The Geologic Story of Glacier National Park
Special Bulletin No. 3
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For the succeeding several hundred million years the geologic history of Glacier National Park is rather obscure, but additional Belt sediments apparently were deposited before uplift of the area caused the sea to withdraw. Following this event many feet of the younger Belt sediments were removed by erosion. The sea probably returned and received more sediments during much of the Paleozoic Era, although no trace of these rocks has been found inside the park boundaries.

CRETACEOUS ROCKS. Not until the Cretaceous period of Earth history, about 100 million years ago, did the geologic record again become clear. At that time a great thickness of mud and sand was deposited in the geosyncline burying deeply the ancient Belt and other rocks which had accumulated as sediment during the preceding several hundred million years. Life had made tremendous advances in this interval, and the abundance of fossils in Cretaceous rocks indicates that the sea swarmed with shelled creatures during that period.

THE LEWIS OVERTHRUST. Toward the end of Cretaceous time tremendous crustal forces, principally from the west, were directed against the geosyncline with the result that its rocks were compressed and uplifted, converting the site of the former sea into a mountainous region. Similar activity took place throughout the length and breadth of the entire geosyncline, which resulted in the formation of the Rocky Mountain system stretching between Mexico and Alaska. A number of mountains were formed on other continents during this period. So widespread and tremendous was the deformation, especially in the present day Rocky Mountain region, that it is known as the Rocky Mountain, or Laramide (after the Laramie Range in Wyoming), revolution. Mountain-building forces continued for several million years in the Glacier Park area, finally squeezing the rocks into a great fold (anticline). Continued pressure from the west overturned the fold and put additional strain on the rock layers, eventually causing them to break along a great low-angle fault. The western limb of the fold, now a great slice of the crust, was driven upward and eastward over the eastern limb ultimately reversing the order of rock layers by placing older on top of younger ones (Figure 3). These younger layers are Cretaceous shales and sandstones underlying the plains immediately east of the mountains. The mountains themselves have been carved by streams and glaciers from the Belt formations comprising the upper block of older rock, that slice of the crust which has been moved more than 15 miles toward the east. The surface over which it was pushed is the Lewis overthrust. At the time this great break occurred the part of it now exposed in Glacier National Park was deeply buried. It was long after that when removal by erosion of overlying Belt rocks, possibly several thousand feet of them, finally exposed the fault.


Movement along this fault was slow — so slow that had people been present at the time they probably would not have been aware that anything of an unusual nature was occurring. Occasionally along many large faults, however, there is sudden movement of small magnitude, usually not more than a few inches, but strong enough to vibrate the crust. These vibrations are earthquakes, and their frequent occurrence in California and elsewhere along the Pacific coast indicates the presence of numerous active faults. Their occurrence also in the northern Rockies, as at Helena, Montana in 1935 and 1936, attests to the fact that some of the faults here are still active.

The Lewis overthrust comes to the surface at the base of the Altyn formation along the entire precipitous east front of the Lewis Range and can be traced nearly 100 miles northward into Canada and for almost an equal distance south of the park. The section lying within the park is tilted very gently toward the southwest, the angle of dip seldom exceeding ten degrees. In some places it is practically horizontal. For this reason the lower courses of all the largest, and some of the small, valleys on the east side of the Lewis Range have been cut entirely through the upper block (overthrust) of Belt rocks down into the weak Cretaceous shales underneath. This causes the trace of the overthrust to be very sinuous and also accounts for the deep indentations in the mountain front formed by Swiftcurrent, St. Mary, Two Medicine, and other valleys. The floors in the lower courses of these valleys, because they lie below the level of the thrust surface, are composed of Cretaceous shales. In most places these rocks are covered by glacial moraine, but they are exposed along the highway from Babb into the Swiftcurrent Valley, especially along the shore of Sherburne Reservoir and near the entrance station. Because these shales readily disintegrate when exposed to the atmosphere they give rise to slumps and landslides which, although of small proportions, cause a great deal of damage to the highway, sections of which must be rebuilt annually. At most damaged spots along the route the shales appear as a dark mud or clay in the roadcuts. The bumpy topography of the whole slope lying north of the road has been formed by innumerable such small landslides.

