LEWIS OVERTHRUST FAULT

One of Montana's more famous geologic features and one that has given rise to some of its most spectacular geology is the Lewis Thrust Fault. Ever since Bailey Willis, an intrepid geologist and early explorer of northwestern Montana, discovered the Lewis Thrust Fault in 1901, geologists have considered it one of the world's classic geologic structures.

In a thrust fault, a fracture forms in the earth's crust nearly parallel to the surface of the earth. The rock on one side then begins to ride up and over the rock on the other side. The Lewis Thrust Fault sliced slightly diagonally through thick layers of sedimentary rocks. All the layers on top of that fault line slid as a series of slabs as much as 50 miles across western Montana, pushed along by the same compressive forces that caused the fracture. They came to rest where Glacier National park and the Great Bear, Bob Marshall, and Scapegoat Wilderness Areas are today.

Viewed from above, the Lewis Thrust Fault surfaces along a line that runs from Mount Kidd, British Columbia in the north to Steamboat Mountain in west-central Montana in the south. West of it are Belt sedimentary rocks and the start of the Rocky Mountains. East of it are Cretaceous shales and sandstone hills and the beginnings of the northern Great Plains.

The mountains along the fault line -- known collectively as the Overthrust Belt -- consist of long, north-south ridges. The ridges are the upturned edges of the chunks of sedimentary rock that slid eastward and overlapped the ones before them like the shingles on a roof. They stand like a parapet against eastern Montana, but are rootless in the sense that they are not anchored, not sunk into the earth like most mountains. Rather they perch on top of it, unconnected except by juxtaposition to the rock beneath.

Geologists from around the world come to visit the Lewis Thrust Fault because of its size (it is as long as Colorado is wide, which makes it one of the largest thrust faults in the world) and because of the difference between the ages of the rocks on either side of it. The rock on top is 1,300 million years older than the rock underneath, violating the natural law that says rock gets older as you move down through the earth's strata.


-- Excerpt from:
Montana Fish, Wildlife, & Parks Website, 2002

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