90 GUIDEBOOK OF THE WESTERN UNITED STATES. 
core of it still remain to tell the story to the geologist. There are on 
the flanks of the large fold a number of small folds, and the rocks 
that have just been described constitute such a wrinkle. This 
minor fold, as shown in the section (fig. 13), has been pushed over 
toward the south beyond the vertical, so that the beds on the south 
side dip toward the anticline instead of away from it, as they would 
had they not been overturned. This fold is bounded on the south 
by a fault (a break in the rocks), and near Brisbin it 
is succeeded by a smaller fold of the same type. The 
ppt Meret Madison limestone making the core of this smaller 
air ' fold forms the high, straight ridge or spur that 
trends at right angles to the railway. Beyond this ridge there are 
traces of another kind of fold—a trough or syncline which lies at the 
base of the limestone ridge and extends far to the northwest where 
it contains the Trail Creek coal field. Coal is being mined from Cre- 
taceous rocks in this syncline at the present time, but the product 
of the mines reaches the railway on the other side of Bozeman Pass. 
Beyond Brisbin the rocks on the west forming the Gallatin Range 
are made up of volcanic materials, some of which consist of fine frag- 
ment (tuffs) blown out of some crater with explosive violence or of 
coarse angular blocks derived from the same source or from the 
breaking up of partly cooled lava flows. 
The rocks on the east side of the valley from Deep Creek, opposite 
Brisbin, on the north to Mill Creek on the south are very ancient 
gneiss and schist. This great mass of crystalline rock constitutes the 
central part of the large anticline already described, from which all 
the younger sedimentary formations have been removed by erosion. 
Traces of the sandstones and limestones that once constituted the 
south flank of the fold are to be found up Mill Creek, but they are 
so badly faulted and covered with voleanic breccia (rock composed 
of angular fragments) that they can not be easily recognized from 
the train. The most prominent peak in this part of the valley is 
Mount Cowen, which has an altitude of 11,190 feet and stands 6,400 
feet above the bottom of the valley. 
As far as milepost 16 the railway is on the flat surface of a terrace 
50 to 75 feet above river level. This was formed by the glaciers that, 
: long ago, came down Yellowstone and Mill Creek 
Se valleys and joined near the village of Chicory. The 
St. Paul 1,028 mies, ®*reams flowing from the melting ice carried large 
quantities of gravel and sand and dropped them in 
the open valley, filling it to a considerable depth. Since the ice 
melted away the river has cut a deep channel in this filling, leaving 
remnants of it here and there in the form of terraces. As the terrace 
was built only below the limit of the glacier, the railway is forced, 
Brisbin. 
