Fossil Ice-Wedge Polygons in the Colorado Front Range: Origin and Significance

Fossil ice-wedge polygons in the Colorado Front Range: Origin and significance

J. B. BENEDICT Department of Anthropology, Colorado State University, Fort Collins, Colorado 80521

ABSTRACT of 3,390 to 3,395 m above sea level, the polygons are above present timberline but below the absolute upper limit of tree growth; Polygonal patterned ground in a tundra valley near Sawtooth krummholz spruce and fir trees survive in sheltered locations along Peak, Boulder County, Colorado, is unique in the Front Range be- the moraine crest and in a glaciated col a short distance to the cause of its large size and its resemblance to fossil ice-wedge poly- southwest. gons. The polygons occur in a terminal moraine of latest Pinedale The Sawtooth valley is windy and cold, with locally variable age (about 15,000 to 10,000 B.P.), at an altitude of 3,390 m. They winter snow accumulation. Prevailing westerly winds are funneled range from 10 to 25 m in diameter and are outlined by shallow, into the valley through a trough on the western side of the conti- vegetated troughs that show no surface indication of sorting. The nental divide, depositing snow in the shelter of the cirque headwall polygonal ground pattern is attributed to thermal contraction but sweeping the polygon locality bare. Erosion of the winter cracking and ice-wedge formation during a late Pinedale or early snowpack by wind reduces effective precipitation on the surface of Holocene cold interval in which permafrost existed at least 350 m the moraine to less than the 800 to 900 mm of direct annual pre- below its present lower altitudinal limit. Frost-sorted rubble ac- cipitation normally characteristic of the Front Range timberline cumulated in troughs above the melting ice wedges; evidence of ecotone (Benedict, 1975, Fig. 2A). The estimated mean air tempera- sorting, however, is concealed by humus-rich loessal or slope-wash ture at the elevation of the polygon locality, extrapolated from sediments that bury the stones and fill interstices between them to nearby weather stations, is —1 °C (Benedict, 1975, Fig. 2B). form wedges of silty material as deep as 1.0 m. Organic matter in Although relict glacial or snowbank ice may be preserved beneath the humus-rich silt is believed to have eroded from an Altithermal insulating debris higher in the cirque, no permafrost was found dur- A1 horizon during early Neoglaciation; radiocarbon dates of 5750 ing excavations in the moraine. Ground-temperature measure- ± 110 and 5765 ± B.P. apply to formation of the Altithermal soil and are maximum ages for its erosion and redeposition, but they have no bearing on the time of patterned ground formation. Stones in the polygon borders have experienced no important frost sorting since the close of the Triple Lakes stade of Neoglaciation, which ended prior to 2855 ± 90 B.P. in the Sawtooth valley.

INTRODUCTION

Polygonal patterned ground on the floor of a high-altitude valley in western Boulder County, Colorado, closely resembles fossil ice-wedge polygons. The polygons are more than 100 km south of the Laramie Basin, Wyoming, the nearest locality at which Quater- nary ice-wedge casts have been previously identified (Mears, 1973). The polygons were studied in the hope that they would provide paleotemperature information to supplement other approaches to the reconstruction of former Front Range climates (Benedict, 1970, 1973; Maher, 1972). In this paper I discuss their geologic setting, surface appearance, and internal structure, speculate about their origin, age, and paleoclimatic significance, and attempt to explain the rarity of such patterned ground along the crest of the Colorado Front Range. Figure 1. Quaternary geology of Sawtooth Peak area. Deposits include moraines (m), protalus ramparts (p), talus sheets and cones LOCATION AND ENVIRONMENT (t), rock glaciers (rg), debris flows (f), and an avalanche boulder tongue (av), dating from Arapaho Peak (AP), Audubon (A), Triple The polygons are located in the Middle St. Vrain drainage basin, Lakes or older (TL), Satanta Peak or "late stade" Pinedale (SP), and 0.6 to 0.7 km southeast of the 3,750-m-high summit of Sawtooth earlier Pinedale (P) glacial advances. Striae on bedrock outcrops Peak (Fig. 1). They occur in a moraine on the floor of an unnamed (lines with dots) indicate directions of ice movement. Arrow points hanging valley (referred to informally as the Sawtooth valley), at to location of large polygons. Photograph taken September 1, approximately lat 40°07'30"N, long 105°37'00"W. At an elevation 1970.

