POST-BLACKFOOT EROSIONAL FEATURES
Although the Blackfoot surface may have reached its culmination as recently as the early part of the Pleistocene epoch, it has since been attacked so vigorously that in many places its remnants have not been identified with assurance. Only the major features of the somewhat complex post-Blackfoot history are known. Those features related primarily to stream erosion are described here; features of glacial origin being touched upon in later pages.
After the Blackfoot surface was formed, rejuvenation took place, and streams cut deep valleys in the mountains, reducing the nearby plains to a level close to the present one and well below the ridge tops regarded as remnants of the Blackfoot surface. As some of the ridges are capped by early Pleistocene drift, much of the rejuvenation that largely destroyed the Blackfoot surface took place after the first Pleistocene glaciation. The mountain valleys produced by this rejuvenation were modified by and hence are older than the glaciers of the Wisconsin stage. This indicates that much of the downcutting took place near the middle of the Pleistocene.
The larger mountain valleys contain terraces that record pauses in their formation. Most of the higher terraces are poorly preserved, and many are concealed or obscured by forest growth and landslides. Probably there are three and possibly more terraces between the flood plains and the Blackfoot surface. On the plains likewise, there are bevelled ridge tops that correspond to one or more pauses in down-cutting later than the Blackfoot cycle (Alden and Stebinger, 1913, p. 532-542; Alden 1932, p. 14-15).
The fact that the downcutting did not extend to the heads of present streams is shown by the absence of incised gorges in most of the higher remnants of the Blackfoot surface. This is illustrated diagrammatically by figure 31, which shows a gorge in the foreground that dies out before reaching the higher part of the broad valley carved in Blackfoot time. In a few places the gorges of middle Pleistocene age must have reached almost to the range crests, for Alden mentions one such gorge. His unpublished manuscript on Glacier National Park calls attention to a place at the head of Quartz Creek in the northern part of the Livingstone Range. Here a valley whose shape testifies to its fluviatile origin is incised into relatively level terrain interpreted as a remnant of the headwater part of a surface of Blackfoot age. Although surrounded by cirques, the V-shaped stream valley bears no evidence of glaciation other than smoothed and striated surfaces on its rock walls.
Much of the spectacular character (figs. 3, 16, 27, 28, 30-32) of the present topography of the Glacier Park region results from glaciation. The Flathead region (figs. 1, 2, 22, 23) has also been glaciated, but with less spectacular results, which is largely because much of it is lower and some of the rocks are not adapted to the production and preservation of cliffed forms. While the principal glacial features of the two regions have long been known, the summaries in the present report can do no more than point out salient features. Detailed description and interpretation require much more information than is available.
Alden's descriptions prove that glacial deposits of early Pleistocene age are plentiful on ridge tops in a rather small area in the plains east of Glacier National Park that escaped later glaciation. Evidently when glaciers of the Wisconsin stage spread over the landscape, they removed, or at least rendered unidentifiable, the products of the early glaciers. The known remnants of the earlier deposits range up to more than 200 feet in thickness and reach altitudes of 7,000 feet; so the mountain glaciers that dumped these deposits on the plains may have been extensive. Most of the old drift now remaining lies on flat-topped remnants of the Blackfoot surface (the No. 1 bench of Alden), but some appears to be on surfaces which are over 100 feet lower and belong to Alden's No. 2 bench. Glaciation may have begun after erosion had reached the level of No. 2 bench, or there may have been two early glacial episodes, in one of which ice moved over the Blackfoot (No. 1) surface and the second of which did not take place until after that surface had begun to be dissected to depths of one or two hundred feet. In either case, the glaciers presumably extended headward to gathering grounds high on the peaks of that day and covered much of the mountainous region with ice. That region had topography like that shown in figure 30—much less rugged and less deeply incised than it was during Wisconsin time. This, coupled with the fact that all early Pleistocene drift seems to have been removed from the mountains, suggests that the early glaciers were not as thick as the later ones.
Information as to early glaciation west of the Continental Divide is even more scanty than it is to the east. It is a fair assumption that the western valleys contained glaciers similar in magnitude to those to the east. Presumably ice streams from the mountains poured into the valleys of the ancestral Flathead River and its major tributaries.
