Erosional Cycles in the Front Range of Colorado and Their Correlation 1

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)

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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.

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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. The flat summit of Meridian Hill, eight miles east of Mount Evans, with an elevation of 11,400 feet, may also be an isolated remnant. To the southward no other remnants of the old plain were definitely recognized. The wide extent of the Rocky Mountain surface in this region, however, readily explains their absence. It is probable that the great spurs extending out from Pikes Peak at an altitude of about 12,000 feet are remnants of the Flattop plain rather than upfaulted parts of the Rocky Mountain surface, as Finlay supposes. The crest line of the Tarryall Mountains, six miles long, with fourteen distinct knobs, all of which reach elevations between 11,700 and 12,400 feet, is also suggestive. This range lies midway between Pikes Peak and Mount Evans, though not in a direct line. The region west of the Tarryalls is not yet mapped. In this connection credit should be given Richardson for his earlier ob servation that Devils Head (9,3 1-8 feet) and other isolated peaks west of the ar£a of the Castle Rock Folio may be remnants of an older surface that withstood the general denudation of the Rocky Mountain cycle. It is clear from the above that from the northern boundary of Rocky Mountain National Park to a point south of Mount Evans remnants of a mature topography at an elevation of about 12,000 feet are so numerous as to indicate the former presence of a continuous surface. Although south of this area remnants are not so common, its presence as far south as Pikes Peak may probably be assumed. ' Lee has hazarded the suggestion that a peneplain still older than the Flattop surface may be represented by the summit of Longs Peak (figure 2) and other of the higher mountains in Colorado. Davis has pointed out that there are forty-two summits in Colorado between 13,500 and 14,000 feet, thirtj'-nine between 14,000 and. 14,500 feet, and none higher than 14,500 feet. These facts are suggestive, although the remnants seem too few and the summit areas too small to permit of a definite hypothesis. TI1E ROCKY MOUNTAIN PENEPLAIN The writer has not been able to discover the first use of the term Rocky Mountain peneplain, but Lee refers to it in 1917 as a term commonly in use. In contrast to the Flattop peneplain, it finds its best development in the southern part of the area studied—the Pikes Peak region. Near the abandoned mining town of Gillett, four miles northeast of Cripple Creek, it stretches two miles north and south over the old granite surface

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F igure 4.—Large remnant of Flattop Peneplain, west of Berthoud Pass Elevation, about 11,500 feet.

F igure 5.—Rocky Mountain Peneplain Surface On this surface lies the town of Gillett, near the southwest base of Pikes Peak. Elevation, 10,000 feet.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/495/3429942/BUL36_3-0495.pdf by guest on 30 September 2021 502 II. P. LITTLE----EROSIONAL CYCLES IN THE FRONT RANGE with an almost tablelike flatness (figure 5). Here its altitude is 10,000 feet. Farther north, opposite Palmer Lake, it is about 9,200 feet in alti tude. Instead of being continuous here, it has been dissected by a dendritic drainage into a maze of promontories rising sharply above the recent valleys (figure 6). Viewed from a distance, these promontories merge to give the effect of a continuous surface. North and west of here, toward the South Platte River, jhe surface of the plateau drops to an altitude of 7,400 to 8,000 feet. This sag in the surface can be clearly seen from various points; it was observed especially clearly from the knoll where the road bends toward Palmer Lake, as shown at the eastern edge of the Platte Canyon sheet ten miles south of Stone Canyon. Farther north, in the Bergen Park area, the peneplain surface is far less well preserved. For the purpose of classifying the region physiographically, the general view from Genesee Mountain, five miles southwest of Golden, is most satisfactory. From here it is evident that the peneplain is repre sented only by most discontinuous fragments, which rise abruptly from a younger late mature erosion surface. The eastern and western spurs of Genesee Mountain and the summit areas of Lookout Mountain, Bear Mountain, and Doublehead Mountain, as well as other unnamed summits, are probable remnants; also other areas to the south, which occur at alti tudes of from 7,600 feet to 9,000 feet. If these areas are correctly iden tified, the surface of the peneplain has been considerably warped or faulted in this area. Farther west, toward the crest of the range, are flattish remnants which may represent its inner margin. The chief area which may be so interpreted is the large flat surface immediately south west of Meridian Peak, at an altitude of about 10,800 feet. Accepting this as a remnant, the plain in this region has a slope of 175 feet per mile eastward. The area between Idaho Springs and Central City and westward toward the crest of the range is so dissected that little of it can with any definite ness be considered to represent a previous erosion cycle. To the north east of Central City, however, along the westward margin of the Black Hawk sheet and extending for a number of miles toward Rollinsville, the late mature topography is clearly evident at an altitude of 9,000 to 9,200 feet. Thorodin and Tremont mountains rise from this surface as un usually jagged monadnocks. To the north the elevation declines to about 8.500 feet. The region is quite thoroughly dissected by 'canyons and short steep gulches, but when viewed from a vantage point, as near the town of Ward, the canyons and gulches take on less prominence and the plateau remnants far more, furnishing one of the finest views of a dis-