Geological Society of America Bulletin, Parti, v. 90, p. 173-180, 8 figs., February 1979, Doc. no. 90211.

173

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ments on Niwot Ridge, 7.5 km to the south (Ives, 1973), suggest 40 cm of peat and muck, the basal 1 cm of which (NaOH-insoluble that the polygons are 350 m below the approximate lower fraction) has been radiocarbon dated at 2855 ± 90 B.P. (1-7461). altitudinal limit of permafrost in well-drained Front Range soils This is a minimum date for cessation of Triple Lakes glacial activity without appreciable surface slope or winter snow cover. in the cirque and for deposition of the till that underlies the outwash. GEOLOGIC SETTING The northeast-facing half of the cirque (Fig. 1) contains an av-

alanche boulder tongue of Audubon and younger age (Aav), which The Sawtooth valley is underlain by Precambrian gneisses and partially buries a Triple Lakes protalus rampart (TL„) but does not schists (Gable and Madole, 1976), intruded by small dikes of fully hide its topographic form. Stones in the avalanche boulder quartz monzonite, quartz latite, aplite, and pegmatite. Bedrock tongue are sparsely lichen-covered and show no appreciable differ- outcrops are uncommon on the valley floor because of an extensive ential weathering; surface instability suggests that snow or ice is cover of moraines, rock glaciers, and avalanche debris (Fig. 1). present at shallow depth within the rubble. The striated appearance The sequence of Quaternary deposits in the valley appears gener- of the avalanche deposit (Fig. 1) is caused by ridges of cobbles and ally similar to a published sequence for the Arapaho Pass region small boulders that extend downvalley in the lee of larger rocks. (Benedict, 1973), 11 to 14 km to the south (Fig. 2). Similar deposits farther to the south have been misidentified as The southeast-facing half of the cirque (Fig. 1) contains a fluted ground moraine (Benedict, 1968, p. 83-84).

moraine or protalus rampart of Arapaho Peak age (APm in Fig. 1), a In addition to the Neoglacial deposits described above, the valley rock glacier of Audubon age (Are), and a massive rock glacier of contains evidence of earlier and more extensive glacierization. Dur- Triple Lakes age (TL rg). The Triple Lakes rock glacier is composed ing a late phase of the Pinedale glaciation, ice advanced beyond the of several overlapping lobes, with steep, unstable margins that threshold of the cirque, impinged against the cleaverlike western suggest continuing motion and the possibility of a surviving ice end of a sharp-crested bedrock ridge (Fig. 1; summit elevation core. Boulders at the surface of the rock glacier are pitted and 3,530 m), and split into two lobes. The southern lobe overflowed a etched by differential weathering, and they bear a fully developed low col to the southeast, depositing a jumble of morainal debris on lichen cover. Approximately 150 m downvalley from the rock the steep slope leading to the floor of the Coney Lake valley. The glacier front (Fig. 3), outwash from the deposit is overlain by 35 to northern lobe flowed a short distance to the northeast, then stag- nated to form the broad-crested moraine (SPm) in which the polygonal patterned ground has developed.

