144 GUIDEBOOK OF THE WESTERN UNITED STATES. 
these blocks, as, for example, the one which lies between Plains 
and Thompson Falls (see sheet 20, p. 152), the rocks are slightly 
bent into broad, open folds. The structure in this block is repre- 
sented by figure 31. 
The rocks in sight east of Plains belong to the Prichard formation, 
which dips to the west and passes below water level, and at Plains 
the thin-bedded gray quartzite and argillite of the overlying (Ra- 
valli) formation come into view. This formation continues with 
fairly regular dip to a point about one-half mile beyond Weeksville. 
At this place the Newland limestone, overlying the Ravalli, appears, 
dipping in the same direction and at about the same angle as the 
TIT /. 77; Ny 
oe 
FIGURE 31.—Great folds in the rocks bet Plains and Thompson Falls, Mont. 
Ravalli. Within a short distance the dip flattens, and at milepost 
15 is reached the point toward which the beds dip from both direc- 
tions; that is, the axis of the syncline. The beds here are nearly 
horizontal, but toward the west they begin to rise, and near mile- 
post 18 the Newland limestone disappears from track level, though 
still present in the tops of the hills, and the Ravalli 
Eddy. formation beneath it again comes into view. From 
=item this place past Eddy and Frost to the mouth of 
Thompson River, near milepost 26, the cliffs are 
made up of Ravalli rocks thrown into folds or wrinkles too small to 
be shown in the diagram (fig. 31). The walls in this part of the can- 
yon are probably more rugged and more nearly vertical than those 
of any other part of its course. 
Beyond Thompson River the dip of the rocks brings the outerop 
of the Newland limestone down from the tops of the hills (see fig. 31), 
zontal space after the movement than | In the region west of Missoula there 
they did before the faulting occurred, as | have probably been two principal move- 
shown by the diagram. An overthrust | ments—(1) a movement of compression, 
fault is generally produced by the break- | which threw the rocks into broad folds, 
ing ofafold. The fold and fault are due | the compression in some places, a8 in 
to compression in the earth’s crust, and | the Glacier National Park, being 8° 
the result of the movement is that the intense as to produce a great over- 
older rocks are shoved upward and for- | thrust fault; and (2) a movement of ten- 
ward over the younger rocks, thus giving | sion or stretching, by which the some- 
them an inverted relationship. Another what folded mass of rock was broken by 
result is that the faulted mass occupies | a great many normal faults, afew of 
less space than it did before the move- which are shown on the accompanying 
