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Marcel Kornfeld,2 Richard G. Reider,3 and George C. Frison2. 1Department of ... Folsom assemblage (~10,500 14C yr B.P.) occurs within a buried soil.
Paleoindian Environmental Change and Landscape Response in Barger Gulch, Middle Park, Colorado James H. Mayer,1,* Todd A. Surovell,2 Nicole M. Waguespack,2 Marcel Kornfeld,2 Richard G. Reider,3 and George C. Frison2 1

Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Department of Anthropology, University of Wyoming, Laramie, Wyoming 82071 3 Department of Geography and Recreation, University of Wyoming, Laramie, Wyoming 82071 2

Middle Park, a high-altitude basin in the Southern Rocky Mountains of north-central Colorado, contains at least 59 known Paleoindian localities. At Barger Gulch Locality B, an extensive Folsom assemblage (~10,500 14C yr B.P.) occurs within a buried soil. Radiocarbon ages of charcoal and soil organic matter, as well as stratigraphic positions of artifacts, indicate the soil is a composite of a truncated, latest-Pleistocene soil and a younger mollic epipedon formed between ~6000 and 5200 14C yr B.P. and partially welded onto the older soil following erosion and truncation. Radiocarbon ages from an alluvial terrace adjacent to the excavation area indicate that erosion followed by aggradation occurred between ~10,200 and 9700 14C yr B.P., and that the erosion is likely related to truncation of the latest-Pleistocene soil. Erosion along the main axis of Barger Gulch occurring between ~10,000 and 9700 14C yr B.P. was followed by rapid aggradation between ~9700 and 9550 14C yr B.P., which, along with the erosion at Locality B, coincides with the abrupt onset of monsoonal precipitation following cooling in the region ~11,000–10,000 14C yr B.P. during the Younger Dryas oscillation. Buried soils dated between ~9500 and 8000 14C yr B.P. indicate relative landscape stability and soil formation throughout Middle Park. Morphological characteristics displayed by early Holocene soils suggest pedogenesis under parkland vegetation in areas currently characterized by sagebrush steppe. The expansion of forest cover into lower elevations during the early Holocene may have resulted in lower productivity in regards to mammalian fauna, and may partly explain the abundance of early Paleoindian sites (~11,000–10,000 14C yr B.P., 76%) relative to late Paleoindian sites (~10,000–8000 14C yr B.P., 24%) documented in Middle Park. © 2005 Wiley Periodicals, Inc.

INTRODUCTION Middle Park, one of three high-altitude basins in the Southern Rocky Mountains of north-central Colorado, contains an unusually high density of Paleoindian sites, many of which have been subject to archaeological testing (Naze, 1986; Kornfeld et al., 1999; Kornfeld and Frison, 2000; Surovell et al., 2001; Kornfeld, 2002). Nevertheless, with a single exception (Kornfeld et al., 1999), no detailed descriptions of the soil

*Corresponding author; E-mail: [email protected]. Geoarchaeology: An International Journal, Vol. 20, No. 6, 599–625 (2005) © 2005 Wiley Periodicals, Inc. Published online in Wiley Interscience (www.interscience.wiley.com). DOI:10.1002/gea.20070

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Figure 1. Map showing location of Middle Park in north central Colorado. Also shown are locations of other Paleoindian localities discussed in text. UTM  Upper Twin Mountain. Modified from Waguespack et al. (2002).

