116 GUIDEBOOK OF THE WESTERN UNITED STATES. 
LOGAN TO GARRISON BY WAY OF HELENA. 
A short distance west of the station at Logan (see sheet 16, p. 112) 
the Helena line crosses Gallatin River and then follows this stream 
to its junction with the other rivers that form the Missouri. At the 
bridge and for a short distance beyond it the 
railway skirts the foot of a bluff of Madison 
limestone, but this rock dips below river 
level and beyond it the bluff is composed 
of the overlying Quadrant formation. Al- 
though the Quadrant resembles limestone it 
contains little of that rock, being generally 
composed of quartzite or flinty beds that are 
much harder and generally of a lighter color 
than the limestone. These beds in turn dip 
below water level and the rocky bluffs give 
way to a low rolling country with swampy 
land near the track. 
The railway follows the flat bottom of 
Gallatin River for some distance and thence 
once more follows the cliffs of the Madi- 
son limestone, which is brought up from 
below water level by a great anticlinal 
fold. On the left (west) there is’ an ex- 
tensive flat valley through which Gallatin, 
Madison, and Jefferson rivers flow on their 
way to join forces and form Missouri River. 
They unite at the entrance to the gorge which 
the combined stream has cut through the 
limestone and which the train is about to 
enter. This junction, known as Threeforks, 
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Saul has attracted the attention of every trav- 
Rea eler who has entered this region since 1805, 
: San when it was first seen by Lewis and Clark. 
‘See’ (See p. 100.) 
Ze North of Threeforks the river has cut a 
Rowena g canyon along a great arch or anticline which at 
oe the entrance to the canyon brings up to view 
only the Madison limestone, but the fold increases in magnitude north- 
ward (downstream) and lower formations are successively brought 
above water level. As shown in figure 25 this fold is not a simple arch, 
but the force which bent the rocks was so great that the fold was 
pushed over to the east, oroverturned. The rocks were then broken 
or faulted, as shown in figure 26 (p. 117). 
URE 25.—Section between Logan and Trident, Mont. The rocks, as seen from the railway, appear to be jumbled, but 