AGE OF THE DEPOSIT

Efforts to date the moraine have not been entirely successful, although they indicate an approximate age of 15,000 to 10,000 radiocarbon years. There is no direct radiometric control, other than the minimum age of 2,855 ± 90 yr (1-7461) previously described, and an even less closely limiting minimum age of 1,400 ± 80 yr (1-9004) for archeological site 5BL97, on correlative till in the col leading to the Coney Lake valley. The moraine is almost certainly older than the Altithermal interval: its topography is gentle and subdued and its surface better vegetated and less bouldery than the surfaces of early Neoglacial protalus ramparts, rock glaciers, and debris flows that overlap its margins (Fig. 1). Deep weathering pits, Rhizocarpon geographicum thalli larger than 100 mm, and mature lichen communities suggest that these stratigraphically younger features, together with the sorted nets and stone-banked solifluction lobes that occur in till on the floor of the overflow col, are at least as old as the Triple Lakes stade of Neoglaciation (Ben- edict, 1973, p. 591). A maximum age for the deposit is more difficult to determine. The altitude of the moraine is 670 to 960 m higher than terminal moraines of middle and early Pinedale age mapped by Richmond (1960, Table 1) in Rocky Mountain National Park, and an estimated 425 to 910 m higher than terminal moraines of comparable age in the upper St. Vrain drainage basin (Madole, 1969, Fig. 2). Considering the depth and orientation of the Sawtooth cirque, the source of snow to windward, the size of the modern snowbank, and I I I I L— . 1 0 100 200 300 400 500% the degree of development of Neoglacial deposits in the valley, it seems unlikely that glaciers of the early or middle stades of Pinedale Length relative to Triple Lakes maximum glaciation would have terminated at such a high elevation. Figure 2. Time-distance diagram for glaciers in Arapaho Pass More probably, the moraine was deposited during the Satanta region, Colorado Front Range (from Benedict, 1973, Fig. 1). Heavy Peak advance of latest Pinedale time (Benedict, 1973), or during the lines indicate positions of terminal moraines, protalus ramparts, preceding "late stade" of Pinedale glaciation (Richmond, 1960). Its and rock glaciers in Sawtooth valley. Large polygons are at crest of location in the forest-tundra ecotone, a short distance beyond the

moraine labeled SPm - most extensive Neoglacial valley-head deposits (Figs. 1, 2), is re-

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miniscent of radiometrically dated Satanta Peak moraines a few others, 1965, p. 24). The centers of the polygons are gently convex kilometres to the south (Benedict, 1973). Compatible with a (Fig. 4), with an open cover (20% to 50%) of fellfield herbs in Satanta Peak or "late stade" Pinedale age assignment are such age- which Silene acaulis, Selaginella densa, Hymenoxys acaulis, related characteristics as maximum weathering pit depth (10.6 cm), Minuartia obtusiloba, Trifolium dasyphyllum, and Bistorta percentage of weathered granitic boulders (93%), mean and vivipara are conspicuous. maximum weathering rind thicknesses in aphanitic rock types (4.6 Shallow troughs, their floors 15 to 75 cm below the level of the and 13.0 mm, respectively), maximum R. geographicum diameter polygon centers, outline the polygonal pattern (Figs. 4, 5). The (139 mm), total lichen cover (10% to 95%), B-horizon thickness plant cover in the troughs is denser (60% to 100%) and more (35 to 55 cm), reddest B-horizon hue (7.5 YR), and depth of loess luxuriant than that of the centers, reflecting favorable moisture admixture (0 to 40 cm). Unfortunately, relative dating criteria such conditions and shelter from the wind. Vegetational contrasts are as these are influenced by factors other than deposit age, making most prominent in autumn, when polygon borders are emphasized the distinction between "late stade" Pinedale, Satanta Peak, and by the red leaves of Acomastylis rossii, Bistorta bistortoides, and early Neoglacial deposits a difficult one in the absence of radiocar- Heuchera parvifolia. A tiny open crack, resulting from desiccation bon evidence. or seasonal frost cracking, can be found by probing the bottoms of many of the troughs; the width of the largest opening observed in DESCRIPTION OF THE PATTERNED GROUND August 1972 was 6mm. Viewed from the ground surface, there is no evidence that stones have been sorted into the borders of the The polygons in the Sawtooth valley are 10 to 25 m in diameter, polygons (Figs. 4, 5). more than twice the size of the largest patterned ground previously The polygons occupy the crest and gently sloping sides of two reported in the Southern Rocky Mountain region (Osburn and adjoining till knolls (Fig. 3). Slope angles range from 0° to 10°.

Figure 3. Enlargement of Figure 1, showing location of radiocarbon-dated basal peat sample 1-7461 and soil-profile localities A, B, and C. Borders of large polygons outlined for clarity. Dashed lines enclose areas in which small (1.6 to 5.5 m) sorted nets occur; circles indicate positions of craterlike depressions.

