THE BELT SEA AND ITS INHABITANTS
The predominant rocks of Glacier National Park, the Belt series, of almost inconceivable antiquity, were formed from sediments laid down more than half a billion years ago. Even then, the earth had been in existence about two billion years, long enough so that it had acquired, in a general way, its present pattern of continents and ocean basins. Thus, in that remote period at which the geologic history of the region begins, North America was already in existence, although its appearance then was very different from its appearance today. Then, as now, the earth was subject, somewhat intermittently, to great strains that significantly changed its crustal features. The surface features, the topography, of the earth have undergone practically continuous modifications.
During the long history of the continent, shallow seas repeatedly encroached upon it, submerging great areas and at times forming long seaways through the present interior. At times more of the continent might have been above sea level than at present, but there certainly were many periods in which vast expanses of what is now dry land were covered by water and received deposits from that water. Now advancing, now retreating, changing ceaselessly in position and configuration but so slowly that scarcely any differences could have been discerned within the three score years and ten of a man's lifetime, many of the seas persisted for millions of yearsperiods long even in terms of geologic timeonly to become restricted or to disappear at last.
Changing sea basins were among other results of the strains from which the earth suffered. At intervals the rocks along certain stretches of the surface of North America were subjected to such great pressures that the were warped and flexed over rather wide expanses, or folded, crumpled, and fractured in narrower zones of more intense movement. Effects of these kinds influenced the rocks for miles beneath the surface of the ground. Naturally, deformation tended to be concentrated in relatively weak zones. Many of these were in places where depressions in the surface had been filled with exceptional thicknesses of sedimentary rocks.
These deposits had been laid down in the temporary seas, in some places in a succession of such seas. Originally the deposits included mud, sand, lime, and other materials, mostly derived from the adjacent lands. Even when hardened into sedimentary rocks, such as shale, sandstone, and limestone, these materials yield more readily when force is applied to them than do the great bodies of massive crystalline rocks that make up so much of the earth's crust.
Just how and why the forces originate and are applied and how the depressions or basins were localized where they were are matters that are still open to debate. We are learning some things about these matters, but much remains unknown or uncertain. Theories are plentiful but proof is being gathered slowly. Notwithstanding these uncertainties as to causes, it is clear that such forces, acting parallel to the earth's surface, have from time to time been able to overcome the resistance of rocks, particularly in sediment-filled depressions.
The Belt series and the younger beds that, as indicated in appendix A, originally overlay them constituted a thick accumulation of comparatively weak rocks, ready to yield when adequate pressures were applied. The younger rocks are thick and weak but the Belt series is even thicker. At the present time the beds composing the Belt series are hardened into argillite, quartzite, and crystalline limestone, but they were originally soft muds and sands. The surprising thing is not that the rocks of the region yielded in spectacular fashion when suitable pressures were applied, but that events of this kind were so long delayed.
In the mountains of Glacier National Park the sedimentary rocks that are younger than the Belt series, and even parts of the latter, have long been eroded away. To study the younger rocks, one must journey beyond the park into areas where they are still preserved.
Belt sediments were laid down in a basin, or perhaps at times a group of basins, that extended from the Arctic Ocean, in the region of Hudson Bay, southward through Canada and into western Montana and Idaho. The southern margin might have been in southern Idaho, at about the latitude of the Snake River Plain. Rocks that somewhat resemble the Belt series are exposed at intervals far to the south of the Snake River Plain, but little is known in regard to their relationships to that series.
The western and eastern margins of the depressions cannot be accurately determined. Rocks belonging to the Belt series extend far to the west of the park and are buried under the extensive lava flows of eastern Washington. East of the park some rocks correlated with the Belt series extend well into eastern Montana, but most of these probably belong to the younger parts of the series. Early in its history the eastern edge of the Belt sea might have been between the park and the Sweetgrass Hills, a hundred miles to the east.
