountain thrust was 
traced. This group of faults, all of which are associated with 
