In Rocky Mountain Montana the mountain ranges are aligned generally from north-northwest to south-southeast. They are made up of ancient, hard rocks that were compressed, folded, faulted, and otherwise contorted by the mountain-building forces that created the Rockies.

During the last Ice Age, glaciers carved the mountain crest lines and high valleys from rounded, convex terrain into sharp, rugged, concave topography and, when they melted, left the loose earth material that they had gouged out of the mountains as glacial deposits in the valley bottoms. The glaciers in Montana today are very small compared to the great tongues of ice of the past. The bottoms of the valleys between the mountain ranges consist mainly of alluvial floodplains and terraces; of benchlands and foothills carved on young, soft rocks; and of plains, terraces, and foothills made up of glacial deposits.

There is a contrast within Rocky Mountain Montana between mountains with narrow valleys and those with broad valleys. In the narrow-valley regions, which are the most rugged and spectacular of the state, the valley floors are humid and forested. There are two narrow-valley regions. One is northwestern Montana, which includes Glacier National Park with most of Montana's glaciers. The other lies in south central Montana at the northern end of Yellowstone National Park; this area contains the highest point in Montana, Granite Peak, which has an elevation of 12,799 feet (3,901 metres) above sea level. These two narrow-valley regions are separated by a broad-valley area in west central and southwestern Montana. There the valley bottoms are wide, dry, and grassy, permitting sweeping panoramic views of the mountain ranges.

Most of Great Plains Montana is rather rough land. The country south of the Yellowstone River is mainly scattered hills, which make up the Breaks of the Missouri around Fort Peck Lake, with genuine plains found in the “Golden Triangle” north of Great Falls and plateaus elsewhere. Some of the hills, breaks, and valley bluffs form rugged badlands. The valleys of the major rivers flowing from the Rocky Mountains across eastern Montana are deeply incised. Scattered upon the plains and plateau surfaces are eight small mountain masses called Rocky Mountain outliers, which are like islands of the Rockies set out upon the plains.

The rocks underlying Great Plains Montana, except for the mountain outliers, are young, soft, and more or less horizontal. Roughly north of the Missouri River the plains rocks are covered by glacial deposits left by the continental ice cap, which occupied the area at the same time that alpine glaciers were sculpting the mountains to the west. The bottoms of the incised valleys are made up of alluvial floodplains, terraces, and soft-rock benchlands.