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Along the north flank of the deposit, where the slope is steepest, the wedge show a preferred subhorizontal orientation that continues centers of the polygons are slightly terraced, and there is internal laterally into the host material. evidence of downs lope soil movement. Although the polygons give the appearance of being nonsorted The troughs that outline the polygonal pattern are diffuse and (Fig. 5), subsurface evidence of frost sorting was found at all three wide, making it difficult to determine whether polygon junctions profile localities. Polygon borders are underlain by linear zones of are orthogonal or nonorthogonal (Lachenbruch, 1962, p. 59). rubble enrichment, which are broader and older than the wedges of Two additional kinds of patterned ground occur in the moraine dark silty material that transect them. Stones in the rubble show no (Fig. 3). Small sorted nets, 1.6 to 5.5 m in diameter, occupy moist tendency to become smaller with depth, and they lack the on-edge terraces and swales; clusters of craterlike depressions, 1.0 to 5.0 m orientations commonly associated with the borders of large-scale in diameter and 10 to 45 cm deep, occur in more exposed localities. sorted patterned ground (Washburn, 1973, p. 123). Fines are pres- Similarities in scale suggest the possibility of a common origin; ent as a matrix in most parts of the frost-sorted rubble, although however, the depressions may simply be the locations of old mar- areas of openwork debris also occur. mot burrows that have collapsed and become revegetated. Neither Soils in the centers of the polygons are weakly developed Typic the sorted nets nor the depressions have been studied in detail. Cryumbrepts (Retzer, 1974, p. 793). A1 horizons are 5 to 15 cm thick, very dark brown in color, and similar in texture to underly- INTERNAL STRUCTURE ing material. B horizons are weakly oxidized, with 10 YR hues pre- dominating; there is no evidence of pedogenic clay accumulation. Trenches were dug across the borders of three representative B-horizon thicknesses range from 35 to 55 cm in polygon centers, polygons (Fig. 3). Profile drawings, textural analyses, and becoming thicker in coarser textured border areas. Manganese organic-matter determinations are given in Figures 6, 7, and 8. stains occur on the lower surfaces of stones in the Al, B, and IIB Stratigraphie units are described in Appendix 1. horizons, and manganese mottles occur on the undersides of stones Conspicuous in all three profiles are wedges of humus-rich, for an additional 20 to 25 cm below the depth of 10 YR oxidation. dark-brown to ver)' dark brown silty soil, finer-textured and more compact than the material that surrounds them, and tapering ir- ORIGIN, AGE, AND SIGNIFICANCE regularly downward beneath the borders of the polygons. The un- OF THE POLYGONS dersurfaces of stones in the wedges are stained purplish-black by manganese. There is no evidence of vertical foliation, inclusions of Patterned ground with a clearly defined polygonal outline (as host material within the wedges, or deformation in flanking opposed to the irregular outline of sorted nets and certain other coarse-textured sediments. At locality A (Fig. 6), pebbles in the patterned-ground forms) is generally considered to be a contraction

Figure 4. Telephoto view of large polygons, taken from east slope of Sawtooth Peak. Standing figure indicates scale. Poly- gons are 10 to 25 m in diameter, with sparsely vegetated convex centers. Photo- graph taken August 7, 1972.

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phenomenon (Washburn, 1973, p. 139). Although a variety of pro- than of frost cracking due to rapid cooling of frozen ground. Frost cesses can cause ground contraction and polygonal cracking, pat- cracking can occur in ground that is frozen only seasonally. How- terns as large as the polygons in the Sawtooth valley, developed in ever, it is improbable that polygons with such well-developed bor- coarse-textured sediments in a cold, high-altitude environment, are dering troughs could have formed without the existence, and sub- less likely to be the result of desiccation cracking or salt cracking sequent melting, of ice wedges. Thus, the polygons are considered to indicate former permafrost conditions. Development of the initial polygonal crack network was followed by frost sorting, frost creep, and perhaps shallow solifluction. Stones that were heaved to the ground surface in polygon centers moved radially outward, along local gradients, to form concen- trations of rubble in polygon border areas. Pebbles in the moving surface layer may have acquired their preferred subhorizontal orientations at this time, or during early Neoglaciation. As lateral movement became less effective, cobbles and small boulders were left stranded as lag accumulations in the centers of the polygons. Next to develop were the wedges of humus-rich silty soil that underlie polygon borders. There is no reason to believe that the wedges are true ice-wedge casts: virtually all of the criteria considered essential for the recognition of ice wedges in fossil form (Black, 1976, p. 11-12) are absent. An abundance of silt and a scarcity of fine gravel suggest a loessal or slope-wash origin for the sediments. The silt is believed to have filtered downward through intercon- nected openings in the frost-sorted rubble, perhaps encouraged by Figure 5. Shallow troughs, floored with loess and/or slopewash renewed frost cracking along zones of weakness formed by the sediments and overgrown with tundra herbs and grasses, outline sorted borders of the polygons. borders of polygons. Borders show no surface indication of the Radiocarbon dates of 5765 ± 110 B.P. (1-7459) and 5750 ± 110 sorting that occurs at depth. Photograph taken August 7, 1972. B.P. (1-7460) were obtained for samples of humus-rich silt collected

PARTICLE SIZE

100 <~ II SAMPLE GRAVEL O.M • 1 24 5% 15 5%

2 52 7 20 0 Figure 6. Profile across polygon border at locality A. Textural analyses (hydrometer, wet sieving) and organic-matter determi- 3 30.8 4.2 • nations (loss on ignition) are shown for seven samples. A1 hori- 4 54.0 1.7 • , zon (unit I) is gradational into gravelly loam fill of recent frost or 5 62,5 1.7 desiccation crack. Subhorizontal orientation of pebbles in stratigraphic unit HI continues into wedge of silty, humus-rich material $ 50 5 0.5 y (unit V); pebbles in younger crack fill lack preferred orientation. 7 IT.a 14.0 See Appendix 1 for description of stratigraphic units.

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deep within the polygon borders (Figs. 7, 8). The significance of the ply of organic carbon and fine-textured clastic sediments for rede- dates depends upon the mechanism by which organic matter be- position by wind and slope wash processes in sediment traps such as came incorporated into the unit. the sorted borders of the polygons near Sawtooth Peak. If, for example, the humus were a result of soil-forming processes Deposition of the silt was followed by formation of the modern operating in the vegetated borders of the polygons, the dates should A1 horizon, continued oxidation of the till, and deposition of man- be interpreted as minimum ages for deposition of the silty material. ganese on the undersurfaces of stones. Little is known about the Reasons for rejecting this interpretation include the increase in chemical behavior of manganese oxides in Front Range tundra organic-matter content with depth observed at localities B and C soils. However, manganese staining is rarely, if ever, developed in (Figs. 7, 8) and the abruptness of the change in color and organic- openwork rubble, suggesting that the purplish-black coatings must matter content that takes place at the margins of the wedges. postdate deposition of the: interstitial silt. Because only the under- Neither would be expected to occur if the humus were a product of surfaces of stones are discolored by manganese, the stones cannot root penetration and decay in situ. have been reoriented by frost disturbance since the most recent in- If, on the other hand, the humus were eroded from a nearby turf terval of staining. Stratigraphic studies in the Arapaho Pass region horizon or peat bog and redeposited concurrently with the silt, the suggest that staining was last important during the soil-forming in- radiocarbon dates should be interpreted as maximum ages for terval that separated the Triple Lakes and Audubon stades of Neo- filling of the polygon borders. This interpretation is consistent with glaciation (Fig. 2); thus, a minimum age of approximately 3,000 the homogeneity, fine texture, and sharply defined boundaries of radiocarbon years can be taken for silt accumulation in the borders the wedges, and with events thought to have occurred in the Front of the large polygons and for the end of significant frost sorting in Range alpine region during mid-Holocene time. Dates for the the moraine at this locality. humus relate to a time during the Altithermal interval in which Virtually all patterned ground in the Sawtooth valley has been patterned ground was inactive even in very moist micro- inactive for 3,000 yr or longer. Relative-dating criteria suggest that environments, and a thick, very dark brown to dark brown A1 hori- the borders of small sorted nets in the col connecting the Sawtooth zon was developing above present timberline (Benedict and Olson, and Coney Lake valleys have been stable since the Triple Lakes in prep.). During the Triple Lakes stade of Neoglaciation, large stade of Neoglaciation; the centers of the sorted nets have experi- areas bearing this Altithermal soil were disrupted by nivation, enced no important frost disturbance for at least 1,400 ± 80 patterned-ground reactivation, and wind erosion, providing a sup- radiocarbon years (1-9004), the age of charcoal associated with

