At Sun River, Montana, the Rocky Mountains meet the Great Plains, and the Overthrust Belt of Montana meets the Disturbed Belt. In the zone known as the Disturbed Belt, the huge thrust faults and tight folds of the Overthrust Belt (which were caused by tectonic compression) taper out into smaller thrust faults and more open folds to the east. The Disturbed Belt extends eastward beneath the edge of the Great Plains. This is as far inland as the crust was compressed and its geological structure was altered by plate convergence and terrane accretion on the west coast.

In this picture of the Sawtooth Range, looking south across the Sun River, each high ridge is made up of the same formation of dolostone. The single layer of sedimentary rock has been stacked up repeatedly along a series of thrust faults, forming an imbricated sequence--like several overlapping shingles. Thus, in this scene, there is a large thrust fault at the base of each high ridge of white rock. The layer of rock that forms each ridge was thrust from the right (west) to the left (east).

French Gulch Thrust

This is a close-up of a thrust fault, the French Gulch thrust, exposed along the road up Sun River Canyon. The white rock on the right is dolostone, of Mississippian age, thrust up over the dark, finely layered Cretaceous shale on the left. Select the image to see a larger view. Use your browser's back button to return to this page.

During a period of motion on the fault, a piece of the dolostone broke off in the fault zone and became embedded in the shale. Close inspection reveals that the shale has developed slaty cleavages (layering caused by the faulting) at two different angles. The slaty cleavage provides information about the direction of stress and motion on the fault.

From the ridges seen at a distance (first picture, above), to the outcrop scale (this picture), to the scale of rocks seen in the microscope, Sun River Canyon provides detailed evidence of a sequence of thrust faults and how the rocks deformed during motion on the thrust faults.

Castle Reef

The upper layer of rock in this view is the Castle Reef Formation. Castle Reef itself is the cliff and flying buttresses of rock that form the steep upper part of the ridge. At the base of Castle Reef, along the bottom of the cliffs and on top of the talus slope, is the Sawtooth thrust fault. Select the image to see a larger view. Use your browser's back button to return to this page.

Sawtooth Thrust Fault at base of Castle Reef

This view is a close-up of the Sawtooth thrust fault at the base of Castle Reef. The thrust fault is a layer of breccia that spans from the ankle level to the elbow level of the person in the picture. Unlike the French Gulch thrust fault (above), which put dolostone on top of shale, the Sawtooth thrust fault at the base of Castle Reef put limestone on top of dolostone. As a result of the different minerals and mechanical properties of limestone on dolostone as compared to dolostone on shale, the Castle Reef fault zone has a different structural style than the French Gulch fault zone. Select the image to see a larger view. Use your browser's back button to return to this page.

The tremendous amount of west to east compression in this region of the Rocky Mountains, which created the multiple thrust faults and many folds in the rock layers, occurred between the Late Cretaceous period and the late Eocene epoch, or between 70 and 40 Ma. This is the same interval of time that the Lewis thrust fault developed in what is now Glacier National Park, 100 miles north of Sun River. The Overthrust Belt that affected a large area of the northern Rocky Mountains, from Wyoming far north into Canada, was part of the Laramide Orogeny.

Glossary terms that appear on this page: thrust fault; tectonic; compression; dolostone; shale; cleavage; stress; talus; breccia; limestone; orogeny