Clearly the Belt series, like many other widespread sedimentary deposits, was laid down in a great depression. The floor of this depression was of such extremely low relief that for all practical purposes it might be considered to have been a vast, almost featureless plain. The Belt sea, which by no means occupied the entire depression, was much shallower than later seas that invaded Montana and other parts of North America. The evidence for the shallow nature of the Belt sea led some earlier geologists to believe that the sediments were deposited in lakes rather than a sea. Recent studies of the sediments, however, indicate that they were laid down in water bodies that were so salty and so extensive that they are better thought of as seas than lakes. All the seas that spread over continental areas at intervals during geologic time were shallower than the present oceans. Their maximum depths might have been hundreds rather than thousands of feet. Perhaps parts of the Belt sea attained depths of a few hundred feet, but during much of its existence the average depth was probably less than a hundred feet. Such things as ripple marks, mud cracks, clay spalls, raindrop impressions, certain kinds of fossil algae, and a few casts of salt crystalsall point to very shallow conditions and even exposure, at times, in mud flats from which the water had retreated.
Shallow though the Belt sea must have been, its deposits now have an aggregate thickness of more than four miles and in regions south of the park the thickness is even greater. How is it possible to reconcile the concept of deposition in a shallow sea with the fact that strata more than 20,000 feet thick accumulated in that sea? At first glance such a situation seems impossible, yet a reasonable solution is reached if one assumes that the marine strata were deposited on a subsiding sea floorone that sank, as the sediments accumulated on it, slowly enough so that the shallow depths of water were maintained.
Why did the basin sink? Geologists study the earth's crust and its properties, as revealed not only by the rocks but also by precise surveying and interpretation of earthquake, gravity, and magnetic data. As a result of such studies, it is now generally believed that beneath an outer skin of rather rigid rock lies a thin layer that behaves like a stiff liquid. Parts of the crust that are overloaded sink in this liquid, and parts that are lighter rise proportionately.
Although this mechanism may operate in a broad way, it might not have been the only factor in the deposition of the Belt series. For the process to work perfectly every time that a layer of mud or similar sediment is spread over the sea bottom the increase in the load that results must cause subsidence. At the same time, the land from which that mud was removed must be lightened correspondingly and therefore must rise. Considering the wide areas of the earth's crust involved and the small weight of any individual layer of mud deposited, it does seem to be asking a good deal of the process of adjustment to regard it as the sole factor involved. Granting that it is a factor, and probably a very important one, other factors might have contributed also. Conceivably, lateral pressures, possibly in continuation of those that produced the basin in the first place, had a significant effect. Tidal measurements in the Gulf of Bothnia, the northern arm of the Baltic Sea, indicate, even today, the dynamic character of the earth's crust. In that region that crust is rising at a rate of about three feet per centurywhile deposition is also going on. If this uplift continues, the floor of the Gulf of Bothnia will become dry land. The fact remains that throughout the earth's history regions of the crust have repeatedly risen and subsided.
Earlier in this discussion we mentioned that large areas of the Belt sea were at times exposed as mud flats. The exposure of these large areas must have resulted from a combination of circumstances. The influence of tides might have been an important factor, although we have no idea of their magnitude during Belt time. The small vertical movement of the vast, almost reliefless plain that contained the Belt sea was probably not uniform over the entire area, and some tilting of the surface must have occurred. In such a shallow sea, a very small amount of tilting would cause radical changes in the position of the shorelines. To complicate matters, as the basin was subsiding, sediments were being carried to the sea by streams. The volume of the material furnished to the sea by the streams probably varied from season to season. During rainy periods when stream levels were higher a greater volume of sediment would be transported to the sea than during the dry periods. Thus, at times deposition would exceed the rate of subsidence and mud flats would develop close to the mouths of rivers and along the shorelines. When the volume of sediment decreased, during the dry periods, sinking could catch up with and pass the rate of upbuilding of the sea bottom, causing the mud flats to become submerged again. The great thicknesses of mud-cracked rock in the Belt series can be satisfactorily explained in this manner.
