Montana is made up of two main geographic areas: the western two-fifths of the state is part of the Rocky Mountains, and the eastern three-fifths is part of the Great Plains. Rocky Mountain Montana is a land of high mountains, deep valleys, green forests, and treeless crest lines, whereas Great Plains Montana is a vast flat area of yellow rangeland, golden grain fields, and brown fallow strips.
The Rocky Mountains are made up of ancient hard rocks that were compressed, folded, faulted, and otherwise contorted by the mountain-building forces, beginning about 100 million years ago.
During the last ice age, around 11,500 years ago, glaciers carved the mountain crest lines and high valleys into sharp, rugged, peaks and, when the glaciers melted, the loose earth material that they had gouged out of the mountains was left as glacial deposits in the valley bottoms. There are still glaciers in Montana today, but they are very small compared with the great walls of ice of the past. The Continental Divide passes in a jagged pattern through the western part of the state, from the Lewis to the Bitterroot ranges.
In between the mountains of the Rocky Mountain region are two types of valleys. The narrow valleys are the most rugged and spectacular, and the floors of these valleys are humid and forested. Glacier National Park and Yellowstone National Park are both narrow valleys. In west-central and southwestern Montana are broad valleys. These are wide, dry, and grassy areas. The highest point in the state is Granite Peak, at an elevation of 12,799ft (3,904m), located in south-central Montana, near the Wyoming border. The lowest point, at 1,800ft (549m), is in the northwest, where the Kootenai River leaves the state at the Idaho border.
Montana is the only state which has waters that flow to the Hudson Bay, the Gulf of Mexico, and the Pacific Ocean. The northwestern section of the state is west of the Continental Divide and waters from this region flow to the Columbia River, and then to the Kootenai River and the Flathead River, and finally to the Pacific Ocean. On the eastern side of the Continental Divide, streams flow to the Missouri River and its principal tributary, the Yellowstone. The Missouri, in turn, flows into the Mississippi River, and then into the Gulf of Mexico and the Atlantic Ocean. The land on the eastern slope of Glacier National Park drains into Hudson Bay. Ft. Peck Reservoir is Montana's largest lake, covering 375sq mi (971sq km); Flathead Lake is the largest natural lake.
The climate of most of the Great Plains region of eastern Montana is semiarid, with hot summers and cold winters. This area gets only about 13in (330mm) of rain and have cycles of drought followed by periods of very heavy rainfall and flooding. There is little snow in this region, but frost or freezing occurs more than 200 days of the year. The chinook, a warm winter wind that blows on the plains near the foot of the Rockies, sometimes interrupts the bitter cold -January temperatures average in the upper 10s F (about -8°C).
In the Rocky Mountain region there are several different climates, depending on how high up in the mountains you are. The climate of the lowest zone - the dry valley bottoms - is similar to that in the plains. The climates of the other zones become cooler, wetter, and more snowy the higher up you go. The all-time low temperature in the state, -70°F (-57°C), registered at Rogers Pass on 20 January 1954, is the lowest ever recorded in the lower 48 states; the all-time high, 117°F (47°C), was set at Medicine Lake on 5 July 1937.
