ROCKY MOUNTAIN A Guide to the Geology of Rocky Mountain National Park, Colorado

WE NOW come to a consideration of the formation of the Rocky Mountains, which are entirely distinct from the older mountains of the pre-Cambrian and Paleozoic, their only relation to the earlier ranges being that they were formed along the same zone of weakness in the earth's crust. It would have been possible to omit, in this brief account, all mention of the earlier mountains had it not been that their formation, particularly that of the pre-Cambrian ranges, explains the origin and presence at the surface of so large an area of the schists and granites which form the principal rocks of the park.

TIME AND FORM OF THE MOUNTAIN FOLDING

The growth of the Rocky Mountains began about 60,000,000 years ago at the close of the Mesozoic era or the Age of Reptiles. In fact, the great changes in climate and vegetation produced by the mountain uplift may have had much to do with the extinction of the cold-blooded dinosaurs and the rapid development of the smaller but more intelligent and warm-blooded mammals.

The uplift of the mountains took place very slowly but was of magnificent proportions. It assumed the form of a great fold or arch (fig. 3) which involved not only the pre-Cambrian schists and granites but the thousands of feet of sedimentary beds which had been deposited upon them by the seas that repeatedly covered the region.

Figure 3.—Profile section across the Rocky Mountains east and west through Longs Peak and Grand Lake, showing the present surface in solid line and the hypothetical ancient surfaces which were warped by the rise of the mountains. The single line of dashes is the position which might have been assumed by the surface that emerged from the Cretaceous sea had it not been destroyed by erosion. The double line of dashes is the position that the base of the Cretaceous rocks—Dakota sandstone—might have assumed had it not been eroded away. (click on image for an enlargement in a new window)

EROSION FOLLOWED BY REGIONAL UPLIFT

No sooner had the first Rockies begun to rise than erosion began its work of destruction. During the uplift and in the long period which followed, some 10,000 feet, or a thickness of almost 2 miles of sedimentary rock, was washed away from the top of the arch, until the ancient schists and granites which formed the core were uncovered and carved by the mountain streams. This core was bordered on either side by the eroded edges of the sedimentary beds which were left dipping away from the axis of the great arch of which they had formed a part.

The final result of this erosion was the formation of a rolling plain of moderate elevation, above which rose low, rounded mountains 1,000 to 2,000 feet in height. The disintegrated rock which was washed away by the streams was spread as a blanket of sand and clay east of the mountains and today forms part of the rocks of the Great Plains.

The erosion of so vast an amount of rock by streams may seem fantastic, but figure 3, page 5, illustrates the evidence upon which the statement is founded. It will be noted that the thickness of the sediments upturned along the flanks of the range can be measured. Most of these sediments belong to the Mesozoic era and, since they have much the same characteristics on both sides of the range, they probably were put down in the same body of water rather than in two separate seas divided by a land barrier. In other words, there were no mountains here when these sediments were laid down, and consequently the sediments were deposited across what is now the range. As they are 10,000 feet thick on the sides of the present range, they probably had about the same thickness over what is now the crest, and inasmuch as they are not found on the crest of the mountains the entire 10,000 feet of rock must have been removed by erosion. The time required for so gigantic a task as the removal of a lofty mountain range is difficult to comprehend. It may well have been many millions of years.

The evidence that a rolling plain was developed by the erosion of the Rockies after their first uplift can be seen on the mountains themselves. Trail Ridge Road above timber line traverses a rolling upland (fig. 4) which is in marked contrast to the rugged canyons cut into it from the east. Part of this upland surface forms the top of Flattop Mountain (fig. 5) and for this reason the entire rolling upland has been called the Flattop peneplain, the term meaning almost a plain. There are really two surfaces, an upper and a lower, closely related in origin (p. 10). They are evidently erosional and appear to have been cut by streams which were flowing at a lower elevation and consequently had but little fall. As the surfaces now stand from 11,500 to 12,000 feet above the sea, they must have been raised from the lower level at which they were formed to their present elevation by a series of uplifts which did not greatly fold or distort them. These uplifts affected not only the Rockies but also the adjacent Great Plains.