After the early glaciers retreated vigorous stream erosion, as already noted, incised the mountain valleys to close to their present depths—locally as much as 3,000 feet below the Blackfoot surface. Figure 31 illustrates the character of the topography that resulted close to the range crests at an early stage in the rejuvenation. Before the return of glaciation, the incisions were somewhat deeper than figure 31 indicates. The deepened valleys were filled with glacial ice, which widened and somewhat deepened them. Most of the spectacular topographic features for which Glacier National Park is noted were formed at that time, and the higher parts of the country came to resemble the scenery shown in figure 32. The glaciers poured far out into the lowlands beyond the mountains. The largest glaciers on the eastern slope were about 50 miles long. On the west the mountain glaciers of the Glacier National Park and Flathead regions joined ice streams from the north that extended as far as the present outlet of Flathead Lake, a distance along the course followed of more than 50 miles from the border of the park. All of the mountains except the higher peaks and ridge crests were buried under the ice streams. Probably few peaks rose more than 1,000 feet above the ice.
A general idea of conditions during the Wisconsin stage of glaciation can be obtained from Alden's map (1932, pl. 37), which shows much of the eastern part of the Glacier Park region and the country toward the east past the site of the town of Cut Bank. Glaciers extended into Canada along the valleys of Belly and Waterton Rivers. Another, fed largely from gathering grounds in the headwaters of Kennedy Creek and St. Mary River, reached the edge of the continental ice sheet near the international boundary. South and east of the site of St. Mary, a large area of the plains does not appear to have been covered by ice during Wisconsin time. The glacier that issued from the valley of Cut Bank Creek was able to push only a few miles east of the right of way of the present highway that skirts the mountain front. The glaciers emerging from the valleys of forks of Two Medicine Creek and from valleys farther south coalesced to form a large glacier that stretched more than 35 miles east of the mountain front and almost reached the lobe of the continental glacier that passed east of the site of Cut Bank. According to Alden, that town and much of the country north and west of it are within the limits of a lake that formed west of the continental ice sheet. It is somewhat strange that the glaciers that emerged from the southern tip of the present park and areas farther south were able to maintain themselves farther across the plains than those from the more rugged, intensely glaciated mountains farther north. The reason may be in part that numerous individual mountain glaciers were able to unite on the plains drained by Two Medicine Creek. Also, the glacial divide, according to Alden's unpublished data, was farther southwest in the general vicinity of Marias Pass than the present Continental Divide.
West of the Continental Divide the effects of mountain glaciation of the Wisconsin stage are somewhat less spectacular than they are to the east, where already steep slopes on hard rocks lent themselves to the formation of cliffs. Nevertheless, all of the valleys were filled almost to their tops, and the ice streams in each united to form large glaciers in the valleys of the principal forks of the Flathead River. Most, if not all, of the spurs whose crests mark the approximate position of the old Blackfoot surface were inundated by the ice and cloaked with morainal material. Data gathered by members of Campbell's parties and by Alden in later visits show that the ice covered the Apgar Mountains completely and left striae on their crests. Thus the glaciers must have attained thicknesses in excess of 3,000 feet. The ice streams joined and passed westward through Bad Rock Canyon and other gaps into Flathead Valley (pl. 3). There they joined large glaciers moving down from Canada. Together the ice streams pushed their way southward to the site of Polson. The terminal moraine near that town forms the southern shore of Flathead Lake and marks the most southerly point reached by the ice during the Pleistocene. According to J. T. Pardee (1942) the ice there met and dammed glacial Lake Missoula, a large water body that was fed by melting glaciers and in part contained by walls of glacial ice. Lakebeds supposed to have been deposited in Lake Missoula are plentiful south of Polson and extend somewhat north of that town on both sides of Flathead Lake. According to unpublished maps made in 1911 by Eugene Stebinger, the moraine at Polson is the younger of two terminal moraines deposited during Wisconsin time. The older moraine has its center about at Charlo, 18 miles farther south and is now surrounded by the deposits (largely silt) laid down by glacial Lake Missoula. The glacier that occupied Flathead Valley must have been one of the principal ones in the Rocky Mountains region. The great masses of ice from the vicinity of Glacier National Park were merely tributaries. Probably the largest single contribution to the ice in Flathead Valley came from Canada through the depression that has been termed "the Rocky Mountain Trench." Other substantial contributions came from the west. The glacier in Flathead Valley was in places more than 15 miles wide and presumably over 3,000 feet in maximum thickness; and according to W. M. Davis (1916), it was powerful enough to shear the ends off the spurs of the Swan Range.