The occasional presence of salt-crystal casts also seems to indicate changes from warm moist seasons to warm dry seasons. The salt-crystal casts are not common and seem to be restricted to the more inaccessible areas of the park. Sparse occurrence of these casts seems to indicate that at no time during its history was the Belt sea extremely saline. It is believed that the beds containing these salt crystals were deposited in lagoons, completely separated from the sea by land-surface irregularities along the borders of the sea during the early parts of the dry cycles. At that time the mud flats were still exposed, and here and there behind the shoreline such lagoons might have existed. Evaporation of these small bodies of water would cause an increase in the salinity of the water and eventually salt crystals would have formed.
The foregoing discussion has given us a general idea of the environment in which the Belt rocks were deposited. In brief, the Belt rocks were deposited in a shallow, shifting sea that lay on a vast, almost featureless plain, in a climate that was generally warm and alternately humid and semiarid.
The Belt series is older than the rocks in which fossil remains are generally known to be plentiful. However, the presence in it of traces of organisms has long been known. The most abundant and most definitely recognizable are records of the former presence of certain primitive plants known as algae. Fossils of this kind are especially well displayed in Glacier National Park and have received much study there. They are visible at a number of places along Going-to-the-Sun Highway (fig. 124).
Algae are of many kinds and grow under many different environments. Everyone, whether he realizes it or not, comes into contact with certain kinds of algae almost daily. Webster's New Collegiate Dictionary defines the word "alga" as, "Any plant of a group (Algae) comprising practically all seaweeds, as rockweed, sea lettuce, etc., and allied fresh-water or nonaquatic forms, as pond scums, stoneworts, etc."
There is a great range in size and complexity among algae. Some are microscopic unicellular plants; others are large masses, colonies consisting of millions of cells and reaching lengths of several hundred feet. Some of the unicellular algae impart a characteristic color, ordinarily green, to the water in which they live. The pond scums and allied forms are of this type. Most algae are water dwellers, although some grow on soils, others on the stems of trees, and one species of red algae may be seen growing on the snowfields of Glacier National Park Algae constitute most of the vegetation of the ocean. These marine algae serve as one of the most important sources of food for other life in the sea.
Most modern algae lack characteristics which make possible their preservation as fossils. Only algae that cause the precipitation of lime from the surrounding sea water and those that actually secrete lime or silica in their cell walls can be preserved after burial in the form of fossils. Algae of these types are responsible for the fossil records in the Belt series.
Appendix C gives the classification of fossil algae which is based on that of modern algae. The fossil algae of the Belt series belong to the group called Spongiostroma. As indicated in the table, microscopic plant structure is rarely recognized in these fossils. As a consequence of this apparent absence of microscopic structures, the algae of the Belt series have been classified on the basis of the external form and growth habit of the colonies.
The ancient algae of the Belt sea probably lived under environmental conditions not greatly different from those of the marshy flats around the Bahama Islands or parts of some of the coral islands of the southwest Pacific. True, some of the latter are commonly regarded as coral reefs, but actually they have been built up of limy materials, including skeletal parts, contributed by many kinds of sea-living organisms, and lime precipitated by algae is among the most abundant of these.
If this comparison is valid, what a contrast the present scene offers to the scene which imagination conjures out of the geologic past! Where the snow-clad peaks of the Rocky Mountains now stand, then stretched the monotonous reaches of the Belt sea. In its clear waters grew brightly colored algae, their varied colors made even brighter by sunlightfor algae can grow only in water sufficiently shallow and free of sediment to permit the penetration of sunlight. Under favorable conditions much of the sea bottom was covered with closely packed algal colonies. At other times there were isolated reefs and patches of algal growths. Around and above the colonies, each with its core of hard, precipitated lime, delicate tendrils floated. The drab gray and buff hues of present outcrops give no hint of the vivid colors that the soft tissues of the living plants must have displayed: shades of green and, perhaps, red and brown.
Once established on the sea bottom, the algal colonies were strong enough to resist attack by waves and currents. They tended to maintain themselves and grow upwards, making the water even shallower than when their growth began. At times of low water, algal heads would have been exposed. The reefs served, furthermore, as gathering grounds for sediment, and their volume was also increased where evaporation of sea water or other factors caused the chemical precipitation of dissolved lime. Under modern conditions palms or other plants would have taken root, but this aid to island growth was absent in Belt time. The lands bordering the sea likewise could not have had a vegetable cover like that of the present day. They must have presented a bleak and desolate appearance for no fossils of land life have been found in rocks older than Silurian. (See app. B.) Conceivably some of the hardier plants had begun their struggle for existence on the land during Precambrian time, but these could be found only along the shores of the sea where there was an abundance of the moisture so necessary for their survival.