and stratigraphic settings of Paleoindian sites in Middle Park exist in the published literature. Barger Gulch, a heavily incised tributary system of the Colorado River in the western portion of Middle Park (Figure 1), contains several Folsom (~10,900–10,200 14 C yr B.P.) localities (Naze, 1986; Kornfeld and Frison, 2000; Surovell et al., 2001). Of these sites, Locality B has been the subject of survey and excavation since 1995, yielding an extensive Folsom lithic assemblage from a thin (~0.75 m) layer of unconsolidated sediments overlying bedrock (Surovell et al., 2001). Geoarchaeological investigations in Barger Gulch were conducted to assess landscape evolution during, and subsequent to, the Folsom occupation. Procedures focused on: (1) description of the physiography, (2) development of a late-Quaternary geochronology through radiocarbon dating of charcoal and soil organic matter, and (3) comparison of the geoarchaeological context of Locality B with other Paleoindian sites in Middle Park (Miller, 1998; Reider, 1998; Kornfeld et al., 1999). Moreover, establishing a geomorphic history of the site area allows correlations with proxy records of late Pleistocene and early Holocene environments from the Front Range and other areas of the Southern Rocky Mountains in Colorado (e.g., Benedict, 1973, 1985; Markgraf and Scott, 1981; Elias, 1983, 1985, 1996; Short, 1985; Short and Elias, 600

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1987; Menounos and Reasoner, 1997; Vierling, 1998; Reasoner and Jodry, 2000), thus enabling us to construct a paleoenvironmental scenario for the Paleoindian period (~11,000–8000 14C yr B.P.) of Middle Park. This paper is one of two aimed at describing the geoarchaeological context of Locality B. The focus here is a description and interpretation of the soil- and lithostratigraphic setting at and around Locality B, as well as elsewhere in Barger Gulch. A companion manuscript by Surovell et al. (2005) pursues the issue of natural siteformation processes in the excavation area and the context of the archaeological assemblage. While these papers have slightly different scales of inquiry, they are not mutually exclusive, each having obvious relevance to the other. Both studies address issues critical for understanding the context of the Folsom occupation at Locality B, and in order to discuss the results and implications in adequate detail, we have decided to present them separately. PHYSICAL SETTING Physiography and Geology Middle Park is one of many high-altitude structural basins in the Southern Rocky Mountain physiographic province (Fenneman, 1931), and is bound by the Front, Vasquez, and Williams Fork Ranges to the east and southeast, the Park and Gore Ranges to the west and southwest, and the Rabbit Ears Range to the north (Figure 1). Middle and North Parks are part of one contiguous synclinal basin separated by Oligocene and Miocene volcanics of the Rabbit Ears Range (Izette, 1968). Relief in North and South Parks is relatively subdued. Middle Park, however, displays substantially more relief due to north-trending Laramide overthrust faults (Izette and Barclay, 1973), and elevations range from about 2300 m along the Colorado River to 2560 m at lower treeline. The average floor elevation is about 2350 m, and peaks at the eastern edge of the drainage basin reach over 3960 m. In the Hot Sulphur Springs area in the eastern half of the park, volcanic breccias, arkosic sandstones and conglomerates, and mudstones of the Paleocene Middle Park Formation comprise much of the bedrock (Izette, 1968). In the western part of the park, bedrock is comprised of tuffaceous claystones, siltstones, and conglomerates of the Miocene Troublesome Formation (Izette and Barclay, 1973; Izette and Obradovich, 2001). Precambrian igneous rocks, and Paleozoic and Mesozoic sedimentary rocks crop out throughout the basin, the result of late Mesozoic and early Cenozoic thrust and reverse faulting (Izette, 1968). A notable example of this is Wolford Mountain, where Precambrian granite overlies Cretaceous Pierre Shale. Regional uplift during late Miocene block faulting caused incision and established the course of the Upper Colorado River system (Larson et al., 1975), leading to the formation of broad Miocene and Pliocene erosion surfaces in Middle Park (Izette and Barclay, 1973). A number of late Pliocene and Pleistocene pediments and strath terraces capped with Colorado River gravels formed as a result of downcutting of the Upper Colorado River throughout the late Cenozoic. Strath terraces in the GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL

