I noticed that the Sacred-Texts.com website now has the full text of Walter McLintock's The Old North Trail (1910) now online! This is a great read about the great prehistoric trail corridor called "The Old North Trail" leading from the Arctic North down to Mexico along the Continental Divide, which passes through Montana. Rather than one trail, it is actually a corridor, with at least two major tracks, one going through the foothills and the other along the edge of the open plains.
"There is a well known trail we call the Old North Trail. It runs north and south along the Rocky Mountains. No one knows how long it has been used by the Indians. My father told me it originated in the migration of a great tribe of Indians from the distant north to the south, and all the tribes have, ever since, continued to follow in their tracks. The Old North Trail is now becoming overgrown with moss and grass, but it was worn so deeply, by many generations of travellers, that the travois tracks and horse trail are still plainly visible.
"On Crow Lodge River, just across from our present camp, a lone pine tree once stood. It was a land-mark for people travelling north and south along the Old North Trail, because it stood upon the plain and could be seen from a long distance. Finally the Lone Tree fell, but two children took its place. They have grown large and now they mark the former course of the North Trail. The Indians still speak of the spot as the Lone Tree. In many places the white man's roads and towns have obliterated the Old Trail. It forked where the city of Calgary now stands. The right fork ran north into the Barren Lands as far as people live. The main trail ran south along the eastern side of the Rockies, at a uniform distance from the mountains, keeping clear of the forest, and outside of the foothills. It ran close to where the city of Helena now stands, and extended south into the country, inhabited by a people with dark skins, and long hair falling over their faces (Mexico). In former times, when the Indian tribes were at war, there was constant fighting along the North Trail. In those days, Indians, who wanted to travel in peace, avoided it and took to the forest. My father once told me of an expedition from the Blackfeet, that went south by the Old Trail, to visit the people with dark skins. Elk Tongue and his wife, Natoya, were of this expedition, also Arrow Top and Pemmican, who was a boy of twelve at that time. He died only a few years ago at the age of ninety-five. They were absent four years. It took them twelve moons of steady travelling to reach the country of the dark skinned people, and eighteen moons to come north again" (McLintock 1910:434-436).
I know some sections have been mapped and marked in Montana, especially up around Choteau and the Rocky Mountain Front country. And the archaeologists working for the Helena National Forest are reported to be familiar with portions of the trail. "The Bear's Tooth" near Helena is a landmark on the trail; it is better known today as the "nose" of "The Sleeping Giant" geographic feature visible from Helena.
Below are some sites mentioning the Old North Trail, and you can Google more: