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

The deposition of the Belt series fulfilled one of the requirements for the crustal deformation that is related to mountain building: the accumulation of a large thickness of sedimentary rocks mechanically weaker than the parts of the earth's crust that are composed of massive crystalline rock. Obviously other requirements were lacking. A vast interval of time, spanning more than 450 million years, was to elapse before the changes set in that gave rise to mountains. During this interval, the deposition of sedimentary rocks continued, so that eventually the ancient Belt series came to be buried beneath younger strata throughout the broad region that included the site of Glacier National Park. As these younger rocks have been stripped from the surface in the park by erosion it is now necessary to go outside the park to study them.

Processes which made possible the accumulation of sediment to form the Belt rocks continued for the later deposits; that is, space for the later deposits was made available in part by the sinking of the basin floor as the load of sediments increased and possibly in part by broad downward flexure of the earth's crust under lateral pressures. As these adjustments took place, the originally soft sediments were packed under the weight of successively younger deposits and gradually hardened and changed into the rocks we find today. They remained relatively nonresistant, however, so that they would yield when adequate pressures were built up. Eventually, as will be shown below, deformation did take place.

Between the pause at the end of Belt sedimentation and the beginning of the Tertiary period, at least six interruptions in the deposition of sedimentary rocks took place in the region. Facts of this kind are learned mainly from studying the fossiles in the rocks. Evolution brings about changes in the character of plants and animals as the years go by. In consequence, the fossils in old rocks may be radically different from those in younger ones. Gaps in the record of progressive changes of this kind in any particular locality demonstrate that during the corresponding part of geologic time no sediments were deposited there.

None of the interruptions to sedimentation before the beginning of the Tertiary period was accompanied by mountain building. If enough deformation to lead to the development of mountains had taken place, clear evidence of it would be visible in the rocks. On the contrary, those of one age rest on those beneath them in nearly perfect parallelism, even where the fossils prove that major interruptions in sedimentation have occurred. Mountain-building deformation would have disturbed the rocks so that parallelism would have been impossible.

It is probable that half of the time interval between the end of Belt sedimentation and the beginning of the upheavals related to the Lewis overthrust is not represented by sediments in the general vicinity of the park. Whereas the older formations deposited during this interval were laid down in seas that had greater average depths than the Belt sea, some of the younger formations are nonmarine, having been laid down in lakes, swamps, or on river flood plains. Those among the younger sedimentary units that are of marine origin are thinner and probably less widespread than the older marine deposits. Seemingly as time went on the region was encroached upon by the sea to a decreasing degree.

The long period of accumulation of sedimentary strata was drawing to a close, and the forces of uplift were gaining ascendency over those of depression. Plate 53A shows diagrammatically conditions as they are believed to have been at this time. Slow oscillation of the earth's crust continued, and for some 30 million years before violent deformation took place the region remained dry land much of the time. Near the close of the Mesozoic era preliminary folding might have taken place. The preparations had been long, but finally the stage was set for those dramatic events that may now be recounted.