In the preceding summary the glaciation of Wisconsin age has been treated essentially as a unit. This is because of lack of information on which to base division. Most of the glacial sculpture manifest in the present mountains was performed during the Wisconsin. Two or more major fluctuations must have occurred, but adequate discussion cannot be made until detailed studies with this objective in mind have been undertaken.
It appears that the glacial activity of the Pleistocene epoch came to an end roughly 10,000 years ago and was followed by a period of increasing warmth in which most of the glaciers in the Western United States, probably including all in the vicinity of Glacier National Park, melted (Matthes, 1942, p. 190-215; Dyson, 1949b, p. 7-11). About 4,000 years ago another climatic change permitted the reappearance of mountain glaciers in many parts of the Western United States. The present glaciers in and near Glacier National Park were born at that time. They have fluctuated in size but have never approached the dimensions of the glaciers of Wisconsin age. Mainly on the basis of comparison with data assembled by F. E. Matthes for other regions, they are thought to have reached their greatest extent during the middle of the nineteenth century. The glaciers in the park were certainly larger at the beginning of the 20th century (Dyson, 1949b, p. 7-16) than they were 48 years later. However, about 1950 such extensive snow banks began to be recorded during the winter that there is hope that the glaciers may have a period of renewed growth. Matthes (1942) states that retreat of the mountain glaciers at the rates observed during much of the first half of the 20th century might well mean their obliteration within a few decades. He emphasizes, however, that in Europe and elsewhere, where glaciers have been under observation for much of historic time, fluctuations are the rule. Whether or not it has already begun, resurgence of the glaciers in and near Glacier National Park is to be expected before the present ones have disappeared.
RECENT EROSIONAL FEATURES
When the glaciers of Wisconsin time retreated, they left the bottoms of the mountain valleys broadened and mantled by glacial debris. Streams, which were at first fed by melt water from the glaciers, reworked the debris and produced flood plains. Since then in nearly every valley the flood plain has been cut into by a narrow gorge one to several score feet in depth that contains the present stream. These inner gorges, rarely more than 75 feet deep except near and along the major branches of the Flathead River, commonly penetrate short distances into bedrock. They are too narrow to show distinctly in the contours of the topographic maps. In a few places such as upper Nyack Creek, the rock revealed in the gorges has been shown on the geologic maps (with slight exaggeration in width) (pls. 1, 2). Attention was first called to inner gorges of this character by Deiss (1943b, p. 1164), who worked south of the Flathead region. They are not uniformly developed, in part because of the difficulty experienced by some streams in cutting through thick and irregularly distributed glacial debris. Along some stretches of such tributaries as MacDonald, Nyack, and Ole Creeks, the inner gorges are quite large enough to attract the attention of travelers. They are not conspicuous in many parts of the Flathead region, but are, nevertheless, present there. Among the streams in that region that display them may be mentioned Graves and Sullivan Creeks in the southwest part of the Nyack quadrangle and Badger Creek and its tributaries in the eastern part of the Marias Pass quadrangle.
The inner gorges along the main branches of the Flathead River are over 100 feet deep and have in places been obscured as a result of the meandering of these powerful streams. Parts of the gorge walls have been cut back and even obliterated in places. Local flood plains have formed close to the modern rivers and within the inner gorges. Where these are broad enough, the river has meandered. The detailed topographic maps of the South Fork illustrate these features (U. S. Geol. Survey 1937, 1939, 1947, 1950). They also show the inner gorges along the lower reaches of some tributaries, although not as effectively as if the contouring had been carried a little farther above present channels. Similar features are shown by the detailed maps of the main river and of the Middle Fork. For several miles above West Glacier (Belton) the meanders of the Middle Fork are deeply incised in a rock gorge and thereby differ sharply from the wanderings of the river over modern flood plains farther upstream and from most meanders of the other branches of the Flathead River. Presumably the incised meanders originated on a former flood plain at an earlier date, perhaps when the stream flow was checked during one of the readjustments in drainage commented on above.