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Kremmling area range from approximately 10–170 m above the current valley bottom of the Colorado River (Scott, 1965; Izette and Barclay, 1973; Madole, 1991), and probably formed during Pleistocene glaciations in the Southern Rocky Mountains. Climate and Vegetation As with most high-altitude settings, annual climate in Middle Park is highly variable. Based on instrument data from 1948 to 2003, average annual maximum and minimum temperatures in Kremmling (2245 m) are 13.0 and –5.8ºC (Western Regional Climate Center, 2003). Average annual total precipitation is 29.2 cm, about half of which falls between June and September. Climate in the region is classified as a dry midlatitude steppe (Strahler and Strahler, 2000). Most of Middle Park below 2600 m is covered by cold steppe vegetation, dominated by sagebrush (Artemisia), grasses (Poaceae), and serviceberry (Amelanchier), with north-facing slopes containing isolated groves of pine (Pinus) and aspen (Populus) with sagebrush understory (Reider, 1998; Kornfeld et al., 1999). Larger stands of Douglas fir (Pseudostuga menzeiseii), aspen, and subalpine fir (Abies) are found above 2600 m (Scott-Cummings and Moutoux, 1998; Kornfeld et al., 1999). Hydrology The Middle Park basin contains a number of high-order streams that comprise the headwaters for much of the Upper Colorado River (Figure 1). Ephemeral stream systems, or gulches, are common in the western half of Middle Park, where extensive gully systems developed primarily in fine-grained rocks, such as the Pierre Shale (Cretaceous) and Troublesome Formation (Miocene). Locality B, at an elevation of about 2320 m, sits on the north-facing shoulder of an east–west oriented interfluve between tributary gullies of Barger Gulch, a gully system deeply incised into Troublesome Formation (Figure 2). METHODS The gully systems in Middle Park prove useful to geoarchaeological investigations in that many have incised through latest Pleistocene and early Holocene deposits, exposing Paleoindian-age alluvium and buried soils. The excavation area at Locality B and manually excavated trenches provided additional subsurface exposures (Figure 2). Geologic sections were measured, described, and sampled in the field, and descriptions were carried out using terminology following standard geologic and pedologic nomenclature (Compton, 1985; Soil Survey Staff, 1993; Birkeland, 1999). Buried soils are designated in field descriptions by the letter “b” at the end of all other horizon letters and numbers, and Arabic numbers are placed after the b to differentiate buried soils occurring in vertical sequence from the top down (after Holliday [1985] and Birkeland [1999]). When making correlations across the study area, soils are referred to using uppercase Roman numerals, with the stratigraphically oldest buried soil referred to as soil I, and so 602

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Figure 2. Topographic map showing locations of profiles in Barger Gulch area discussed in text. Locality B 02-4 refers to excavation area mentioned in text.

on. We classified buried soils to the suborder level based largely on diagnostic surface and subsurface horizons (Soil Survey Staff, 1999). It should be stated here that we recognize certain assumptions must be made when attempting to classify buried soils. Classification systems, such as Soil Taxonomy (Soil Survey Staff, GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL

