CONTRIBUTIONS TO THE GEOGRAPHY OF THE UNITED STATES, 1922. PENEPLAINS OF4 THE FRONT RANGE AND ROCKY MOUNTAIN NATIONAL PARK, COLORADO. By Wnxis T. LEE. PURPOSE OF THE PAPER. The purpose of this paper is to call attention to some of the major surface features in the Rocky Mountain National Park and to point out their probable correlation with similar features in neigh­ boring regions. The observations on which the paper is based were made in the summer of 1916, during an investigation in which other work demanded first consideration. This paper may therefore be considered a by-product. For the same reason many of the observa­ tions were not followed to conclusions, yet the data obtained seem to be sufficient to establish a certain order of events, the recognition of which may be of assistance in working out in detail the geologic and geographic history of the Rocky Mountain region. TWO CYCLES OF EROSION PREVIOUSLY RECOGNIZED. In an account of the general geology of the Georgetown quad­ rangle, Colo.j Ball x describes three distinct types of land forms " an old, mature mountainous upland; younger V-shaped valleys in­ cised in this upland; and glacial cirques developed at the heads of some of the streams, passing downward into U-shaped valleys." Ball describes the mountainous upland as consisting of highlands of rounded or gently sloping tops which are remnants of an ancient peneplain or old surface of erosion. Most of these remnants in the Georgetown quadrangle lie above an altitude of 11,500 feet. On thig old peneplain there were many monadnocks, some of which are now recognizable as the summits of the highest mountains. Similar sur­ faces of erosion occur elsewhere in the mountains, among them the well-known upland plain near Pikes Peak, recognized by Cross.2 1 Spurr, J. E., and others, Economic geology of the Georgetown quadrangle (together with the Empire district), Colo. [General geology, by S. H. Ball] ; U. S. Geol. Survey Prof. Paper 63, ?. 31, 1906. 2 Cross, Whitman, U. S. Geol. Survey Geol. Atlas, Pikes Peak folio (No. 7), 1894. 1 2 CONTBIBUTTOETS TO GEOGBAPHY OF UNITED STATES, 1922. Later Davis 3 accepted Ball's description and applied to the high­ lands the term " a two-cycle mountain mass." This term and other expressions in his paper indicate that Davis believes that only two main cycles of erosion are represented in the Rocky Mountains in Colorado an older one resulting in the formation of a peneplain and a younger one during which the canyons and cirques were excavated. Hence the remnants of the highland plain found at many places in these mountains are referred to as parts of "the peneplain." A' POSSIBLE THIRD CYCLE. The dissected plateau near Pikes Peak which Cross described as practically the same as the floor of South Park was recognized by Eichardson 4 in the Castle Rock quadrangle and by Finlay 5 in the Colorado Springs quadrangle. This plateau lies at altitudes between 9,000 and 9,500 feet and is extensively developed in the Front Range in Colorado. From localities at some distance from the mountains this plain appears to be an almost horizontal shelf that extends along the mountain front as far as the eye can reach. The area in which it is best developed and best preserved extends from Denver south­ ward to Pikes Peak. In areas farther north, such as the moun­ tains west of Golden, it is not so readily recognized, but it seems to be represented by surfaces much lower than the highland plain described by Ball as lying above 11,500 feet. Remnants of the lower plain may be recognized from some commanding eminence, such as Mount Lookout, near Golden, Colo., whose summit appears to be a part of the lower peneplain. This plain has become known somewhat widely as the Rocky Mountain peneplain, possibly because it is the most extensive land form in the Front Range. From Mount Lookout may be seen at nearly the same level numerous flat tops, benches^ and smooth-floored reentrants (see PI. I), which are rem­ nants of the Rocky Mountain peneplain. But the best-known view of this peneplain is that which may be had from the side of Pikes Peak at an altitude of about 10,000 feet. From this point the ob­ server looks to the north and west over the tops of the mountain ridges as over a level plain. The reentrants seem to furnish indubitable proof of a distinct cycle of erosion. The older peneplaned surface was elevated, tilted, and dissected; the plain formed by the coalescing valleys has a slope notably different from that of the higher and older peneplain. A reentrant, possibly modified to some extent by glaciers, seen near Jenny Lake, Colo. (PI. I), is a part of this new plain. This is one 3 Davis, W. M., The Colorado Front Range: Assoc. Am. Geographers Annals, vol. 1, p. 42, 1911. 