Geological Survey Professional Paper 294—K The Rocks and Fossils of Glacier National Park: The Story of Their Origin and History

In the development of the topography of the Glacier National Park region, deformation of the earth's crust by titanic pressures and sculpture of the surface through the agencies of weathering and erosion went hand in hand. Streams, glaciers, and wind; heat, cold, and frost; rain and avalanches—various forces of this kind have affected the region at one time or another, and all have played a part in giving character to the landscape. But these forces would not have been able to bring into being the majestic scenery of the present if the earth's crust had remained quiescent since the dramatic disturbances associated with the Lewis overthrust died out. No comparably violent earth movements have taken place. On the contrary, the relatively recent crustal movements have been mainly intermittent, broad uplifts in which the mountainous area must have risen at a faster rate than the plains to the east. Nearly all the movements were so gentle that they left little discernible imprint on the rocks; their record is to be sought in complexities and apparently anomalous features of the topography.

Throughout the late Tertiary and probably somewhat beyond the close of that period the process of carving and modifying the topography continued steadily. Uplift persisted enough so that the streams remained vigorous. Erosion worked faster than crustal rise, as illustrated by the difference in relief above sea level between plate 53 C and D. The streams that existed when movement along the Lewis overthrust came to an end were guided to a large extent by structural features and by any irregularities that may have persisted from the time, long previous, when the land emerged from the sea. Many streams had sought out courses along faults and related features, including minor fracture zones. As the principal faults and folds trended toward the northwest, similar trends had become evident also in the orientation of major valleys and ridges. Many of the other mountain valleys trend roughly at right angles to these. A few, such as the valley of Ole Creek, follow known subsidiary faults that can be mapped with some degree of certainty. Others may follow undetected faults or fracture zones that have weakened the rocks without noticeably moving them. Many of the valleys began to be carved before the post-Belt rocks were eroded away and might have been controlled by features in those rocks not detectable in the resistant rocks that remain.

With continued modification of the landscape, the streams gradually deepened their channels and, in consequence, flowed more and more slowly. Meanwhile the principal valleys acquired gentle slopes and broad, flat floors, and the uplands between the valleys assumed rounded forms. If this had continued long enough the valleys would have been widened until the ridges between them were all but consumed, so that the region would have been essentially reduced to a plain, across which the streams would have meandered sluggishly. Actually, this did not happen. The principal mountain masses were not obliterated or even extensively subdued. Although the valleys were far broader and their sides gentler than they are at present, the streams remained vigorous enough to carry gravel from the mountains outward to mantle the plains to the east. The landscape forms more nearly resembled those near and west of the town of Kalispell (pl. 52) than the cliffs, canyons, and pinnacles of the present park. For convenience, the old surface to which those forms belonged has been called the Blackfoot surface. Figure 142 is an attempt to reconstruct the appearance of a part of that surface. At present the maximum relief within the park is nearly 7,400 feet. In contrast, the maximum relief there in Blackfoot time might have been between 2,500 and 3,000 feet.

Remnants of the Blackfoot surface are widely scattered over the park and the surrounding country. All of them, of course, have been modified by later events, many so much so that recognition is difficult or doubtful. The largest existing remnants of lowlands of Blackfoot time within the mountains are the broad, nearly level summits of Flattop Mountain and West Flattop Mountain. In fact, these mountains were so named because both are surmounted by remnants of what probably was once a very wide valley floor. The preservation of features such as these in the midst of a region as rugged as Glacier National Park seems extraordinary and anomalous, but several reasons for it can be offered. The lowland of which they are remnants was one of the largest in the mountain area because it was developed along one of the greatest of the downwarps in the region, a downwarp so broad that the rocks along its lowest part were scarcely flexed at all. The nearly flat strata that are exposed in the bottom of the downwarp in the locality of the two Flattops are hard enough to have resisted erosion. The canyons of McDonald Creek, Mineral Creek, and others have cut deeply, but substantial patches of the old surface, held up by resistant rock, are preserved. Smaller flat areas, such as Granite Park, The Hanging Gardens, and many others, originally merged with the plain on the summits of the two Flattops. Undoubtedly the plain or broad valley once extended northward over the present Waterton Valley, for the flat area south of Kootenai Peak, the nearly flat crest of Porcupine Ridge, and other similar features near the Canadian border closely correspond to it in altitude.

Beyond the mountain front, in the Great Plains to the east and southeast of the park, are hills with flat tops bearing gravel. These flat summits likewise represent erosion surfaces, a few small patches of which have survived here whereas elsewhere they have been worn away. The most prominent summit flats are probably remnants of the Blackfoot surface. St. Mary Ridge and Milk River Ridge are among the larger hills thus capped. The flat ridge tops correspond in position to portions of a nearly flat plain believed to have been present at the time that the broad valleys were forming within the mountain mass. Streams emerging from the mountain valleys deposited gravel on the plain at the eastern border.

Another broad valley lay along the west border of the park, where now is situated the southeastward-flowing part of the main Flathead River, locally called the North Fork. Here the long, smooth-topped spurs between the river and the southwest side of the Livingstone Range are so different from the rugged mountains of that range as to demand an explanation. Unlike the two Flattops, the spurs are composed of soft strata of Tertiary age and are mantled by still younger glacial debris. The spur crests are believed to be modified remnants of the floor of a valley of the Blackfoot surface, now cut by streams from the Livingstone Range.

The topography of Glacier National Park has been so greatly modified during the last million years that it is now difficult to reconstruct all the features of the Blackfoot landscape. Nevertheless it is certain that that landscape differed from the present one in having lower local relief and more subdued contours and also in details of the drainage pattern. A striking instance of the latter difference is to be found along the western border of the park. When one views the present topography from the lookout above Hidden Lake near U. S. Highway 2, or from a similar vantage point, he is struck by the fact that the valley of the upper part of the Flathead River (the North Fork) lines up so perfectly with the valley of the Middle Fork of the Flathead that they look like a single valley. This is far from being true nowadays. The two branches of the Flathead now flow in opposite directions, and both swing sharply westward before they come together southwest of the Apgar Mountains. East of the Apgar Mountains lies a broad, high depression which is wholly streamless in that central part called McGees Meadow and which elsewhere is drained only by streams so tiny that they could have had little to do with the carving of the depression. One can hardly escape the conclusion that formerly a single river flowed northwest from near the mouth of Bear Creek down the valley of the Middle Fork, over McGees Meadow, and thence into Canada.