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F ig u re 6.—Surface of Rocky Mountain Peneplain west of Palmer Lake Elevation, 9,200 feet. The valleys do not belong to the present cycle, but were probably eroded in the Park Stage.

F igure 7.— Dissected Rocky Mountain Peneplain Surface overlooking Ward Elevation, 8,500 feet

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/495/3429942/BUL36_3-0495.pdf by guest on 30 September 2021 504 H. P. LITTLE----EROSION A L CYCLES IN THE FRONT RANGE sected peneplain to be seen anywhere (figure 7). As Ward is approached, the elevation increases to upward of 9,300 feet or perhaps more. About one mile to the northeast the highway runs for a mile over a wholly un dissected remnant of the plateau (figure 8). From Ward northward for twelve miles toward Allens Park the remnants become increasingly small, until they completely disappear in the modern mature topography. The area to the east is unmapped. No further remnants to the northward are identified with certainty until the Rocky Mountain National Park is entered. Immediately west of the Rollinsville-Ward area just described are three striking remnants of the Rocky Mountain peneplain, known as Caribou Flat, Chittenden Mountain, and Guinn Mountain—Bryan Mountain (figure 9). Because of increased elevation of the peneplain surface toward the Continental Divide, these remnants are much higher than those found farther east—10,250 feet for much of Caribou Flat and 11,000 feet for much of Chittenden and Guinn Mountains. The sharp break between these high level remnants of the Rocky Mountain surface and the Flattop surface at Corona near by furnishes one of the' chief ar guments in favor of the existence of two separate peneplains (figure 10). The Rocky Mountain peneplain is represented by no continuous sur faces north of Allens Park, at least in none of the area yet mapped. In Estes Park, however, as Lee has pointed out, numerous peaks reach alti tudes corresponding to those of areas farther south. This includes Giant- track Mountain, Deer Mountain, the ridge of which The Needles is the culminating point, and Prospect Mountain. These range in elevation from 8,800 to 10,000 feet. There is a small, strikingly flat area at the very head of Tahosa Valley,., midway between Allens Park and Estes Park, which the writer is inclined to regard as a remnant, adding one ipore link to the chain which connects the various areas and shows them to be remnants of a common surface. THE PARK STAGE The name Park Stage is proposed for a mature to late mature topog raphy which has been eroded below the Rocky Mountain surface at various localities (figure 6). Its presence seems to have escaped the notice of previous workers, although in several instances it is a most striking feature and contrasts strongly with the canyons downstream and the isolated remnants of the Rocky Mountain peneplain which rise above. There are three regions in which it is best developed: the Wood land Park-Divide area north of Pikes Peak, the Evergreen-Bergen Park