Floating, swimming, and crawling among the algal growths were probably extremely primitive forms of animal life. The presence of such life during Belt time, however, is known only by indirect evidence. In many places the Belt rocks contain burrows that are very similar to those made by modern worms and trails that resemble very closely trails made by modern mollusks. During the following Cambrian period, when the first deposits containing an abundance of animal fossils were laid down, animal life was already diverse and complex.
The plants grew as thin mats of fine threadlike filaments. Each filament was made up of a row of closely packed cells and was covered with a sheath of sticky matter. In the process of photosynthesis, plants absorb carbon dioxide and give off oxygen. The solubility of lime in sea water is proportional to the amount of carbon dioxide present in the water. We may recall, here, a simple experiment that is usually included in children's chemistry sets. A milky solution of lime is cleared by the bubbling of carbon dioxide from one's breath. What has happened is that the increase of carbon dioxide in the solution has increased the solubility of the lime, causing the disappearance of the lime precipitate that produced the milky appearance of the solution. When the algae remove carbon dioxide from the water around a colony, that water becomes saturated with lime, and a very fine precipitate is formed. This fine precipitate of lime adheres to the sheath of the algae and forms a thin coating over the colony. After the coating of lime is formed, the old algal mat dies and a new one begins to grow on the new surface of the colony. In this way the colonies continue to grow and expand until a change in environment occurs. Perhaps an influx of very fine clayey material may cause the water to become murky and decrease the penetrating power of the sunlight. With a lack of sunlight, the algae die and the growth of the colony is brought to an end. Changes in environment such as this occurred several times during the deposition of the Belt sediments and account for the many beds of algal colonies separated by various thicknesses of barren rock.
Fossil algae occur in eight distinct layers or zones in the Belt series of the park area. (See app. D.) Each zone is characterized by the dominance of one or two kinds of algae. The zones bear the names of the species that occur in greatest abundance. Thus, the Conophyton zone 1 consists primarily of Conophyton inclinatum Rezak, but this is not the only species that may exist in the zone. In addition, we find Collenia frequens Walcott, Collenia multiflabella Rezak, and Cryptozoon occidentale Dawson, all of which are less common.
The individual masses of algae, ordinarily called heads, may assume any one of a number of shapes. The heads may be dome-shaped, columnar, fan-shaped, or conical. They are composed of layers of limestone, separated by layers which contain more silt. On weathered surfaces the impure layers tend to stand out in relief, because they yield to weathering processes more slowly than the layers of pure limestone. Thus the structure of the heads is clearly revealed on the rock surfaces.
Scientific names have been used for the heads for lack of other names. Insofar as possible, the descriptions of species have been written in nonscientific language so that anyone interested in visiting fossil localities can recognize the various species. A few of the technical terms are defined in the glossary, appendix E. A key to the identification of the fossils is included as appendix D.
Nowhere in or near Glacier National Park has erosion laid bare the rocks that underlie the Belt series and constitute the floor upon which the Belt sediments were laid down. This floor probably consists of rocks like those that crop out near the southern border of Montana, far to the southeast of the park. These are the oldest rocks known in Montana, and many of them are of granitic character. Layers of sandstone and conglomerate (consolidated gravel) interbedded in the Altyn limestone, the lowest exposed unit in the park, show from their composition that they must have been eroded from granitic rocks. Hence, rocks of that kind must have been exposed on the land areas that bordered the Belt sea. Much the greater part of the Altyn limestone consists of crystalline magnesium-rich limestone.