The present drainage pattern presents features of some interest. As already noted, it is in many aspects inherited from the pattern whose development began as a result of uplift after the deformation near the beginning of the Tertiary period. As no general peneplain has been produced and no regional mantle of deposits, either sedimentary or volcanic, has been laid down since that deformation, the concept of superposition is not applicable to any large part of the drainage pattern. Deiss (1943b, p. 1162-1163) has offered suggestions of that kind to explain features in the mountains south of the eastern part of the Flathead region. He calls attention to streams that flow toward the Great Plains directly across the major structure. His explanation is that these streams originated on an erosion surface that had attained maturity after the major deformation and which was subjected to broad upwarp near the end of the Tertiary period. He assumes that northeastward-flowing streams were established on a plain on the site of the present mountains, maintained themselves during uplift, and were accelerated when the mountain area rose above the level of the present plains to the east. If the Blackfoot surface, or a surface closely allied thereto, was very extensively developed in the area described by Deiss, it is possible that some streams that originated on the more thoroughly beveled parts of that surface might have survived in the manner he postulates. Both in his area and in the eastern part of the Flathead region, some of the major streams find their way to the plains through gorges that cut directly through large ridges in a manner that seems to call for some such explanation as the one he offers. However, that explanation, if valid at all, can be applied only to limited areas. Bear Creek, Ole Creek, and others that head close to the border of the plains but nevertheless flow southwest transverse to the regional structure do not fit Deiss' hypothesis. Ole Creek in part coincides with a transverse fault, and some of the others may have been excavated along similar lines of weakness. Transverse streams that flow either northeast or southwest are so numerous and demonstrated transverse faults are so rare that excavation along lines of weakness is not entirely satisfactory as a general explanation—especially as the faults are old and recent revivals of them are unproved. Probably the present drainage system has multiple origin. Many of the streams have survived, with modifications from those of early Tertiary age, related in one way or another to the structure of the rocks; some may have originated on the Blackfoot surface; many of the more abnormal-appearing ones may result from the linking together of parts of streams of diverse origin through piracy.
In a region of long-continued uplift, piracy is likely to be widespread. Probable examples of it are numerous, and the process is still going on. For example, Badger Creek and its forks, with numerous right-angled turns and an exit to the plains through an impressive gorge, seems clearly to be made up of parts of several streams. How the stream managed to cross the steep mountain front in the northeast corner of T. 29 N., R. 11 W., is not known. Some factor, possibly difficulty in traversing the glacial deposits so liberally dumped on the nearby plains, checked the flow of the creek in fairly recent time. Before it reestablished flow northeastward, the stream deposited a broad expanse of alluvium just inside of the mountain border. This alluvium is still dotted with marshes and small ponds, resulting from imperfect drainage. The interference with the development of Badger Creek thus recorded may explain why its tributaries have failed to extend themselves beyond Muskrat and Badger Passes in competition with the tributaries of the Middle Fork of the Flathead. The long distance the latter has to wander through the mountains would seem to put it at a disadvantage. Even so, its headwaters have penetrated country underlain by hard limestone, and it seems likely that the stream will soon acquire the upper tributaries of the South Fork of Badger Creek. The somewhat greater precipitation on the southwest side of the Continental Divide is a contributing factor. There is also a contest between Badger Creek and the South Fork of Two Medicine Creek. The undisturbed glacial deposits on the divide between them suggest that neither has made much headway against the other since the mountain glaciers retreated. The North Fork of Badger Creek is in an area of highly disturbed rocks. It may have been aided in excavating its canyon by a transverse zone of fracture. Many other oddities in the drainage pattern of the Flathead region suggest readjustments in the streams during the Quaternary period. Among these is Lodgepole Mountain, an island of hard rock surrounded by soft deposits.
The Glacier National Park region contains similar features. The transverse streams flowing southwest have made greater inroads on the mountain block than their counterparts with northeasterly flow—possibly because of the difference in precipitation on the opposite sides of the Continental Divide. Their headwaters are now spreading up longitudinal valleys. If this tendency persists, it seems possible that they may become so linked as to split the part of the Lewis Range in the southern part of the park by means of a series of longitudinal valleys, permitting the Livingstone Range to extend to the valley of Bear Creek. If so, the Livingstone Range would continue to be cut by McDonald Creek but would be a more impressive topographic feature than it now is.