0 em r

$0 Figure 7. Profile across polygon border at locality B. Slope is to northwest at 10°. Wedge of humus-rich silt (unit V) has been deformed by I 00 downslope soil movement, probably during Neoglaciation. Strati- 5765 ± 110 B R (1-7459} graphic units are described in Appendix 1. Radiocarbon sample was pretreated for removal of 1 II III IV V carbonates and humic acids.

SAMPLE GRAVEL OM

50 8* ISO* 207 S-l 50.2 49 63.4 12 5 556 1.4 37 i 115 50 100 250 500 1000 2000." 72.5 300 PARTICLE SIZE

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dry-laid stone game-drive structures that overlie the patterned present levels is implied for initial development of the polygonal ground. Cavernously weathered and heavily lichen-covered upper crack network. As emphasized by Black (1976), the opportunities surfaces indicate that stones in the borders of sorted nets at two of for error involved in such conclusions are legion. Because of am- the localities shown in Figure 3 have been in their present orienta- biguities in dating and interpretation, the Front Range polygons tions since the close of the Triple Lakes stade of Neoglaciation; the are, at best, imprecise paleothermometers. third locality, a closed depression that holds meltwater for a brief Formation of the polygonal fracture pattern was followed by an period in spring, may have been reactivated more recently. interval of minor frost sorting, indicating an active-layer thickness Climatic implications of the stratigraphic sequence in the Saw- of at least 1 m, and a freeze-thaw regime in which autumn soil tooth valley can be summarized as follows. moisture was more abundant than at present. The additional mois- The moraine in which the large polygons are developed was de- ture may have been supplied by melting icy permafrost, under posited during a late phase of the Pinedale glaciation, inadequately warming conditions, or by increased precipitation. The melting- dated, but almost certainly occurring between 15,000 and 10,000 permafrost hypothesis is particularly attractive because (by the radiocarbon years ago. Pinedale valley glaciers had receded sig- ablation of ice wedges) it accounts for the troughs in which the nificantly by this time, and active ice bodies were restricted to high sorted rubble accumulated. elevations and favorably oriented cirques. Development of the Altithermal A1 horizon implies conditions of Deposition of the moraine was followed by a warm interval in soil stability, resulting from increased warmth and/or decreased which the till became sufficiently stable to allow preservation of the precipitation. The approximate age of the soil is indicated by polygonal pattern. radiocarbon dates of 5750 ± 110 and 5765 ± 110 B.P.; both dates Conditions at the time of frost cracking were colder than the fall near the middle of a 500-yr period thought to represent the present, and permafrost is likely to have existed at least 350 m culmination of Altithermal warmth and aridity in the Rocky lower than it exists today. If ice wedges were involved in polygon Mountain region (Benedict and Olson, in prep.). formation (which is suspected, but unproven), and if criteria de- During the Triple Lakes stade of Neoglaciation, snowbanks ex- veloped by Pewe (1966) for ice-wedge polygons in Alaska can be panded and rock glaciers developed in the Sawtooth valley. In extrapolated to the high Colorado mountains (which is unlikely), a addition to rockfall from the valley walls, the Triple Lakes stade was decline in mean air temperature on the order of 5 to 7 °C below characterized by renewed frost activity, erosion of the Altithermal

NW mmmm

Äri1 * ® » ?

