Geologic History of Western Montana

geology of western montana

Geology of Western Montana

Earth processes play an essential role in ecology. Ecosystems are dependent upon the structural foundation determined by the dynamic physical earth. A fundamental step in studying ecology is to understand the physical processes that shape landscapes.

Western Montana provides an excellent setting to witness and experience a wide variety of earth processes and their ecological implications. These processes range from the effect of plate tectonics on mountain habitats to the effect of river processes on fish migration. The Bitterroot, Sapphire, and Mission mountains, and the Clark Fork, Bitterroot, and Blackfoot rivers, are in close proximity to Missoula. This proximity provides the opportunity to easily observe and experience connections between physical processes and local ecosystems. Additionally, western Montana contains one of the richest geologic histories of any area in the country. From 3.3 billion year old rocks exposed in the Beartooth Plateau to 12,000 year old Glacial Lake Missoula sediments deposited in the Missoula Valley, to the volcanic activity of Yellowstone, the rocks in Montana record a vast array of numerous tectonic environments, climate changes, and even a meteorite impact!

Highlights of Montana's geologic history


Period of time

Years before present

Glacial Lake Missoula



Yellowstone volcanism

Latest Tertiary, and Quaternary

2.1, 1.3, and 0.63 million

Tropical Montana

Late Tertiary

~20 million

Coal production

Early Tertiary

~60 million

Lewis Thrust Fault

Late Cretaceous – Early Tertiary

~72 million – 58 million

Idaho Batholith


~73 million

Belt Supergroup


~1.5 billion – 800 million

Geologic time is divided into smaller periods, each with different defining characteristics. Click on any colored time period to see what was happening in Montana's geologic history at that time.

Text only version

geolgic timescale protoerozoic eon precambrian eon paleozoic era mesozoic era cenozoic era paleozoic era mesozoic era cenozoic era cretaceous period early tertiary period late tertiary period quaterary period quaternary period