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F igure 9.—Looking Southwest over Caribou Flat toward Corona Showing inner edge of Rocky Mountain peneplain.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/495/3429942/BUL36_3-0495.pdf by guest on 30 September 2021 506 H. P. LITTLE EROSIONAL CYCLES IN THE FRONT RANGE area, about eight miles southwest of Golden, and the Estes Park area. In the first two regions especially' it is of considerable extent and is the most evident of all the surfaces. In contrast to the older surfaces these remnants can not, because of their less complete development, be traced from basin to basin. Their slope is in all cases toward'existing valleys. In the vicinity of Woodland Park and Divide is clear evidence that this stage was interrupted by a glacial episode. On the retreat of the ice, erosion began again and has dissected the glacial deposits into a stream- cut topography of dendritic type. All trace of glacial action has been

F igure 10.— Looking Northwest from Flattop Peneplain at Corona, over the inner Edge of the Rocky Mountain Peneplain Elevation of inner edge, 11,000 feet. Difference in elevation of the two surfaces, about 1,000 feet. removed save a few erratics, but from numerous road cuts striated cobbles have been collected, leaving no question as to the origin of the materials. The elevation of this surface varies approximately from 8,300 to 9,300 feet. The transition from the Park Stage surface to the Rocky Mountain peneplain is in some places almost imperceptible; at other localities, where distinct monadnoeks occur, it is abrupt. In the Bergen Park-Evergreen area the surface occurs at elevations varying by gentle slopes from 7,000 to 8,000 feet. The canyons occupy ing the lower ends of the valleys flow in meandering courses probably

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 5 07 inherited from this stage. Wide terraces in the rock high above the present stream but below the Rocky Mountain surface are visible in the canyons, as in the canyon of Bear Creek below Evergreen. These are best seen, however, in the Estes Park region. In Estes Park the areas representing this episode lie 1,000 feet or more below the Rocky Mountain peneplain, in a series of late mature valleys (figure 3). The road south from Estes Park traverses one of these val leys, and the valley of Fish Creek southeast of Prospect Mountain is of a similar nature. On leaving Estes Park by Big Thompson Canyon the pronounced meandering nature of this stream is evident, some of the in cised meanders being of striking perfection. Wide benches appear above the stream—sometimes on one side, sometimes on the other, sometimes on both. These benches are higher above the present stream the farther downstream one progresses. The meandering course is evidently in herited from a time when the upper mature valleys of Big Thompson River had its outlet at the level of this bench, and the present intrenched mean ders may be inherited from that stage, due to recent uplift. The absence of a topographic map of the canyon to the east, however, makes it im possible to say how the level of the bench compares with the level of the wide valley of Estes Park, and it may represent the equivalent of the Fountain Creek (which see) rather than the Park stage. A hurried trip to the north showed the presence of a similar cycle just to the west of Livermore. This area is described in detail by Mather in a paper abstracted in the current volume of this Bulletin. In it he recog nizes an upper bench within the canyons, which he makes part of the canyon cycle rather than correlating it with the Alford stage, which cor responds to the Park stage. THE FOUNTAIN CREEK STAGE Here and there evidences of an additional minor stage are found inter mediate between the Park stage and the present Canyon-cutting stage. At only one place was the presence of such a stage clearly shown, namely, in Fountain Creek, which runs from Woodland Park through Manitou to Colorado Springs and beyond. Because of this fact, the name Fountain Creek stage is suggested. It is represented by the flat inner valley of this creek, which persists for several miles upstream from a point a mile above Green Mountain Falls (Pikes Peak sheet). It merges upstream with the Park stage, while downstream the canyon has destroyed all traces of its presence. This stage is, as said, of minor importance, but is interesting as occupy-

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. . . _ iEARTOOTH W ESTERN WYOMING SOU THE AST WYOMING FRONT RANGE CO L OR AOO SAN JUAN MOUNTAINS SOUT H E A S T-ID A H O ** CENTRAL 0UTTE-*MONT«- MTS WESTGATE FINIA* L t T T L £ AN» ATWOOO AND 8 RANSON B l ACKWLLDER UCKWE.LDER RI(HAffO»4 N EPOCH 8 MATHER LE E. ATWOOO MATHER MANSFIELD umpleby BUWALDA ATWOOD RANGE BE.VAN FRONT THE IN CYCLES AL LITTLE------EROSION P. H.