5750 tuo BP. (1-7460)

SAMPLE GRAVEL OM 1 1 69 2% 17 7* III 2 26 9 172 3 600 292 IV 4 59 1 4 9 3 52 4 2.1 V 6 74 8 4 3 7 40 6 2.1 Stones 8 31.1 1 9 SO 100 250 500 IOOO 2000-" 9 65 1 14 4

PARTICLE SIZE Tenturol sample 10 31 5 23.1

Figure 8. Profile across polygon border at locality C. Units are described in Appendix 1. Note parallelism between texture of unit I and that of underlying material. Unit II is absent at this locality. Preferred subhorizontal orientation of pebbles in unit III is conspicuous only above dashed line. Material below dashed line is stony and structureless, with local pockets of openwork debris; stones show no visible preferred orientation. Surface concentration of gravel and cobbles occurs at each end of profile line.

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soil, and deposition of humus-rich silt in the sorted borders of IICox horizon, developed in glacial till. Unit is very 'compact, with gravelly polygons that remained open at the surface, or were freshly reacti- sandy clay loam texture and weak to moderate blocky structure. It is firmer, finer textured, less bouldery, and less gravelly than stratigraphic unit III. vated. The climate is thought to have been colder and snowier than Stones have thin clay caps. Change in color from unit III to unit IV is rea- the present. The Triple Lakes glacial interval ended, in the Saw- sonably abrupt and is the basis for horizon boundaries shown in Figures 6, tooth valley, prior to 2,855 ± 90 radiocarbon years ago. 7, and 8; change in texture is more gradual. Soil formation in polygon centers and manganese staining of V: Very dark brown (10 YR 2/2 m) to dark brown (10 YR 3/3 m) stones in polygon borders suggest that the Triple Lakes—Audubon humus-rich silt loam to gravelly loam or gravelly sandy loam soil, forming matrix between abundant stones. Moderate to strong fine (~1 mm) platy interstade was a time of patterned-ground stability, probably structure, with very fine (—0.1 mm) spherical vesicles, and widely scattered caused by warmer temperatures. With minor exceptions, the end of flecks of wood or shrub charcoal. Unit becomes increasingly organic with the Triple Lakes glacial interval marked the cessation of depth and is siltier and more compact than adjacent material. Undersur- patterned-ground activity in the Sawtooth valley. Although snow- faces of stones stained with manganese. banks expanded in phase with glaciers elsewhere in the region during the Audubon and Arapaho Peak stades of Neoglaciation (Fig. REFERENCES CITED 2), frost processes were important only in moist micro- environments. Benedict, J, B., 1968, Recent glacial history of an alpine area in the Col- orado Front Range, U.S.A., II. Dating the glacial deposits: Journal of The question arises as to why no other polygons comparable in Glaciology, v. 7, p. 77—87. size and appearance to those in the Sawtooth valley have been re- 1970, Downslope soil movement in a Colorado alpine region: Rates, ported in the intensively studied Indian Peaks region. A possible processes, and climatic significance: Arctic and Alpine Research, v. 2, explanation is that the moraine at the base of Sawtooth Peak is one p. 165-226. 1973, Chronology of cirque glaciation, Colorado Front Range: of the only land surfaces in the area that was simultaneously (1) Quaternary Research, v. 3, p. 584-599. high and windy enough to have been deeply frozen, snow-free, and 1975, Prehistoric man and climate: The view from timberline, in Sug- treeless at the time of initial cracking, (2) level and extensive gate, R. P., and Cresswell, M. M., eds., Quaternary studies: Royal So- enough to accommodate the formation of very large polygons, (3) ciety of New Zealand Bulletin 13, p. 67-74. young enough that pattern dimensions were not conditioned by the Black, R. F., 1976, Periglacial features indicative of permafrost: Ice and soil wedges: Quaternary Research, v. 6, p. 3-26. previous development of small sorted nets, and (4) dry enough that Gable, D. J., and