THE NORTHERN PACIFIC ROUTE. 101 
most branch in honor of President Jefferson. The western branch, 
by reason of the volume of water and its greater length, is gotiorally 
regarded as the real head of the Missouri. 
It was here that Sacajawea, the Indian woman who was the real 
guide of the expedition, found the place of her capture several years 
before. In recognition of her services in the early exploration of the 
country a bronze tablet, presented by former Senator W. A. Clark, has 
recently been placed on a large bowlder in the public park of Three- 
forks by the Montana Daughters of the American Revolution. 
_ Beyond Threeforks the railway leads up the broad flood plain of Jef- 
ferson River and is paralleled by the Chicago, Milwaukee & St. Paul 
Railway as far as Cardwell. The flood plain is com- 
Willow Creek. —_ posed largely of sand and gravel washed down from 
Elevation 4,165 feet. the mountains on either side, consequently the soil 
Popation 3. is thin and the region not well adapted to farming. 
t Willow Creek, however, soft lake beds cover the 
coarse gravel, and the country is more fertile. 
Beyond Willow Creek the railway approaches the bluffs on the 
south side of the valley, which consist of quartzite, shale, and lime- 
stone, of the Quadrant formation, backed by a higher ridge of Madison 
limestone. At milepost 17 the river flows in a narrow canyon through 
a point of this limestone which projects into the valley, and the two 
railways follow the passageway that the river has cut through the 
hard rock. 
? It is by no means uncommon to find | a deep channel in the soft rocks of its val- 
that a river has cut its way through hard | ley bottom and revealed the hard rock 
rock which it might have avoided by | beneath, but is was confined by walls of 
pursuing’a slightly different course. The | its ownslow cutting and could not dodge 
explanation for Jefferson River lies in the | aside whenit tered istant 
fact that when the present course was | rock. As the channel was worn lower, the 
established the valley was not cut to its | stream became intrenched so deeply in 
* FIGURE 19.—Diagram to explain the course of Jefferson River west of Willow Creek, Mont. The 
river cut through soft lake beds ators to hard limestone (AB), but was then so deeply intrenched 
that it could not avoid the hard 
present depth, and the hard limestone | this hard tock as to make direct escape 
that now seems like a barrier in the path- | impossible, even after the whole aspect 
way of the stream lay concealed below | of the country n changed by 
_ the surface. Under such conditions the | erosion and easier routes which the river 
as free to meander from side to | might have taken had become evident. 
“side of its flat-bottomed valley, and one | The situation is represented by figure 
of its loops was located directly above the | 19, in which ABC represents the cross 
estone ledge. In time the stream cut | section of the valley which one sees on 