A deep well located near Cameron Falls in Waterton Townsite (Waterton Lakes National Park) about one mile west of the edge of the mountains passes through 1,500 feet of Belt rocks and then penetrates the Lewis overthrust and the Cretaceous shales beneath.

In the southern part of Glacier National Park just north of Marias Pass, Debris Creek has cut a hole or "window" (known as a fenster by geologists) through the overthrust block (Figure 2). Thus a small area of Cretaceous rock completely surrounded by the Belt series lies within the mountains. This is the only such Cretaceous outcrop in the park, but like the well at Waterton, it serves as a reminder that the rocks of this period are everywhere present under the mountains, and their surface constitutes the "sliding board" over which the upper, more massive block of Belt rocks was pushed. And so we see that the mountains of Glacier National Park, unlike many of the world's great ranges, have no roots, for they rest on a base of greatly different and much less resistant material, the Cretaceous shales. Presumably the Lewis overthrust and Cretaceous rocks beneath it would be penetrated by a well drilled anywhere within the mountains, although in the Livingstone Range the depth of such a well would be very great (Figure 3D).


Erosion in the eastern part of the overthrust block, in addition to producing its crenulated edge, has left several isolated remnants (outliers) east of the main mass of the mountains. The best known of these is Chief Mountain situated near the northeast corner of the park several miles west of the Chief Mountain International Highway. It is a mass of Altyn limestone rising vertically on its east, south and north sides for a distance of 1,500 feet. The Lewis overthrust is well exposed all around its base. Two smaller pinnacles immediately to the west are similar outliers, and, like Chief Mountain, were once part of the main mass of the Lewis Range (Figure 3D and cover sketch). Divide Peak, at the west end of Hudson Bay Divide, is another outlier. It, too, is composed entirely of the Altyn formation.

Although the Lewis overthrust is exposed in a great number of places very few of these are easily accessible, and at only one does a trail provide a close approach to the actual contact between Belt and Cretaceous rocks. The latter site lies along Roes Creek only a few hundred yards from East Glacier Campground. Before reaching the fault at the base of a high cliff of Altyn limestone, the trail crosses several outcrops of Cretaceous sandstone replete with fossil pelecypods (clams) and gastropods (snails). The fault surface is covered by loose rock where the trail crosses it, but on the opposite side of the stream a zone of crushed Altyn limestone and Cretaceous shale is visible.

From U. S. Highway No. 2 just cast of Marias Pass an excellent distant view of the thrust may be obtained. About three miles to the north it appears as a nearly horizontal line high on the side of Summit Mountain. Above it is a vertical cliff in which white Altyn and red Grinnell are prominent, and below is a gentler slope composed of gray-brown Cretaceous shale.

Cretaceous rocks with relatively low resistance to Earth stresses, were strongly crumpled and folded during the period of overthrusting. The folded zone extends several miles eastward from the mountains (Figure 3D), and may be seen to good advantage along Blackfeet Highway on the north side of Two Medicine Ridge, where a series of thin shales and sandstones has been squeezed into anticlines and synclines.

It is because of the Lewis overthrust that there are no significant foothills on the east side of the Lewis Range. The fault has brought into direct contact the massive and resistant Belt rocks which stand up as mountains, and the relatively weak shales of the plains which are carved into subdued landscape features by erosion.


After the Lewis overthrust had taken place, and probably following a period of erosion, the western part of the block broke along a vertical fault and sank several thousand feet. For a short period, of time a lake, in which clay was deposited, covered the floor of this depressed area. The present valley of the North Fork of the Flathead River lies on this downfaulted block (Figure 3D), and the western boundary of the Livingstone Range marks the trace of the fault. Because the fault is of the high-angle variety the front of this range is much straighter than that of the Lewis Range which is formed by the notched eastern edge of the relatively thin overthrust block. The Belton Hills and Apgar Mountains near the park's west entrance are isolated blocks separated from the Livingstone Range by normal faults probably dating from the time the North Fork Valley subsided.

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Last Updated: 11-Jul-2008