4 Richardson, G. B., U. S. Geol. Survey Geol. Atlas, Castle Rock folio (No. 198), 1915. B Finlay, G. I., U. S. Geol. Survey Geol. Atlas, Colorado Springs folio (No. 203), 1916. TJ. 8. GEOLOGICAL SURVEY BULLETIN 780 PLATE I IOS'4-o' 105*35' : EMora. 5^ x I05'40' !05'35' FLATTOP PENEPLAIN 11,000 ROCKY MOUNTAIN PENEPLAIN 10,000 9,000 9,000- 8,000 FEET ABOVE SEA LEVEL 8,000- SECTION ALONG LINE A-B MAP AND PROFILE SECTION OF A PART OF CENTRAL CITY QUADRANGLE, COLO. PENEPLAINS OF THE FBONT RANGE, COLO. of the points of particular interest to tourists on the Denver & Salt Lake Railroad (Moffat road). *9 N ro This road winds over a surface_ rlB .gVr 4»V -g*?* -9V -gx^ niS of slight relief on the Rocky Mountain peneplain, enters the 3 S.« 5 * V glacial cirque around Yankee I" 6' g- 3 p Doodle Lake, and then ascends NS the steep slope leading to the high peneplain at Corona, % &-a 2 which is described in this paper ?' 8.5 H L3 O P as the Flattop peneplain. Here B 3" 3 a short walk brings the ob­ (8 M- B " £. 01 rt. server to the brink of the cliff, 3 3 p J" p C p- where he stands on the older I2 gP r»:a g-ff" s- w peneplain, with the younger « one in full view a thousand feet away. Farther north there are old surfaces of considerable area, on which the rock is deeply disintegrated, such as that near Ward, at altitudes be­ g e. tween 9,000 and 10,000 feet. g. ^ These areas are parts of the Rocky Mountain peneplain. Still farther north, in the 1$ Rocky Mountain National Park, there are many rem­ nants of a high plain, the 5' g- 2. Flattop peneplain, which cor­ 9-« E= responds to Ball's highland plain, and of a lower one, at o/ an altitude of 10,000 feet or ££ less (fig. 1), which corre­ q P sponds to the Rocky Moun- p- ;/! CP O IT r*- rtO M tain peneplain. Both have .y ff B been deeply dissected by the erosion of deep canyons dur­ ing the present cycle. Some geologists and geog­ raphers are disposed to regard the upland surfaces of both levels as parts of the same plain displaced by faulting; others see evidence here of two distinct peneplains which, together 4 CONTRIBUTIONS TO GEOGRAPHY OF UNITED STATES, 1922. with the incised valleys and glacial cirques of the cycle not yet finished, make this a three-cycle rather than a two-cycle moun­ tain mass. CYCLES OF EROSION IN THE ROCKY MOUNTAIN NATIONAL. PARK. FLATTOP PENEPLAIN. In the Rocky Mountain National Park near the crest of the highest range there are many flat or gently rounded surfaces that seem to be remnants of a peneplain. Most of them lie above timber line, which is here about 11,500 feet above the sea. One of the largest and most conspicuous of these remnants is on Flattop Mountain, whose name was suggested by its flat surface. (See PI. II.) These remnants are so numerous and so close together that it is not difficult to reconstruct the old surface which they represent. Some of the gentler slopes are covered with residual soil, and the steeper ones with angular fragments of rock. A surface now connecting the several remnants would appear greatly warped and broken. Pos­ sibly the differences in altitude due to warping and faulting would in some places amount to a thousand feet or more. There are also peaks which were doubtless monadnocks on the old peneplain. Hallett Peak is plainly such a inonadnock. (See PI. II, A.) As viewed from the general level of the old peneplain this peak appears to be a dome rising about 500 feet, but from certain directions it appears to be a half dome, for the ancestor of Tyndall Glacier carried part of it away. Several smaller monadnocks are shown in Plate II, B. The nearly level top of Longs Peak may be a part of the Flattop peneplain warped up more than 2,000 feet, but it seems more likely that this mountain was a monadnock, although the nearly level surface of its summit, about 1,500 feet long and 1,000 feet wide, has been regarded as possibly a remnant of a peneplain still older than the Flattop peneplain. The rolling surface of the Flattop peneplain, which may be in part a product of recent warping, may be seen along the Continental Divide north of Longs Peak (PI. Ill, A ). Remnants of the same peneplain occur in many parts of the Rocky Mountain National Park on the highest mountains.