THS NORTHERN PACIFIC ROUTE. 79 
to the surface, but because of its softness it soon weathers down to a 
dark mud that so conesals the rock from which it was derived that 
the rock can not be seen from the train. 
West of Bull Mountain the Northern Pacific Railway crosses the 
northern point of one of the great mountain-making folds of the 
Rocky Mountains. The rocky layers or formations have been forced 
up into a great arch which has a breadth, where crossed by the rail- 
way, of 75 miles and a length of about 180 miles. In the region of 
its greatest development in Wyoming it forms the Bighorn Mountains, 
and it is generally spoken of as the Bighorn uplift or anticline, but 
the northern projection into Montana has a local development in 
Pryor Mountain and for that reason is known as the Pryor Mountain 
anticline. 
As the railway crosses the fold at its north end, where the forma- 
tions swing around in broad curves, it cuts the outcrops at oblique 
angles or follows them for a considerable distance. It is because of 
this fact that the hills on the left are smooth and low, indicative of 
shale, and the bluffs on the opposite side of the river are rugged, 
being composed of sandstone. 
One of the most striking mementos of the early exploration of 
the Yellowstone Valley is Pompeys Pillar (Pl. XIII, p. 77), a lone 
butte, 200 feet high, between mileposts 196 and 197. In descending 
the Yellowston Capt. Clark noted this butte and from its isolated 
position and vertical walls called it Pompeys Pillar. He states con- 
cerning it, ‘I marked my name and the day of the month and year.”’ 
Halfway up on the side near the river is to be seen Clark’s rude 
inscription, now protected by an iron grating. The sandstone 
forming Pompeys Pillar is near the base of the Lance formation, and 
the westward rise of the rocks soon brings up the dark marine Bear- 
paw shale, but near the railway this shale is covered by soil and can 
not be seen from the train. 
From Pompeys Pillar to Huntley the railway line is in the middle 
of a broad, flat bottom, which is irrigated by water taken from the 
river a short distance above the mouth of Pryor 
Nevin. Creek, under the Huntley project of the United 
rag ta eg States Reclamation Service.t. The underlying rocks 
are not visible from the train except at a great dis- 
tance on the right. As shown on sheet 13 (p. 82), the first formation 
to be passed over beyond Pompeys Pillar is dark shale (Bearpaw) of 
*The Huntley project covers an area of { formation wrought by Government irriga- 
33,000 acres in the broad valley of Yellow- | tion is apparent in the present compact, — 
stone River. In 1907 this region was a | intensively cultivated farms, in substan- 
part of the Crow Indian Reservation and | tial farm buildings, and in growing towns. 
was uninhabited. To-day it contains 400 Be few farms under this Project — 
amilies and six towns. The trans- h y under the terms 