The lowest fossil zone in the Belt series of Glacier National Park lies in the upper part of the Altyn limestone. This is the Collenia frequens zone (fig. 123). The zone consists of closely crowded columnar colonies which stand at angles of 35° to nearly 90° to the bedding surfaces. Columns are from 2 to 15 inches in diameter and from 6 inches to nearly 6 feet in height. The laminae are smooth and moderately to strongly convex upward. The zone may be observed along the lower slopes of Altyn Peak near Appekunny Falls, near the Rising Sun campground close to St. Mary Lake, and near the fire tower on Divide Mountain.
The Appekunny and Grinnell argillites, the two formations next above the Altyn limestone, consist mainly of beds that originated as fine sand and silt mixed with clay and a little limy matter. The original sediments hardened into shale and sandstone and, as time went on, were converted into argillite and quartzite. Most parts of the Appekunny and Grinnell argillites contain no known fossils, but a few have been found recently in the Grinnell. The sediments from which the two formations were derived must have been carried into the Belt sea by streams that had flowed long distances across lands underlain largely by rocks like granite. The sediments must have been brought long distances because they consist largely of mineral grains that are resistant to destruction by chemical and mechanical means. Minerals that yielded readily to weathering had not survived the trip.
It is remarkable that rocks as nearly homogeneous as those making up the Appekunny and Grinnell argillites could have accumulated to as great thicknesses as they did. They are not, as the names may suggest, derived exclusively from mudstone or shale, but material of that kind is plentiful in both units. For such an accumulation to have occurred, both the sources of the sediments and the conditions of deposition must have remained essentially unchanged for tremendously long intervals of time. The strata contain ripple marks, crossbedding, and other features that indicate deposition in water ranging in depth from a few inches to as much, perhaps, as a few hundred feet. The volume of sedimentary material may be accounted for in a measure by imagining that silt-laden streams deposited their loads in broad deltas. Some stream mouths might have been close enough together so that their spreading deltas merged with each other. Currents moving along the seashore might have aided in spreading the deposits. The extreme shallowness of the water helped to permit deposition of nearly uniform material over expanses many times greater than the area of Glacier National Park. Both formations are visible along the highway northeast of Lake McDonald.
The three lowest units of the Belt series, which together belong to the Ravalli group, have now been mentioned. Passing upward, we find that the next younger subdivision, the Siyeh limestone of the Piegan group, consists mostly of impure crystalline limestone and some sandy and argillaceous beds. The visitor to the park will have no difficulty in making firsthand acquaintance with the Siyeh limestone, for most of the higher stretches of the Going-to-the-Sun Highway (the only road that crosses the park) have been blasted from this formation, which is widespread throughout the mountains of the park.
Siyeh limestone records recurrence of conditions of deposition similar to those of Altyn time and strikingly different from the conditions under which the Appekunny and Grinnell argillites were laid down. The water continued to be shallow, but instead of being silt laden it was clear and contained abundant carbonates in chemical solution. Possibly the courses of some of the rivers that brought sediment to the Belt sea had changed or new currents had arisen in the sea that diverted much of the sediment to other localities. The relatively pure limestone masses are smaller in area than other units of the Belt series in Montana.
When the great masses of limestone in Glacier National Park were first studied, many years ago, it was commonly supposed that they were so old that no living things could have existed at the time they were laid down. Even those willing to entertain the idea that life of some sort did exist and traces of it might have been preserved would not have supposed that the limestone itself could have been formed from carbonate of organic derivation. Other theories were advanced, involving, for example, speculation that the composition of the atmosphere in Belt time was so greatly different from that of the present time that locally great volumes of limestone could easily be precipitated chemically. None of the theories received general acceptance. Probably the limestone of the Belt series originated in much the same way as later limestones, in all of which carbonate derived directly and indirectly from organic sources is abundant.
Fossil algae occur from the top to the bottom of the Siyeh limestone in three zones.
