The Geologic Story of Glacier National Park

by James L. Dyson

LEGEND 1. TRACE OF LEWIS OVERTHRUST 2. ALLUVIAL FANS 3. RANGE CRESTS 4. HIGHWAYS

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 3]D).

FIGURE 3. HISTORY OF LEWIS OVERTHRUST

A HYPOTHETICAL SECTION NEAR THE CLOSE OF CRETACEOUS TIME. B PRESSURE FROM WEST CREATES LARGE FOLD AND OVERTURNS IT TOWARDS EAST. THE NEW MOUNTAINS ARE CUT DOWN BY EROSION. C CONTINUED PRESSURE BREAKS THE STRATA AND THE LEWIS OVERTHRUST IS FORMED. DOTTED LINE SHOWS APPROXIMATE PRESENT OUTLINE OF SURFACE D SECTION ACROSS THE PARK SHOWING RELATIONSHIPS OF ROCK FORMATIONS AND THE LEWIS OVERTHRUST (VERTICAL SCALE GREATLY EXAGGERATED)

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 3]D 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.