The Lewis Overthrust Fault and Marias Pass

By editor

East Glacier Park, Glacier County, Montana

The mountains that rise abruptly beyond East Glacier Park are more than mere piles of stone; they are chapters in the immense chronicle of Earth’s history, written in rock and folded by the slow, inexorable forces of tectonic motion. These peaks belong to the Belt Supergroup, a vast assemblage of sedimentary rocks laid down in a shallow sea that once stretched across what is now western Montana, northern Idaho, and parts of Canada. These mudstones, limestones, and sandstones were deposited more than a billion years ago during the Precambrian era, making them some of the oldest sedimentary rocks exposed at the surface anywhere in North America. The very whiteness of the limestone layer halfway up the mountainside reveals the long, hidden saga of ancient seas and tectonic upheaval.

What astonishes geologists and naturalists alike is that these ancient rocks do not lie beneath younger formations, as one might expect. Instead, they rest upon a foundation of much younger Cretaceous shales, rocks that formed roughly 70 million years ago. This inversion of the geological order arises from a colossal geological event known as the Lewis Overthrust Fault. During the Laramide orogeny, which began about 80 million years ago and extended into the Paleocene, immense compressive forces pushed a slab of Precambrian crust eastward for more than 50 miles over younger strata, along a nearly horizontal fault plane. This slab, the Lewis Thrust Sheet, measures over a mile thick and encompasses much of what is now Glacier National Park.

The Lewis Overthrust Fault itself is a narrow zone of intensely sheared shale, invisible to the casual observer but holding the key to this geological puzzle. Above this fault lies the out-of-place slab of ancient rock, while below, softer, younger shales have weathered into gentler slopes. Geologists recognize this displacement through careful study of fossil assemblages and metamorphic grades: older, metamorphosed rocks with Precambrian fossils overlie younger sedimentary layers bearing fossils of Cretaceous marine creatures. As J. Hoover Mackin, a geologist who studied the region, remarked, “The importance of the Lewis Overthrust lies not only in its size but in the clarity with which it exhibits the processes of mountain building.”

The origin of this immense thrust fault is tied to the subduction of the Farallon Plate beneath the western edge of North America. As the oceanic plate plunged beneath the continent, compressive stresses folded and fractured the crust, pushing slabs of rock eastward in a series of thrust sheets. These tectonic motions formed the Rocky Mountains, but the Lewis Overthrust is among the most spectacular examples of such structural displacement.

To stand at the foot of these mountains is to witness eons of geological time compressed into a single vista. The white band of limestone visible from the highway marks the thrust fault boundary. Above it, the harder Precambrian rocks form rugged peaks dotted with ancient fossils of stromatolites and algae mats, relics from a world long before complex life emerged on land. Below, the softer shales slope gently, giving way to verdant forests of Douglas fir (Pseudotsuga menziesii), subalpine fir (Abies lasiocarpa), and Engelmann spruce (Picea engelmannii). The air here carries the scent of pine and damp earth, and the call of the Clark’s nutcracker (Nucifraga columbiana) rings clearly across the valleys.

Not far from this grand geological feature lies Marias Pass, a mountain pass that has played an equally significant role in the human history of the region. At an elevation of 5,214 feet, it is the lowest crossing of the Continental Divide in the northern Rockies, providing a natural corridor through otherwise rugged terrain. Native peoples, including the Salish, Kootenai, and Blackfeet, traversed this pass for centuries, using it to hunt buffalo on the eastern plains and engage in intertribal commerce and sometimes conflict.

Despite its utility, Marias Pass eluded European-American explorers for decades. In 1854, Isaac Stevens, then governor of Washington Territory and leader of the Northern Pacific Railway survey, attempted to locate a viable railroad route through the Rockies. His survey failed to find Marias Pass, hindered by dense forest, deep snow, and the formidable terrain. It was not until December 10, 1889, that the pass was rediscovered by John F. Stevens, an engineer for the Great Northern Railway. Stevens faced brutal winter conditions, plowing through four feet of snow and enduring temperatures well below freezing. His persistence paid off when he finally crossed the pass, recognizing its strategic value for rail transport.

Stevens later wrote of that moment: “The pass is the key to the northern Rockies, a gentle gateway through formidable barriers. Its discovery will open the way for commerce and connection.” The Great Northern Railway swiftly took advantage, completing a rail line over Marias Pass by 1891, facilitating the movement of goods and people across the Continental Divide. It was not until four decades later, in 1930, that a highway was constructed over the pass, further integrating this remote region into the expanding networks of the American West.

The natural history and human history here intertwine in a landscape shaped by slow geological forces and swift human endeavor. The Lewis Overthrust Fault reveals the profound power of plate tectonics, while Marias Pass testifies to the ingenuity and determination of explorers and engineers who sought to tame the mountains. Together they form a narrative that spans from the Precambrian seas to the age of railroads, a story etched into the very fabric of Montana’s wilderness.

Today, visitors to Glacier National Park may stand beside the highway, gazing northward at the limestone layer that marks the fault. They may imagine the ancient sea where sediments settled, the grinding forces that thrust these rocks eastward, and the footsteps of native hunters and railway engineers who crossed the divide. In this place, the deep past meets human history, each layer revealing the passage of time in stone and story.

See also

• The Lewis Overthrust Fault and Marias Pass at East Glacier Park, Glacier County
• Time Machines at East Glacier Park, Glacier County
• Glacier Park Woman's Club at East Glacier Park, Glacier County