The first aerial photographs were taken from balloons in the late 1850s, but the process of making maps based on them, called photo- grammetry, awaited the advent of the airplane after 1900. World Wars I and II, plus the accelera- tion of Western resource extraction in the interval between them, provided ample motivation.
Southward they thought they discerned two mountain ranges, one behind the other, and they figured the ranges' bearings--generally from southeast to northwest. They were almost right.
Actually there are three ranges in that direction. The nearest are the Highwood Mountains, roughly 35 miles distant. About 20 miles south of them are the Little Belt Mountains. Still farther south--and overlapping them, from Lewis and Clark's perspective--are the Big Belt Mountains, whose peaks reach up to 9,200 feet. The northwest end of the Big Belt range is intersected by the Missouri River, flowing between the cliffs that Lewis would, 46 days in the unforeseeable future, name the "gates of the rocky mountains."
The Belt Mountains
A conspicuous dark band of Cretaceous sandstone all the way around a prominent hill summoned the name Girdle Butte in the 1880s, which was soon recycled into Belt Butte, which in turn contributed the name to a town and a creek, and two mountain ranges. Geologists began in those mountains to study a huge wedge of Precambrian sedimentary rock that extends from western and west-central Montana through northern Idaho, which they called the Belt Formation.
There are ninety different named mountain ranges in the present states of Montana and Idaho together, and from the linked sketch-map showing their relative locations one can imagine how hard it would be to tell one from another at ground level, even today. Some are well over a hundred miles long; others, such as the Moccasins, consist of just a couple of undistinguished buttes rising a little above the plain.