RECENT Bt I L V U I O Clt CANYON CUTTING CANYON CUTTING SPRING CREEK CYCLE CYCLE “ G fa s o N " IIVE.RMORC L*TE. PLEIST CANYONS CYCLE* FOUNTAIN CREEK OXFORD GLACIAL 0 CYCLE CENE VAL LE YS CYC Lt Pl a in * i A L FORD CYCLE PARKS PLAIN *2 BOULDER-- PUTNAM LESLIE STAGE suBSunnir PLAINSV« MESA GANNt TT VALLEYS ROCKY 1ROSION SURFACi Of ?ENEPLAIN (NtlTHtR OF MTN. MTN PL WIND RIVER ABOVE OC t) Eft PINE PLAIN PENEPLAIN PENEPLAIN PERHAPS 10 T Han Pki OC tNc] SUMMIT C£N£ SUMMIT. TtBMI**ATlb CONPLETED DURIN& AT Cl DSC OF PENEPLAIN PENE PLAIN T iH T tA ftr EROSION PLIOCENE PENEPLAIN TYGEECYCLE [at LEA 6 T PENEPLAIN MIO [p e r h a p s in THIRD P#5T NID-TE.RTMMÌ PART EARLY [NOT OLDER CENÍ PLIO C tN £ ] CYCLE. THAN mioceni OF SUMMIT FLAT TOP SNOWDRIFT PENE PLAIN PENEPLAIN PEN C PLAIN EROSION PLAIN BAS E D ON PENEPLAIN [C0 MKtT£C> [PKt MIDDLE OLI GO NEW AGE £Of Pt£TED C ENE 1 BY C l OSE OF SOMtWHifli DETERMINATION SECOND 6 T nip TlAT|A*i NMR OF PA Y ETTi 0LJGÛCE NE CYC L E OF MlOTERTIAli rûR MA T ION EROSION

£0 MEDICINE BOW FLAT TOP FLAT TOP EOCENE EOCENE EOCENE CE N£ PENEPLAIN PENE.PL AIN PENE PLAIN PENE PLAIN PENEPLAIN PENEPLAIN

F ig u re 11.— Tabic showing Erosion Cycles of Front Range ami adjacent Areas as recognized by various Writer-* Each cycle terminates at approximately the position of the solid lines. AGE AND CORRELATION OF EROSION CYCLES 509 ing approximately the position that has been assigned to minor stages in other areas. THE CANY OX-CUTTING STAGE As is well known, the streams of the Front Range leave the mountains by canyons due to recent uplift. The writer has nothing new to offer in this connection.

A ge a n d C o r r e l a t io n o f E r o sio n C y c l e s The age of the Flattop and Rocky Mountain peneplains was not fixed by the writer’s studies any more definitely' than previously indicated by Lee. Mather, however, who was in touch with the party in Estes Park, has correlated the peneplains previously described by Blackwelder in southeast Wyoming with sedimentary deposits on the plains, and will publish their ages as Eocene and Pliocene or earliest Pleistocene re spectively. He recognizes, however, that the Flattop cycle may have per sisted into early Oligocene. These peneplains are considered by Mather, as well as all who have been in the field with the writer, as undoubted equivalents of Flattop and Eocky Mountain peneplains. Dean Worcester, who has been giving detailed study to the problem, has also expressed orally his view that the Sherman peneplain can be traced directly into the Eocky Mountain peneplain. The writer had hoped to get some clue to the age of the lower peneplain by its relations to the Florissant basin, but nothing definite was estab lished. The Eocky Mountain peneplain comes up sharply to the border of the basin, which contains undoubted Oligocene deposits. If these deposits occur in a basin formed by erosion or down-warping or faulting in the Eocky Mountain surface, that surface is, of course, older than Oligocene. Such an age is entirely out of harmony with that determined for what is considered the northern extension of the same surface. An other possibility is to consider that the Florissant was deposited in a basin formed by warping in the Flattop surface, and that later the Eocky Mountain peneplain was developed by erosion across granite and lake bed surfaces alike. Differential erosion then produced the present topog raphy. If it were not for the clear development of two distinct surfaces to the north, the relations at Florissant might lead one to adopt Finlay’s view, that what is now distinguished as Flattop and Eocky Mountain peneplains are simply segments of the same surface displaced by faulting. The Park stage occupies about the position of the Leslie valleys recog nized by Blackwelder and the Boulder Mesa recognized by Atwood and Mather in the San Juan Mountains, and probably all were formed at

