THE NORTHERN PACIFIC ROUTE. 143 
Plains, formerly known as Horse Plains because it is situated in 
the midst of a broad prairie that was used as a pasture ground for 
Piiiis horses belonging to the trading posts of the Hudson’s 
Elevation 2,182 feet, -P2®Y Co., is a sort of oasis in the desert of rocky 
Population 481. canyons along Clark Fork. Here the valley opens 
St, Paul 1,825 miles. and terraces are well developed, especially one about 
170 feet above river level. East of the station the terrace shows on 
the north, but west of the town a large remnant of the terrace, 
equally well developed, can be seen across the river on the south. 
Fait beach lines also appear on the high, smooth hill slope back 
of Plains, but it is probable that the lines visible from the railway 
are not the highest in this region, the others being obscured by the 
timber and brush growing on the higher hills. 
One of the most striking features of the valley of Clark Fork is 
the fairly regular succession of narrow canyons and broad valleys, 
without any apparent reason so far as the action of the stream is 
concerned. These changes are not due to differences in the hard- 
ness of the rocks, for as a rule all the formations of the Belt series 
have about the same degree of resistance to erosion; but they are 
due to great breaks or faults in the rocks. These faults have broken 
the crust of the earth into huge blocks, some of which have been 
raised, some lowered, and some tilted over so that one edge is very 
much lower than the other. These dropped or tilted blocks form 
the broad valleys, and the raised blocks or the upper edges of the 
tilted blocks have proved to be serious obstructions in the pathway 
of the river, which has succeeded in cutting only narrow canyons 
through them. This explains the broad valley at Plains and the 
narrow canyons which lie directly above and below that place. 
The breaking into blocks and then the tilting of these blocks 
into various attitudes seem to have been the movements that gave 
to this region its distinguishing In some of 
1 Rast of the Mission Range the rocks 
are thrown into great folds or tilted along 
faults having a general northwesterly 
trend, but west of that range the struc- 
ture is less regular and the foldsand faults 
A 
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Us , 
|e Magee 
ree d 
: 
structural features.! 
directions. As a rule, the rocks are not 
very severely folded, and most of the 
due to tension or stretching of the earth’s 
crust, but some of them are distinctly of 
~ 
SS" 
Figure 320. 
do not have a common direction, as they 
do jarther east. In the area about 
the structure is broadly simple and yet is 
rendered complex in detail by 
faults and folds that trend in 
minor 
different | a way 
the overthrust type, d 
stresses of compression. 
