The mountains of Glacier National Park are made up of many layers of limestone and other rocks formed from sediments deposited under water. The rocks show ripple marks which were made by waves when the rock material was soft sand and mud. Raindrop impressions and sun cracks show that the mud from time to time was exposed to rains and the drying action of the air. These facts indicate that the area now known as "Glacier National Park" was once covered by a shallow sea. At intervals muds were laid down which later became consolidated into rocks known as "shales" and "argillites." Limy or calcareous muds were changed into limestone. The geologist estimates that these depositions were made several hundred million years ago.

In the plains area east of the mountains are other lime and mud formations. These are younger and softer than the rocks which make up the mountains but were undoubtedly formed under much the same conditions. These contain much higher forms of life, such as fish and shells.

When originally laid down all these layers must have been nearly horizontal, just as they are deposited today in bodies of standing water all over the world. Then came a time when the sea slowly but permanently withdrew from the area by an uplift of the land, which since that time has been continuously above sea level. This uplift, one of the greatest in the history of the region, marks the beginning of a long period of erosion which has carved the mountains of Glacier National Park.

The geologist observes that the rock layers are no longer in the horizontal position in which they were laid down. There are folds in the rocks and many breaks or faults cutting across the layers. Furthermore, the oldest rocks in the region are found to be resting on the younger rocks of the adjacent plains. One of the best examples of this is to be seen at Chief Mountain where the ancient limestone rests directly on the young shale below (fig. 1). The same relationship is visible in Cutbank, St. Mary, and Swiftcurrent Valleys. In these areas, however, the exact contact is not always so easy to locate principally because of the debris of weathered rocks that have buried them. What has happened? How did this peculiar relationship come about? The answers to these questions unravel one of the grandest stories in earth history. Forces deep in the earth slowly gathered energy until finally the stress became so great that the rocky crust began to move.

The probable results of the movement in the crust of the earth are shown in the diagram (fig. 2). Section A represents a cross section of the Glacier Park region, as it most likely appeared, immediately following the long period of sedimentation. The rock strata are horizontal. Section B shows the same region after the rock layers have been slightly wrinkled due to the forces from the southwest, which, although slightly relieved by the bending, still persisted and the folds were greatly enlarged as shown in section C. At this stage the folds reached their breaking limit, and the strata broke in a number of places as indicated by dotted lines in the diagram. As a result of this fracturing, the rocks on the west side of the folds were pushed upward and over the rocks on the east, as shown in section D. The mountain rocks (represented by patterns of cross lines) were shoved over the rocks of the plains (represented in white), producing what is known as an "overthrust fault." It has been estimated that the rocks have moved a distance of at least 15 miles.

As the rocks were thrust northeastward and upward they made a greatly elevated region, but did not, however, at any time project into the air, as indicated in section D, because as the rocky mass was being uplifted, streams were wearing it away and cutting deep canyons in its upland portion. The rocks of the mountains, owing to their resistant character, are not worn away as rapidly as the plains formations with the result that great thicknesses of limestone and argillite tower above the plains. Where the older, more massive strata overlie the soft rocks the mountains are terminated by precipitous walls as shown in section E. This explains the absence of foothills that is so conspicuous a feature of this mountain front and one in which it differs from most other ranges.

While the region now known as "Glacier National Park" was being uplifted and faulted, the streams were continually at work. The sand and other abrasive material being swept along on the beds of the streams slowly wore away much of the rock. The uplifting gave the streams life and they consequently cut deep valleys into the mountain area. They cut farther and farther back into the mountain mass until they dissected it, leaving instead of an upland plateau a region of ridges and sharp peaks. This erosional process which has carved the mountains of Glacier Park has produced most of the mountains of the world.

Following their early erosional history, there came a period of much colder climate during which time heavy snows fell and large ice fields were formed throughout the mountain region. At the same time huge continental ice sheets formed in Canada and also in northern Europe. This period, during which glaciers, sometimes over a mile thick, covered many parts of the world including all of Canada and New England and much of North Central United States, is known as the "Ice Age." Such a tremendous covering of ice had an enduring and pronounced effect upon the relief of the country.

In Glacier National Park some of the ice still remains in the higher portions of the valleys and a study of these ice fields helps in interpreting the history of the park during the Ice Age. It is evident that ice did not cover the entire range, but that the higher peaks stood out above the ice, which probably never reached a thickness of over 3,000 feet in this region. The V-shaped valleys which had been produced by stream erosion were filled with glaciers which moved slowly down the valleys. The ice froze onto all loose rock material and carried it forward, using it as abrasive to gouge out the rock, the valley bottoms, and sides. Gradually the valleys were molded until they had acquired a smooth U-shaped character (fig. 3). There are excellent examples of this work of ice in the park, among which are Two Medicine, Cut Bank, St. Mary, Swiftcurrent, and Belly River Valleys.

In addition to smoothing the valley down which they moved, the glaciers produced many rock basins called cirques. These are the result of ice plucking in the regions where the glaciers formed. Alternate freezing and thawing cause the rock to break and the resulting fragments are carried away by the moving ice mass. In the majority of cases the cirques have lakes on their floors. The park is dotted with these beautiful little lakes scattered throughout the high mountain country.

The valley lakes are usually larger than the cirque lakes and have a different origin. As the glaciers melted they deposited huge loads of sand, mud, and boulders in the valley bottoms called moraines. Debris of this nature has helped to hold in the waters of St. Mary, Lower Two Medicine, McDonald, Bowman, and numerous other lakes in the park.