Part 3

The Story Continues

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.

FIGURE 2. MAP OF WATERTON-GLACIER INTERNATIONAL PEACE PARK

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 3D).

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 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 east 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 3 D), 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.

The Effect of the Ice Age

In Miocene and Pliocene time the mountains were deeply eroded by streams. It was during this time that Chief Mountain, Divide Peak, and two smaller outliers, and the fenster along Debris Creek were formed. All of the existing mountain valleys were cut out of the overthrust block, although not to as great a depth as they have today. The time required for their formation amounted to several millions of years. The result of all this erosion was a landscape very similar to the present day Blue Ridge in Virginia and North Carolina, the type which geologists call mature.

Near the close of Pliocene time the climate cooled, timberline began to lower, and increasing amounts of permanent snow accumulated in the higher parts of the mountains. Finally glaciers formed from the snow and began to move down the stream-carved valleys. This marked the advent of Pleistocene time (The Glacial Age) nearly a million years ago. Glaciers eventually filled all valleys and covered all the park area except the summits of the highest peaks. Glaciers extended from valleys on the east side of the Lewis Range far out onto the plains, and from the Livingstone Range and the west side of the Lewis Range they moved into the wide Flathead Valley. The forests disappeared and it is probable that not a single tree remained in the area which is now the park. Available evidence indicates that climatic fluctuations during Pleistocene time caused the glaciers to disappear for a considerable period of time, or at least to shrink to insignificant size and then to return. At the end of Pleistocene time they began to shrink and about 9,000 years ago, during what is generally regarded as post-Pleistocene time, disappeared again.

The large Pleistocene glaciers greatly altered the pre-existing landscape of the park by gouging out valleys to much greater depth, and making their sides and heads much steeper than the streams had been able to cut them. Most of the lakes, vertical cliffs, sharp peaks, and waterfalls which constitute much of the park's magnificent scenery were created as a result of intensive glacier action.

The Last Chapter

Although events of the last 9,000 or so years didn't create the large spectacular features of the landscape, this period is nonetheless interesting because it witnessed the birth of all existing park glaciers and the return of the trees composing the present-day forests. As soon as the glaciers began to shrink trees undoubtedly started to reclothe the newly exposed surfaces. New varieties came from areas which had not been glaciated. From the Pacific coast came grand fir, Douglas fir, larch, hemlock, white pine and others. From the east came another group including aspen, paper birch, hawthorn and maple. The native trees driven out by the ice also returned to again become important elements of the flora. These are Engelmann spruce, alpine fir, and lodgepole pine. A few species, among which are the alpine willows, driven southward from the far north during the Pleistocene period still persist at high altitudes but they are always ready to move down into the valleys if the climate should again become cool. Of course, continued warming would cause them to disappear. After the large Pleistocene streams of ice disappeared there followed a period of about 5,000 years during which the climate was somewhat warmer and drier than at present, conditions under which even very small glaciers could not have survived. Then about 4,000 years ago the advent of the cooler climate brought about the origin of the present glaciers. During the period of their existence they have fluctuated in size, probably attaining maximum dimensions around the middle of the last century. Since then they have been steadily shrinking, a sure indication that the climate is becoming milder, as it has so many times in the past.

Surrounding all these small glaciers are recent moraines composed of rock debris eroded from the basins in which glaciers lie. These moraines thus represent the amount of material removed, and then deposited, within the last 4,000 years. They are particularly striking at Grinnell and Sperry Glaciers and at the site of the former Clements Glacier near Logan Pass.

Following disappearance of the large Pleistocene glaciers streams returned to the valleys and began to cut new valleys within the old. Because post-Pleistocene time has been of such short duration these new valleys are small youthful gorges. Interesting examples are Sunrift Gorge, where Baring Creek has cut a narrow channel into the upper part of the Appekunny formation; and the gorge at Hidden Falls on Hidden Creek in the Grinnell Valley. Sunrift Gorge lies only a few feet north of Going-to-the-Sun Highway at Baring Creek bridge, and Hidden Gorge is a stop on the guided trip which Ranger-Naturalists conduct from Many Glacier Hotel to Grinnell Lake. Both of these channels have very smooth, straight sides because they have been eroded along vertical fractures known as joints. The latter are common throughout the mountains and are responsible for the smooth surfaces on some of the highest cliffs. The gorge of Avalanche Creek near Avalanche Campground is another example of post-glacial stream erosion, only here the whirling action of sand and gravel-laden water has carved out a number of cylindrical potholes in the stream course. Some of them, though only 6 to 10 feet across, are 20 or more feet deep.