The mountainside north of this location displays the Lewis Overthrust Fault, where a prominent white limestone layer marks the level beneath which the fault lies as a thin zone of intensely sheared shale. Rocks above that layer were deposited more than 50 miles southwest of here and later moved here as part of the Lewis Thrust Sheet, an enormous slab of Precambrian-age sediment more than a mile thick that encompasses most of today's Glacier National Park. Geologists identify the upper part of the mountain as out of place because the sedimentary layers are out of order: deeply buried and metamorphosed rocks lie atop rocks that were buried only shallowly, rocks with old fossils overlie rocks with young fossils, and former mud above the fault became argillite while mud below remained shale. Below the fault, Cretaceous-age shales and sandstones with abundant marine fossils were deposited about 80 to 110 million years ago, while above it, Precambrian-age argillite and limestone containing stromatolites were deposited about 1.4 billion years ago. Movement of the Lewis Thrust Sheet and many other thrust sheets in the area occurred between 72 and 58 million years ago as the Rocky Mountains were forming, driven by subduction of the Farallon Tectonic Plate beneath the western margin of North America. Marias Pass was long known to the Salish, Kootenai, and Blackfeet, who crossed it to hunt buffalo and raid their neighbors, but by the 1830s few non-Indians dared search for it. Isaac Stevens' 1854 railroad survey failed to find the pass, but Great Northern Railway engineer John F. Stevens located the 5,214-foot pass on December 10, 1889, after pushing through four feet of snow in subzero weather. The Great Northern built its line over Marias Pass in 1891, and a highway was not constructed over it until 1930; before then, motorists on U.S. Highway 2 had to load their automobiles onto railroad cars at East or West Glacier to be shipped around the southern boundary of Glacier National Park.