The lowermost and by far the thickest zone in the Siyeh limestone is the Collenia symmetrica zone 1. This zone comprises the lower two-thirds of the formation. It consists almost entirely of Collenia symmetrica Fenton and Fenton. The colonies are from 1 foot to 6 feet in diameter and from 8 inches to 2 feet in height. They have a dome-shaped or flattened dome-shaped cross section and are subcircular in plan. The laminae are rather smooth and are ordinarily flattened centrally but are sharply downfolded at the margins of the colonies. The basal part generally shows an absence of laminae, which presumably was caused by growth on a mud-cracked surface. This zone is well exposed on The Garden Wall near Logan Pass, along Going-to-the-Sun Highway east of Logan Pass, near the south end of Lake McDonald, and on U. S. Highway 2 about 2.2 miles east of West Glacier.
The second zone in the Siyeh limestone is the Conophyton zone 1. This is the most conspicuous zone in the park. It generally forms massive, grayish limestone ledges that can be seen from several miles away. (See fig. 124.) The zone is made up of three parts, each containing a characteristic species. The lowest one-third of the zone contains colonies of Collenia frequens Walcott. (See p. 9.) The middle one-third of the zone contains large masses of Conophyton inclinatum Rezak. Colonies of Conophyton are conical and inclined at low angles to the bedding surfaces. They range from 2 to 48 inches in diameter, although the average is about 8 inches, and up to 3 feet in length. The laminae are smooth, concentric, and conical. The unique form of this species makes it very easy to recognize. (See fig. 125.)
The upper part of the Conophyton zone 1 contains Collenia multiflabella Rezak and Cryptozoon occidentale Dawson. Colonies of Collenia multiflabella Rezak are generally quite large, reaching diameters of 5 feet and heights of 3 feet. They are roughly circular in plan and flattened or discoid in cross section. The lower part consists of columnar heads that expand upward and are capped by later laminae that are continuous over the columns. The laminae are crenulate, flattened at their crest, and slightly downfolded at the margins of the colonies. (See fig. 127.) Colonies of Cryptozoon occidentale Dawson may also be seen in the upper part of the Conophyton zone 1, but they are less common than Collenia multiflabella Rezak. Colonies of Cryptozoon occidentale Dawson are roughly circular in plan and fan shaped in cross section. They range in height from a few inches to 6 feet, and the maximum width is about equal to the height. Laminae are smooth, flattened at the crest, and downfolded at the margins of the colonies. (See fig. 126.)
Conophyton zone 1 may be seen along Going-to-the-Sun Highway just east of Logan Pass, just west of Logan Pass, and just east of the tunnel below the big switchback on the Garden Wall. The trail from Logan Pass to Granite Park crosses the zone near Logan Pass and also just west of Haystack Butte.
The uppermost zone in the Siyeh limestone lies between the Conophyton zone 1 and the red and green argillites of the Missoula group. This zone contains large beds of Collenia multiflabella Rezak, for which it is named, and minor quantities of Cryptozoon occidentale Dawson. (See p. 10 and fig. 127.)
The Collenia multiflabella zone is well exposed at Logan Pass, where the comfort station stands on one of the beds in the zone. Short walks from the comfort station on the Hidden Lake trail and toward St. Mary valley will reveal exceptionally well preserved heads of Collenia multiflabella Rezak. Here, because of the resistance of the fossil beds to weathering, many ledges containing the fossils may be seen. The zone is also well exposed just northwest of the tunnel below the big switchback on The Garden Wall.
In addition to the masses known to be fossils, the limestone contains many intricate structures such as those shown on figure 128, whose origin is not understood.
Weathered surfaces of most limestone outcrops show such structures as fantastically irregular patterns etched in stone. Certain of them have a fancied resemblance to the grinding surfaces of molar teeth of elephants or similar beasts and are called "molar tooth structures." Figure 129 illustrates structures of this kind.
Some or all of the structures may be of organic origin, presumably mostly algal. If some or all of the forms now preserved seem difficult to account for as algal, perhaps they are traces of some other sort of plants or even of primitive animals. The differences between very primitive plants and animals are slightso slight that the distinction is not easily made. As one thinks over these various ideas, the most logical assumption seems to be that the Siyeh limestone is dominantly of organic origin. The other limestone masses in the Belt series do not contain equally large proportions of recognized fossils, but they too may be largely organic in origin.
