Geological Survey Bulletin 730—A Peneplains of the Front Range and Rocky Mountain National Park, Colorado

The Flattop peneplain is probably to he correlated with the highland plain of the Georgetown quadrangle described by Ball. The canyon cycle is obviously the same everywhere along the Front Range, for the canyons are all of essentially the same degree of development. The intermediate cycle was not definitely recognized by Ball, and it is not easily discernible in the Georgetown quadrangle. However, from Golden southward to the Pikes Peak region the peneplain formed during the intermediate cycle appears as the great conspicuous upland called in this paper the Rocky Mountain peneplain.

A question has been raised as to the meaning of the intermediate highlands. There is little doubt that faulting and warping of relatively recent date has occurred in many parts of the mountains, and some of the surfaces here interpreted as remnants of the Rocky Mountain peneplain may possibly be displaced parts of the higher plain let down by faulting; but it is scarcely probable that the intermediate cycle of erosion can be explained away in this manner. Although my observations were hurried and limited to a small area I am disposed to believe that the Front Range is a three-cycle rather than a two-cycle mountain mass.

Although the physiographic history of the Rocky Mountains is imperfectly known, a probable chronologic order of events can be worked out which may be of value, at least in stimulating observation. A partial list of events, given in order from oldest to youngest, follows.

FIRST MOUNTAIN UPLIFT AND EARLY EROSION.

Reasons for believing that sedimentary deposits of Cretaceous age covered the site of the mountains in Colorado and that the mountains emerged from the sea at the end of the Cretaceous period have been given in an earlier publication.⁷ As that publication is easily available these reasons need not be repeated here.

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⁷Lee, W. T., Relation of the Cretaceous formations to the Rocky Mountains in Colorado and New Mexico: U. S. Geol. Survey Prof. Paper 95, pp. 27-56, 1916.

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After the initial uplift the sedimentary rocks were eroded, at least in part, from the area later carved into mountains. Also during this early period of erosion andesite⁸ was extruded in the mountain region, and this also was eroded away and waste from it was included in the early Tertiary beds (Denver formation).

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⁸Cross, Whitman, Geology of the Denver Basin in Colorado: U. S. Geol. Survey Mon. 27, p. 315. 1896.

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FLATTOP CYCLE.

Perhaps somewhat later, although this can not be asserted with confidence, flows of rhyolite and andesite were formed in the region north of Longs Peak. These flows were later broken up, and the blocks were transported, presumably by water, to the lowlands or broad valleys of that time, which had been eroded in the crystalline rocks of the mountain core. Here the debris of rhyolite and andesite was interbedded with volcanic ash, tuff, and flows of glassy lava, the composite mass occupying a wide area and resting on a floor of old crystalline rock. With the blocks of rhyolite were embedded rounded pebbles and boulders of granite, gneiss, and schist so large as to indicate that highlands of crystalline rock of considerable altitude existed near by at that time. A large mass of this composite material many hundreds of feet thick is preserved in Specimen Mountain, in the northern part of the Rocky Mountain National Park. As the base of the material is at nearly the same altitude as the Flattop peneplain, it may be that the broad valleys in which the material accumulated were due to erosion in the early part of the cycle which produced the Flattop peneplain and that the highlands which furnished the boulders of crystalline rock are represented now by such old monadnocks as Hallett Peak.

The formation of the Flattop peneplain was the third major event in the history of this region. Apparently the mountain region was elevated 2,000 feet or more, and the consequent erosion reduced the greater part of it nearly to a plain, leaving many monadnocks, such as Hallett Peak (Pl. III, A), Longs Peak, and Mount Evans. The bedded deposits of rhyolite and andesite debris which accumulated in the cycle were partly eroded away, and dikes of the same material were planed off even with the general surface. One large mass, that of Specimen Mountain, escaped erosion.

There is little evidence by which the date of this peneplain can be fixed, but the general succession of events indicates the improbability that it was formed before the end of the Eocene epoch, and the equal improbability that it is later than Pliocene.

ROCKY MOUNTAIN CYCLE.