Figure 4.—Ancient erosional plain (the Upper Flattop peneplain) southeast of Iceberg Lake on Trail Ridge Road. This plain is believed to have been formed at a much lower altitude, by streams, and to have been raised to its present elevation of 12,000 feet by the last uplift of the region. Terra Tomah Peak on right, Stones Peak in center, and Longs Peak in left distance. Note slight terracing of surface below road at left due to sliding of the wet soil on sloping bedrock. Photograph by Carroll H. Wegemann

Figure 5.—Ancient erosional plain (Upper Flattop peneplain) from Flattop Mountain, elevation 12,200 feet. Longs Peak in distance at left and Halletts Peak in left foreground, with snow at head of Tyndall Glacier. Photograph by H. Raymond Gregg

In observing the Upper Flattop surface one should try to forget the deep canyons which now dissect it and imagine the old surface as it must have been before the uplift which brought it to its lofty position and inaugurated the present cycle of canyon cutting. That the surface had not been entirely leveled to a plain, at least where the former mountains stood, is evident. Rounded hills perhaps 2,000 feet in height, the stumps of these mountains rose above this surface and are today represented by the higher summits of the range.

It is possible that the present elevation of some of these hills above the old surface has been increased by faulting or bending of the surface during the long period of uplift to which the region has been subjected. For example, the peculiar flat top of Longs Peak, which is considered by some authorities to be part of the old surface, lies some 2,000 feet above that surface and may owe its elevation to greater movement in the vicinity of the peak than has taken place in the adjacent area.

EVIDENCES OF INTERMITTENT UPLIFT

As to the nature of the uplift which followed the formation of the Upper Flattop peneplain some interesting observations may be made. About three-fourths of a mile northwest of Iceberg Lake on Trail Ridge Road the highway turns abruptly to the northeast. From the observation point at the turn there is an excellent view to the southwest across the head of Forest Canyon (see p. 24, point 1.0 mile). On the southwest side and extending to the rim of the canyon is a broad bench at an elevation of about 11,500 feet or about 500 feet below the average level of the higher plain. This lower level appears also on the southeast end of Trail Ridge just above timber line (fig. 6, C—2) and on the crests of several ridges southeast of the Mummy Range (fig. 6, E—2). It is evidently an erosion surface, similar to that of Flattop, and has been termed by Van Tuyl and Lovering (p. 32) the Lower Flattop surface. Its presence may be interpreted as follows: After the formation of the Upper Flattop surface the general region was raised gradually about 500 feet. This uplift increased the fall of the streams and enabled them to begin the erosion and destruction of the Upper Flattop surface which they themselves had made. Their work progressed rapidly in the soft rocks east of the mountains, but proceeded more slowly in the mountain granites. The streams had already deepened their valleys as much as fall would permit, and were widening the valley floors through side cutting in their winding channels, when renewed uplift forced them to begin once more the deepening of their valleys. The bench at the head of Forest Canyon is part of one of the broad valley floors formed after the initial uplift.

Figure 6.—View northwest from Longs Peak. To the right is the Mummy Range (C—F), and to the left in the distance the Never Summer Mountains (A). Between them, above timber line, is the smooth rolling surface of Trail Ridge, the higher part of which (B—1), is part of the Upper Flattop peneplain (p. 7). The southeast end of the ridge (C—2) at timber line is a lower erosional surface corresponding to the bench at the head of Forest Canyon (p. 24). This surface can be traced northeast along the crests of the ridges (E—2, F—2). Sloping surfaces in lowland on right are glacial moraines (F—3) on one of which lies Bierstadt Lake (F—4). Photograph by Carroll H. Wegemann

During the many millions of years which have elapsed since the formation of the Flattop surface the cycles of uplift and erosion have been repeated a number of times. In Rocky Mountain National Park remnants of several erosional surfaces are preserved in the level tops of certain mountains and plateaus. For example, the summits of Deer Mountain (fig. 15) and The Needles are parts of an old erosion surface which now stands at an altitude of about 10,000 feet. The plateau west of Gem Lake, north of Estes Park, and the top of Prospect Mountain just south of the village are parts of another erosion surface which has been raised to 9,000 feet. The even-topped summits 10 miles east of Longs Peak (fig. 7) are at the same level. The top of Castle Mountain represents a surface 300 feet lower at 8,700 feet. Each of these surfaces marks a pause in the great uplift which has raised the Rockies and the western Great Plains to their present altitudes.

Figure 7.—Chasm Lake and valley from Longs Peak. The valley was once occupied by a glacier which excavated the cirque in which the nearer lake lies and deposited the great lateral moraine on the left up which the trail runs. From this trail the view shown in figure 2 was taken. Note that the distant mountains rise to a common level which represents one of the old erosion surfaces described on page 10. Photograph by Carroll H. Wegemann

Today a comparatively recent uplift, which began sometime in the Pliocene and may even now be continuing at intervals, has given the streams power to deepen the mountain canyons, and at the same time plane down the soft rocks of the lowland, which lies east of the mountain front. The mountains tower above the lowland because their rocks are hard and resistant to erosion, and because the broad uplift of the region has given the streams power to wash away the soft rocks of the lowland.