Madole, R. F., 1976, Geologic map of the Ward quad- the polygonal network escaped destructive sorting and subdivision rangle, Boulder County, Colorado: U.S. Geological Survey Geologic during Neoglaciation. Quadrangle Map GQ-1277. Ives, J. D., 1973, Permafrost and its relationship to other environmental pa- rameters in a midlatitude, high-altitude setting, Front Range, Col- ACKNOWLEDGMENTS orado Rocky Mountains, in Pewe, T. L., and Mackay, J. R., eds., Per- mafrost: North American contribution to the Second International Byron L. Olson and Jean C. Florman assisted in field work. Au- Conference: Washington, D.C., National Academy of Sciences, drey D. Benedict identified plants at the polygon locality and col- p. 121-125. Lachenbruch, A. H., 1962, Mechanics of thermal contraction cracks and lected a radiocarbon sample from the bog in front of the rock ice-wedge polygons in permafrost: Geological Society of America glacier. Soil textural analyses were run by James R. Rogers, Front Special Paper 70, 69 p. Range Environmental Laboratory; radiocarbon dating was super- Madole, R. F., 1969, Pinedale and Bull Lake glaciation in upper St. Vrain vised by James Buckley, Teledyne Isotopes. I am grateful to R. M. drainage basin, Boulder County, Colorado: Arctic and Alpine Re- Burke and A. L. Washburn for their helpful review comments. search, v. 1, p. 279-287. Maher, L. J., Jr., 1972, Absolute pollen diagram of Redrock Lake, Boulder County, Colorado: Quaternary Research, v. 2, p. 531-553. APPENDIX 1. DESCRIPTION OF Mears, B., Jr., 1973, Were Rocky Mountain intermontane basins Pleis- STRATIGRAPHIC UNITS tocene tundras?: International Union for Quaternary Research Con- gress, 9th, Christchurch, New Zealand, 1973, Abstracts, p. 233. I: Very dark brown (10 YR 2/2 to 2/3 m) to dark brown (10 YR 3/3 m) Osburn, W. S., Benedict, J. B., and Corte, A. E., 1965, Frost phenomena, gravelly ioam to gravelly sandy loam A1 horizon. Loose and structureless, patterned ground, and ecology on Niwot Ridge: Guidebook for one- with abundant roots. Manganese stains on undersurfaces of stones in lower day field conferences, Boulder area, Colorado: International Associa- part of unit. tion for Quaternary Research Congress, 7th, Boulder-Denver, p. 21- 26. II: Dark brown (10 YR 4/3 m) to dark yellowish-brown (10 YR 4/4 m) B horizon, weakly developed in glacial till, with an admixture of wind- Pewe, T. L., 1966, Paleoclimatic significance of fossil ice wedges: Biuletyn blown^) sand and silt. Gravelly loam to loam texture, with fewer pebbles Peryglacjalny, no. 15, p. 65-72. and stones than occur in underlying material. Undersurfaces of stones Retzer, J. L., 1974, Alpine soils, in Ives, J. D., and Barry, R. G., eds., Arc- stained by manganese. Structureless to weakly blocky. tic and alpine environments: London, Methuen, p. 771-802. Ill: Dark brown (7.5 YR 4/4 m) to dark yellowish-brown (10 YR 4/4 m) Richmond, G. M., 1960, Glaciation of the east slope of Rocky Mountain IIB horizon, weakly developed in bouldery, frost-sorted glacial till, without National Park, Colorado: Geological Society of America Bulletin, an appreciable loess component. Pockets of openwork rubble occur locally, v. 71, p. 1371-1381. although most parts of the deposit have a gravelly sandy clay loam, gravelly Washburn, A. L., 1973, Periglacial processes and environments: New York, sandy loam, or gravelly loamy sand matrix, coarsest in texture near top of St. Martin's Press, 320 p. unit. Strong subhorizontal pebble orientation suggests modification of upper part of deposit by downslope soil-movement processes. Single-grain to moderate fine platy structure. Lower boundary of unit is gradational and MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 24, 1976 poorly defined. Undersurfaces of stones stained by manganese oxides. REVISED MANUSCRIPT RECEIVED OCTOBER 6, 1977 IV: Light olive-brown (2.5 Y 5/3 m) to grayish-brown (2.5 Y 5/2 m) MANUSCRIPT ACCEPTED OCTOBER 28, 1977

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