Renewed uplift still further arched the great mountain anticline, lifting the center 1,500 to 2,000 feet. This uplift rejuvenated the streams, and the Rocky Mountain peneplain was formed. This is probably the most extensive physical feature of the mountain region in northern Colorado. Its formation required a long time. Many facts point to its being the mountain equivalent of the Great Plains, as is suggested by such profiles as those in Plate IV of the Denver Basin monograph,⁹ those of the Castle Rock folio,¹⁰ and those of figure 3 of this paper. Doubtless the erosion that produced this peneplain furnished the material for the beds of late Tertiary age east of the mountains.

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⁹Emmons, S. F., and others, Geology of the Denver Basin, in Colorado : U. S. Geol. Survey Mon. 27, 1896.

¹⁰Richardson, G. B., U. S. Geol. Survey Geol. Atlas, Castle Rock folio (No. 198), 1915.

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Certain gravel beds observed near the tops of the mountains between Palmer Lake and Florissant may be due to the activity of some of the streams that helped to form this peneplain. These were observed many years ago by George H. Stone, who published in the Colorado Springs Gazette in 1900 a sketch map showing a large body of gravel on the highlands east of Florissant, near Divide station on the Colorado Midland Railway, between the headwaters of South Platte River and Fountain Creek. Smaller bodies of gravel which seem to mark the course of an old river lie between this locality and Palmer Lake. On Stone's map the site of the large body of gravel was called Hayden Lake, and the line of gravel beds between this body and Palmer Lake was called Hayden River. These gravel beds suggest the headwaters of one of the streams which in late Tertiary time furnished the pebbles of the gravel beds that still cover vast areas of the Great Plains in eastern Colorado.

CANYON CYCLE.

The Rocky Mountain cycle of erosion was terminated by renewed uplift accompanied by much warping of the surface and great displacement by faulting in many localities. South of Denver the Rocky Mountain peneplain seems to have been lifted without notable warping, the displacement being generally accomplished by slipping along fault planes in the foothill region. In northern Colorado the uplift seems to have partaken more of the nature of arching (see fig. 3), so that the older plain on Hagues Peak is about 5,000 feet higher than the younger plain in the foothill region.

FIGURE 3.—Profile from the crest of the mountains at Hague Peak northeastward to the Great Plains in northern Colorado. A remnant of the Flattop peneplain is preserved on Hagues Peak. Northeast of this peak the slope of both peneplains is greater than it is farther south, and the lower one descends to about 7,000 feet. This slope is such as to bring the restored Rocky Mountain peneplain down to the level of the Great Plains several miles east of the foothills. The space represented between this plain and the present surface shows the amount of degradation of the Great Plains during the canyon cycle. (click on image for an enlargement in a new window)

There are some reasons for believing that this orographic disturbance marks the beginning of the Quaternary period. The surface gravel of the plain, which seems to have been spread out by streams in the later part of the Rocky Mountain cycle, contains fossil bones said to indicate Pliocene or Pleistocene time. If this dating of the last great uplift of the Rocky Mountains at the beginning of the Quaternary is correct, the canyon cycle of erosion is coextensive with Quaternary time. The work of cutting these canyons is comparable to that in the San Juan River region, where canyons 2,000 feet deep have been cut since the oldest Pleistocene glaciation of that region.¹¹ The dissection and degradation of the Great Plains near the mountains accomplished during the canyon cycle is no greater than the degradation in Montana since the early glaciation, now represented by the till which caps certain mesas haying a maximum height of about 1,000 feet above the surrounding country.¹²

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¹¹Atwood, W. W., and Mather, K. F., The evidence of three distinct glacial epochs in the Pleistocene history of the San Juan Mountains, Colo.: Jour. Geology, vol. 20, p. 401, 1912.

¹²Alden, W. C., and Stebinger, Eugene, Pre-Wisconsin glacial drift in the region of Glacier National Park, Mont.: Geol. Soc. America Bull., vol. 24, pp. 529-572, 1913.

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SUMMARY.

There is fairly satisfactory evidence in the Rocky Mountain National Park of three distinct cycles of erosion—one which resulted in the Flattop peneplain and was completed some time near the middle of the Tertiary period; one which resulted in the Rocky Mountain peneplain and was probably terminated at the end of the Tertiary and the canyon cycle, not yet completed. A peneplain still older than the Flattop peneplain may be represented by the top of Longs Peak and other high mountains in Colorado, many of which have nearly the same altitude.