BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA W VOL. 36. PP. 498-512 SEPTEMBER 30. 1925 EROSIONAL CYCLES IN THE FRONT RANGE OF COLORADO AND THEIR CORRELATION 1 BY HOJIKR P. LITTLE (P resented in abstract before the Society December 29, J02J,1) CONTENTS Puice Introduction................................................. ......................................................................... 495 Peneplains of the Front Range................................................................................. 497 General statement...................................................................................................... 497 The Flattop peneplain............................................................................................... 498 The Rocky Mountain peneplain............................................................................. 500 The Park stage............................................................................................................. 504 The Fountain Creek stage....................................................................................... 507 The Canyon-cutting stage........................................................................................ 509 Age and correlation of erosion cycles..................................................... ............... 509 Discussion......................................................: ....................................................................... 510 Bibliography.......................................................................................................................... 511 INTRODUCTION' This paper is the result of work done during July and early August in the summer of 19‘24. The work was financed by (’lark University and received the cordial cooperation of the United States Geological Survey. Particular thanks are due to W. C. Mendenhall, of the Washington office, and C. W. Henderson, of the Denver office. The writer was assisted in the field during the season by Rollin Atwood and in the last few days had the benefit of the field • advice of Wallace W. Atwood and Kirtley Mather. The objective of the work was the study of the peneplains of the Front Range with a view to applying the facts brought out by Lee’s work in Rocky Mountain National Park to other parts of the range and adding such new facts to its erosiona! history as could be discovered. It was especially desired to establish more definitely than previously the age of 1 Manuscript received by the Secretary of the Society April 13, 1025. (495) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/495/3429942/BUL36_3-0495.pdf by guest on 30 September 2021 496 H. P. 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FALLS paKEs«AR'% • Colorado « g i l l e t t s p r i n g s CRI PPLE CREEK • o s to a»© 3o i/o k m* SCALEI INj MILESI = -H F ig u u e 1.—Index Hap Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/495/3429942/BUL36_3-0495.pdf by guest on 30 September 2021 PENEPLAINS OF THE FRONT RANGfc! 4 9 7 the major peneplains, and to establish a correlation with those of other portions of the Rocky Mountains. In this work most of the mountain roads between Colorado Springs and Estes Park were penetrated, as well as much of the adjoining region; a rapid survey was then made of the territory from Estes Park northward to Tie Siding, in southern Wyo- miug, where Blackwelder described the Sherman peneplain, and thence to Glendevey and Green Ridge, a splendid remnant, for knowledge of whose existence the writer is indebted to Dean Worcester, of the Uni­ versity of Colorado (figure 1). South from Colorado Springs the area of the Royal Gorge was penetrated, and for the sake of comparison the writer undertook a three weeks’ trip to the San Juan Mountains with Wallace W. Atwood to examine the peneplains described by him there. 'New evidence of the age of the peneplains from observation within the range itself was not obtained, but Mather, from observations to the north­ east of Port Collins, secured very definite evidence, and it seems safe to correlate most of his stages with those of the Eront Range. Many published articles were of assistance in the work, especially those of Ball, Davis, Blackwelder, and Lee. These and other articles consulted are listed at the end of the paper, but do not comprise a complete bibliog­ raphy of the subject. P e n e p l a in s of t h e F r o n t R a n g e « GENERAL STATEMENT For practical purposes, though not with absolute accuracy, the pene­ plains of the Front Range may be considered as cut wholly across pre- Cambrian granites, gneisses, and schists, with the former especially abundant. Except on the steepest slopes, the region traversed by roads is covered with the coarse sand characteristic of disintegrated but undecom­ posed granite. Soils are occasionally evident, as in road cuts just south of Bergen Park. These, however, are in the majority of cases the prod­ ucts of weathering of the less resistant schistose rocks. The erosional history of the Front Range will be described under the headings Flattop Peneplain, Rocky Mountain Peneplain, Park Cycle, Fountain Creek Cycle, and Canyon-cutting Cycle. Of these terms, the third and fourth are here used for the first time. Discussion of these surfaces groups around four areas where remnants arc particularly conspicuous, namely, the Pikes Peak region, the Bergen Park-Evergreen region, the Rollinsville-Nederland-Ward region, and the Estes Park and Vicinity region. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/495/3429942/BUL36_3-0495.pdf by guest on 30 September 2021 498 H. r. LITTLE----EROSIONAL CYCLES IN THE FRONT RANGE THE FLATTOP PEXEPLAIS The Flattop peneplain was so named by Lee in 1917 because of the wide extent of its development on Flattop Mountain, in Rocky Mountain ATational Park. Here it occurs at an elevation of 12,300 feet. He cor­ related it with the surface described by Ball in the Georgetown Quad­ rangle, and the latter’s description will serve for the region as a whole: “The mountainous upland was an ancient land surface with about the same relief as the present surface. Dome-shaped mountains and broad, 'smoothly contoured ridges, however, stood where sharp peaks and rugged ridges now are. The drainage was dendritic and mature. The remnants of this old topographic surface are covered by rock residuals and in many places by deep soil.” The surface of Flattop Mountain is, as said above, the type area (figure 2). The shoulders of Hagues Peak, when seen from any point in Estes Park, show the old surface almost as well (figure 3), and considerable areas may be seen along Trail Ridge, which is easily accessible from Fall River Pass. Twenty-five miles to the south, along the Continental Divide, are the peneplain remnants at Corona described by Davis, also at an elevation of about 12.000 feet. A few more miles to the southward, and visible from Corona, is the extensive remnant from the western edge of which James Peak rises as a prominent monadnock to a height of over 13,000 feet. A large remnant previously undescribed occurs near the point where the Continental Divide makes its turn to the west, about 10 miles southwest of James Peake. This remnant, which is one of the largest and most level areas in the entire region (figure 4), is easily visible northwest of Berthoud Pass. Its position may be determined from the road by the flat crest line of the large cirque which is cut into its surface. Although this remnant is off the topographic map, its eleva­ tion relative to Berthoud Pass shows that it must be at least 11,500 feet in altitude. Another ten miles to the south is the. Georgetown area described by Ball. This rather inaccessible area was not studied in detail. However, the proposed road to Mount Evans, which now extends to within three miles in an air line from the mountain, was traversed to its end. From this the mature topography of the upland country is strikingly evident. The suggestion is made that the somewhat even-crested ridge extending for three miles west of Chief Mountain at an elevation of 11,000 feet may represent the Flattop surface, and the great mature areas2 south and west 3 This area and much of that to the east and soufh may be best studied from the Denver Moan lain Parks sheet, advance copies of which were available for use in the field through the courtesy of the Denver office of the U. S. Geological Survey. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/495/3429942/BUL36_3-0495.pdf by guest on 30 September 2021 PENEPLAINS OF THE FRONT RANGE F ig u re 2.— Surface of Flattop Peneplain Flattop Mountain in the foreground. Altitude, 12,300 feet. The monadnock of Hallett Peak is at the left and Longs Peak in the distance. F igure 3.—IIagues Peak from Prospect Mountain3 Estes Park The flat shoulders of Hagues Peak represent the Flattop peneplain. The ridge in the middle ground—elevation, 10,000 feet—represents the Rocky Mountain peneplain, and the low area in the foreground the Park stage. XXXIII—B ull. Geol. Soc. A.m., Vol. 36, 1924 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/495/3429942/BUL36_3-0495.pdf by guest on 30 September 2021 500 H. P. LITTLE----EROSIONAL CYCLES IN THE FRONT RANGE of Mount Evans, although not visited, may be conjectured from the map to represent other remnants.