68 GUIDEBOOK OF THE WESTERN UNITED STATES. 
The Pierre shale continues to Cedar Creek, 11 miles beyond Glen- 
dive, where, if the traveler looks ahead on the left at milepost 11, he 
will see on the far side of the valley a large ridge in which the rocks 
dip as much as 20° in the direction in which he is going, the opposite 
direction from their dip between Glendive and Cedar Creek. In 
other words, the train has crossed a great arch or anticline in the 
rocks, the highest point of which is at Cedar Creek. The Glendive 
anticline is the most pronounced fold in eastern Montana. It 
extends from Yellowstone River in a straight line southeastward into 
the extreme northwest corner of South Dakota. It brings to the 
surface the Pierre shale on the center of the arch, and as this shale 
is softer than the rocks on either side, it gives rise to a belt of country 
having little relief. For this reason it was followed by the Chicago, 
Milwaukee & St. Paul Railway from Marmarth, N. Dak., to Baker, 
Mont. The shale is everywhere rimmed about by the hard Colgate 
sandstone, and this in turn by the Lance and Fort Union forma- 
tions. The form of this fold is shown in figure 7, which represents — 
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the strata as they would appear if the observer were in an airplane 
hovering over the flat on the far side of the river and looking up 
the valley of Cedar Creek to the southeast. A short distance beyond 
the mouth of the creek the steep dips die out and the rocks are so 
nearly flat that they seem to be horizontal. 
At milepost 17, between Hoyt and Marsh, there is a large gravel 
pit on the left from which ballast has been hauled as far east as 
Richardton, N. Dak. This gravel, as well as that occurring at other 
places along the river, contains many moss agates which have been 
washed down from the mountains in the vicinity of Yellowstone 
Park, and many fine specimens have been picked up along the track. 
Just beyond the village of Fallon (see sheet 11, p. 72) the Chicago, 
Milwaukee & St. Paul Railway enters the valley of Yellowstone 
River from Fallon Creek, and near milepost 36 it 
Fallon. crosses the Northern Pacific tracks by an overhead 
— feet. bridge. In this vicinity, as elsewhere in the Yellow- 
St. Paul 697 miles, Stone Valley, two plants characteristic of the semi- 
arid West are very abundant, the cottonwood tree 
(Populus) and the sage (Artemisia). The courses of the river and 
its tributaries can be followed across the prairies where the bluffs 