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______ 1949, Interpretations of foothills structures, Alberta, Canada: Am. Assoc. Petroleum Geologists Bull., v. 33, no. 9, p. 1475-1501.
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______ 1922, The historical and structural geology of the southernmost Rocky Mountains of Canada: Royal Soc. Canada Trans., 3d ser., v. 16, sec. 5, p. 97-132.
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______ 1942, Glaciers, chap. 5 in Hydrology, Physics of the Earth, v. 9, p. 190-215, New York, McGraw-Hill Book Co.
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______ 1942, Unusual currents in glacial Lake Missoula, Montana: Geol. Soc. America Bull., v. 53, p. 1569-1600.
______ 1950, Late Cenozoic block faulting in western Montana: Geol. Soc. America Bull., v. 61, p. 359-406.
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Rezak, Richard, 1957, Stromatolites of the Belt series in Glacier National Park and vicinity, Montana: U. S. Geol. Survey Prof. Paper 294—D.
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______ 1950, Fluorspar prospects of Montana: U. S. Geol. Survey Bull. 955—E, p. 173-224.
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______ 1954, Mammalian fauna of the Kishenehn formation, southeastern British Columbia: Canada Natl. Mus. Bull., no. 132, p. 92-111.
______ 1955, Additions to the molluscan fauna of the Kishenehn formation, southeastern British Columbia and adjacent Montana: Canada Natl. Mus. Bull., no. 136, p. 102-119.
Russell, L. S., and Landes, R. W., 1940, Geology of the southern Alberta plains: Canada Geol. Survey Mem. 221.
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______ 1915, Geology of Cranbook Map-area, British Columbia: Canada Geol. Survey, Mem. 76, Geol. Ser. 62, 245 p.
______ 1920, The origin of the Rocky Mountain Trench: Royal Soc. Canada Trans., Sec. 4, p. 73-81.
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______ 1946, Devonian stratigraphy of central and northwestern Montana: U. S. Geol. Survey Oil and Gas Inv. Prelim. Chart 25.
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______ 1916, Geology and coal resources of northern Teton County, Montana: U. S. Geol. Survey Bull. 621, p. 117-156.
______ 1917, Anticlines in the Blackfeet Indian Reservation, Montana: U. S. Geol. Survey Bull. 641, p. 281-305.
______ 1918, Oil and gas geology of the Birch Creek-Sun River area, northwestern Montana: U. S. Geol. Survey Bull. 691, p. 149-184.
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U S. Geological Survey, 1937, Plan and profile of South Fork of Flathead River, Montana, from mouth to mile 44, with dam sites.
______ 1939, Plans and profile of South Fork of Flathead River, Montana, above mile 44, and tributaries.
______ 1947, Plan and profile of Flathead River, Flathead Lake to International Boundary, Montana, with dam sites.
______ 1950, Plan and profile of Middle Fork Flathead River from mouth to mile 49, with dam sites.
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______ 1952, Surface water supply of the United States, 1949, pt. 6, Pacific Slope Basins in Washington and Upper Columbia River Basin: U. S. Geol. Survey Water-Supply Paper 1152, 377 p.
______ 1955, Surface water supply of the United States, 1953, pt. 6A, Missouri River basin above Sioux City, Iowa: U. S. Geol. Survey Water-Supply Paper 1279, 485 p. .
______ 1957, Surface water supply of the United States, 1955, pt. 5, Hudson Bay and Upper Mississippi River Basins: U. S. Geol. Survey Water-Supply Paper 1388, 548 p.
______ 1957, Surface water supply of the United States. pt. 12, Pacific Slope Basins in Washington and Upper Columbia River Basin: U. S. Geol. Survey Water-Supply Paper 1936, 333 p.
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______ 1906, Algonkian formations of northwestern Montana: Geol. Soc. America Bull., v. 17, p. 1-28.
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______ 1927, Normal faulting on upthrust arches [abs.]: Geol. Soc. America Bull., v. 38, no. 1, p. 203-204.
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______ 1938, Lexicon of geologic names of the United States: U. S. Geol. Survey Bull. 896, 2396 p.