In the Bitterroot Valley is a large deposit of Quaternary.
From where the main range turns east from the Idaho boundary line the crest of the Bitterroot Mountains continues on that line with a downward slope to within one degree of latitude from the Canadian border.
In Idaho), Bitterroot (1,180,900 acres), Blackfeet (1,956,340 acres), 1 The St Mary and both forks of the Milk river flow northward into the Dominion of Canada, and as there has been much private irrigation both north and south of the international boundary, the present Federal project and other undertakings in the same region necessitate an international agreement as to the division of the waters, especially of the St Mary, and commissioners representing the Canadian government and the United States conferred in regard to it in May 1908.
To the east and north-east of the Bitterroot Mountains is a considerable basin or peneplain dissected by short ranges having a northwest and south-west trend.
On its slope, which rises abruptly from the Bitterroot Basin, glaciers have cut canyons between high and often precipitous walls, and between these canyons are steep and rocky ridges having peaked or saw-toothed crest lines.