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/495/3429942/BUL36_3-0495.pdf by guest on 30 September 2021 510 II. P. LITTLE EROSION A L CYCLES IN THE FRONT RANGE 'about the same time. The Fountain Creek stage is too local to attempt any definite correlations, but it is interesting to note that a similar short stage occurred toward the close of the Pleistocene in several separated regions. The Canyon cycle has long been recognized as representing the lust great uplift. These relations are brought out more completely on the accompanying table (figure 11). It should be recognized that correlations of these cycles does not in volve the belief that the corresponding present surfaces are of the same age. A great erosion cycle may, for instance, have been brought to a close with the Eocene in one section of the mountains and have persisted into early or even late Oligocene in another. In such a case there may be a rough correlation of the cycles, though not of the surfaces. Viewed thus, it seems safe to recognize three great periods of post-Cretaceous erosion, one cycle persisting through the Eocene over wide areas and brought to a close during the Oligocene, in most eases in early Oligo cene; another cycle, widespread but probably less exactly contempo raneous, occurring ‘in middle to late Tertiary time; and a third cycle, widespread but shorter-lived, occurring in several areas in late Pliocene to early Pleistocene time. D is c u s s io n Prof. F r a n c is P. S iie p a r d : The correlation of peneplains depends on the assumption that they are contemporaneous. If we assume they are not contemporaneous, it is equally easy to show that they are not con temporaneous. Prof. Eliot Blackwelder: Since reference has been made to my paper on the Laramie District (1908), I feel obliged to say that I no longer subscribe to some of the views on physiographic history expressed therein. I *am particularly doubtful about the ages of the earlier stages. Mr. Frederick K. Morris : In one of the illustrations Professor Little showed the Flattop peneplain as a shoulder or bench from which a monadnock rises, apparently with tower-like abruptness. It looked in the picture like the “inselberg” of Passarge or the “girdled mountain” of Keyes. Could such monadnocks point toward an arid climate in the closing stages of the Flattop peneplanation ? To Professor S h e p a r d : Professor Shepard’s position would be well taken if we were trying to show that these peneplains ended contempo raneously. However, we are speaking of the general contemporaneity of the cycle and not the precise end of the cycle. From this point of view it seems to me that they are in a very real way largely contempo-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/495/3429942/BUL36_3-0495.pdf by guest on 30 September 2021 DISCUSSION 511 raneous. The period during which each cycle of erosion occurred has been determined by geologic field methods. To Professor Blackwelder : I have merely attempted to show in out line form Professor Blackwelder’s published statements. It is interest ing to note that Professor Mather, after working on the plains near this region during the summer of 1924, has reached conclusions as to the age of the peneplains quite similar to those expressed by Professor Black- welder in the paper referred to. To Mr. M o r r is : I presume that the illustration to which Mr. Morris refers is the one that shows Hallett Peak rising abruptly above the Flat- top peneplain of Flattop Mountain. I think that this abruptness is due to mechanical weathering at high altitudes, in addition to the fact that a large cirque cuts into the peneplain almost immediately at the edge of Hallett Peak. Near-by extensive areas, of the Flattop peneplain, as at Trail Ridge and the vicinity of Mount Evans, show none of this abrupt ness, but rather well-rounded, dome-shaped hills. I would not regard the monadnock referred to as justifying any conclusions as to the climate in the closing stage of Flattop peneplanation.

B ibliography

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