A very marked, although not abrupt, change in the sedimentary environment ended deposition of the Siyeh limestone, and the rocks of the Missoula group, the last of the major subdivisions of the Belt series, came into being. The striking change in the character of the marine sediments is probably related to crustal disturbances that are recorded by the eruptions of igneous rocks at this time. Low in the Missoula group, lava flows are interbedded with the sedimentary rocks. These dark-colored lavas were originally somewhat like the basalt that is so abundant in eastern Washington and Oregon, except that they were extruded on the sea floor instead of on dry land. This is inferred, in part, from the fact that the lava in Glacier National Park is characterized by curious ellipsoidal structures that give the name "pillow lavas" to flows like these. Such structures are known to originate when lava erupts under water and are only locally present in the basalt of Washington and Oregon. One of the best places to see lava is Granite Park, the flat on which Granite Park Chalet is built. Granite Park supposedly owes its name to the lava once erroneously thought to be granite.
The lava, of course, came from deeper within the earth's crust, and some that did not reach the surface in Belt time is preserved as the nearly black bodies that are conspicuous in many exposures of the Siyeh. Within the park most of these are steep fissure fillings, called dikes, or nearly flat-lying masses called sills, which approximately parallel the bedding in the limestone. Most of the dikes are 10 to 200 feet wide. Among other places, cliffs from St. Mary Lake northwestward past Lake Sherburne reveal conspicuous dikes. Prospectors of the early days found ore minerals close to some of these dikes. The sills are a few score feet to more than 100 feet thick and are even more conspicuous than the dikes, in part because their black rock is bordered on both sides by white zones of limestone bleached by the heat of the intrusion. In most localities only a single sill is exposed and, as it happens, this is commonly a short distance below the massive bed that marks the most widespread of the zones of algal fossils. Sills are well exposed on Clements Mountain and other peaks near Logan Pass. Figure 130 shows a sill and its border of bleached and recrystallized limestone. Other sills are shown in figure 131.
Sills and dikes doubtless were introduced as a result of crustal disturbances. At the levels now open to observation, these disturbances were not violent. They made steep cracks and strained the limestone beds, then newly deposited and relatively soft, so that in a few places the molten material could penetrate along bedding surfaces. The rocks were not compressed enough to be folded or broken, for the beds of the Missoula group lie practically parallel to the limestone beds on which they rest. If violent disturbances had occurred, the limestone, already deposited, would have been tilted and perhaps also broken, so that later sediments, deposited on a flat sea bottom, could not have been parallel to the limestone beds.
The Missoula group consists, for the most part, of argillite beds of purplish red color and some green beds. Much of the rock is sandy and most is somewhat limy. Here and there, throughout the group, are limestone beds and limestone masses, some over 1,000 feet thick. The limestone contains algal fossils. Most of the limestone interbedded with the argillite is so similar to the Siyeh limestone just below that, where relationships to other rocks are not apparent, one might easily be mistaken for the other. The argillite beds show abundant ripple marks, mud cracks, bits of dried mud that had been tumbled about by rills of water while still soft, and other indications that the mud layers, from which the argillite formed, were exposed frequently to the air during the time they were accumulating. The Missoula group is much the thickest subdivision of the Belt series in Montana, but in the part of the park that visitors generally see it is not conspicuous because most of it has been removed by erosion. In many places it is represented only by red summits and pinnacles on some of the higher mountains.
Three fossil zones have been recognized in the Missoula group. The lower few hundred feet of the group consists chiefly of red and green argillite and thin beds of pink limestone. This is the Collenia undosa zone. The limestone beds are about 1 foot thick, and although none has great lateral extent together they are numerous enough to be recognized over a large area. The limestone beds are crowded with Collenia undosa Walcott, Collenia symmetrica Fenton and Fenton (p. 10, 11), and Cryptozoon occidentale Dawson (p. 10). Colonies are composed of alternating layers of pink limestone and green argillite. On the fresh rock surfaces they present a spectacular appearance, with the structure of the laminae made especially conspicuous by the alternating colors. On weathered surfaces the laminae develop strong relief, owing to the more rapid decay of the pure limestone layers.
Colonies of Collenia undosa Walcott generally expand upward to form fan-shaped cross sections. Sizes range from 1 inch high and 2 inches wide to 18 inches high and 20 inches wide. The laminae are coarsely crenulate and dome shaped. They expand upward with growth and unite laterally with adjoining heads to form compound colonies. (See figure 132.)
This zone is well exposed between the comfort station at Logan Pass and the saddle between Mount Oberlin and Clements Mountain, 0.2 mile east of the big switchback on The Garden Wall, 0.6 mile north of the south end of Lake McDonald on Going-to-the-Sun Highway, and 1.2 miles south of Walton on U. S. Highway 2.
The upper half of the Missoula group is exposed only in the southwest part of the park and in the Flathead region to the south of the park. Elsewhere it has been removed by erosion.
The second zone of Collenia symmetrica Fenton and Fenton occurs about 6,000 feet above the base of the Missoula group. This zone, about 50 feet thick, is made up of 3 distinct layers of Collenia symmetrica Fenton and Fenton (p. 10, 11) separated by various thicknesses of barren limestone. The zone differs from its counterpart in the Siyeh limestone in thickness and lateral persistence of the fossil layers. Zone 2 is much more compact vertically than zone 1, and the individual fossil beds cover larger areas than the fossil beds of zone 1. (See fig. 133.) The zone may be seen on the southwest spur of Running Rabbit Mountain about 400 feet below the top of the mountain and also just west of the tunnel at Singleshot, on the Great Northern Railway tracks along Bear Creek.
Conophyton zone 2 lies about 400 feet above the Collenia symmetrica zone 2. It resembles its counterpart in the Siyeh limestone when viewed from a distance; however, it differs considerably in its fossil content. Conophyton inclinatum Rezak and Collenia frequens Walcott are the only species that are recognized in the zone. They occur in 4 alternating layers, each layer containing only 1 species. The lowermost layer consists of Collenia frequens Walcott, and the uppermost layer consists of Conophyton inclinatum Rezak. The second and third layers are separated by about 10 feet of barren, black limestone. This zone may be seen at the top of Running Rabbit Mountain, at the pass between Giefer Creek and Twenty-five Mile Creek on the west side of Baldhead Mountain, and along the Great Northern Railway tracks opposite the point where Devil Creek flows into Bear Creek. The zone on the western slopes of Scalplock, Rampage, and Riverview Mountains may be viewed from U.S. Highway 2, between Essex and Pinnacle. (See figs. 134, 135, and 136).
Deposition of the Missoula group probably marked the final filling of the Belt basin. During this filling, which might have lasted more than half of all Belt time, conditions fluctuated markedly. In some depressions within the basin, the water was clear enough so that much algal limestone could be deposited. On the whole, however, mud flats must have been common features of the region. Possibly the Belt basin was finally obliterated by earth stresses that reversed crustal movement so that downwarping actually gave way to uplift. If so, the movement must have been gentle indeed, for no direct evidence in support of it has been found in nearby parts of Montana. Possibly the movement was so gentle that, locally, deposition was scarcely interrupted at all. In distant areas, such as parts of Idaho and Canada, the evidence of crustal disturbance at about this time is a little more convincing. In some of these areas an interruption in the deposition of sediments, which can be best accounted for by uplift, did take place. Even in these places, however, the rocks were broadly arched or upwarpednot strongly folded or broken.
Let us review briefly the important environmental conditions that existed during the time when the Belt sea spread over much of western North America. Presumably, warm weather prevailed. The abundant algae remind one of the marine algae that in modern seas are most luxuriant in the tropics. Red rocks are plentiful in the Belt series, and red sediments and soils are more commonly formed today under warm than under cold conditions. Very large quantities of sediment were dumped into the Belt basin from land that had such slight relief that the streams carried almost no gravel. This suggests that rain was at times abundant and also that weathering penetrated deeply into the ground, so that the rock was disintegrated and yielded readily to erosion. The fact that at times parts of the floor of the Belt sea were exposed to the air may be accounted for, in part, by long severe dry spells.
Last Updated: 08-Jul-2008