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1999), require direct observations of soil properties, many of which cannot be observed for soils forming under past environments (e.g., soil temperature and moisture). While we do not imply that our classifications of buried soils can be used to estimate climatic parameters, such as moisture and temperature, we do feel that the assumptions are reasonable, and that the classification conveys information useful to the reader. Radiocarbon ages were obtained using charcoal, as well as soil organic matter (SOM) from buried soil horizons (Table I). Although radiocarbon dating of organic sediments and buried soils is somewhat problematic (Scharpenseel, 1979; Matthews, 1985; Martin and Johnson, 1995; Abbot and Stafford, 1996; McGeehin et al., 2001), studies have shown that with proper care in sampling and interpretation, these materials can provide good age control, especially in drier environments (e.g., Haas et al., 1986; Holliday et al., 1994; Rawling et al., 2003; Mayer and Mahan, 2004). Ages of charcoal from sediments provide good minimum estimates for age of deposition, while organic matter from buried soil horizons provides minimum ages of stability and soil formation, as well as maximum ages for subsequent deposition of overlying material. Samples underwent a standard acid-base-acid treatment to remove carbonate and humate contaminants, and to isolate specific fractions of organic matter (after Abbot and Stafford, 1996). Charcoal ages typically were derived for samples pretreated for carbonate and humate removal. However, because of a small sample size, the radiocarbon age of one sample (Beta-171941) was obtained on the combined humate and base insoluble fractions. Samples were processed at Beta Analytic (Beta) and at the University of Arizona NSF-AMS facility (AA). Radiocarbon ages were corrected for isotopic fractionation and are presented in uncalibrated radiocarbon years before present (14C yr B.P.). We decided to present radiocarbon ages in uncalibrated 14C yr B.P. primarily because of uncertainties in the calibration curve during the Paleoindian time frame, as well as to allow comparisons with older studies carried out prior to the widespread use of calibration (Holliday, 2000a). Human occupation of Middle Park during the latest Pleistocene and early Holocene is well represented by diagnostic Paleoindian projectile points occurring in both surface and buried contexts (Naze, 1986; Kornfeld, 1998, 2002; Kornfeld et al., 1999; Kornfeld and Frison, 2000; Surovell et al., 2001). The radiocarbon ageranges of Paleoindian components have been reasonably well constrained at sites across the Great Plains and Rocky Mountain basins (Frison, 1991; Haynes, 1992, 1993; Hofman, 1995; Holliday, 2000a), and the occurrence of these diagnostic projectile points throughout Middle Park provides additional age control for interpreting stratigraphic sequences. RESULTS Summary of Barger Gulch Late-Quaternary Deposits The late-Quaternary stratigraphic sequence in Barger Gulch is comprised of sheetwash, eolian, and alluvial deposits derived primarily from reworked Troublesome 604

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13C

Lab Number

Locality B, excavation area Stratum 3, top of soil III Stratum 3, top of soil III Stratum 3, base of soil III Stratum 3, soil III Stratum 3, upper Stratum 3/1 contact Stratum 3/1 contact Stratum 3/1 contact Stratum 3/1 contact Stratum 3/1 contact Stratum 3/1 contact Stratum 3/1 contact Stratum 3/1 contact Stratum 1 Stratum 1 Stratum 1, base Stratum 1, base

AA45657 AA45658 AA45655 AA45656 Beta-155404 Beta-173389 Beta-173386 Beta-173382 Beta-155403 Beta-173379 Beta-109464 Beta-173387 Beta-173381 Beta-173388 Beta-173385 Beta-173383 Beta-173384

Soil Soil Soil Soil Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal

Residuea Humatesb Residue Humates — — — — — — — — — — — — —

5178  49 5437  45 5890  65 6003  64 6880  60 7510  60 7590  40 7850  40 7880  60 9390  40 9420  50 9450  40 10470  40 8790  40 10770  70 7930  40 7980  50

–25.7 –24.6 –24.6 –25.0 –23.9 –23.8 –22.7 –22.3 –23.2 –21.7 –23.8 –23.9 –24.5 –22.9 –24.3 –22.9 –23.3

Beta-173390

Soil

Bulk soilc

8790  40

–24.0

Beta-173706

Charcoal



9640  40

–23.0

Beta-173391

Soil

10170  60

–23.5

Beta-173707

Charcoal

10680  40

–22.2

Locality F, profile 02-2 Substratum 22a, upper Substratum 22a, middle Substratum 22a, lower Substratum 21a, upper

Beta-171942 Beta-110406 Beta-171943 Beta-171941

Charcoal Charcoal Charcoal Charcoal

8130  40 8510  50 9460  40 9550  40

–22.7 –24.2 –25.1 —

Profile 03-1 Substratum 21a, lower Substratum 1, upper

AA56660 AA56661

Charcoal Charcoal

9714  60 10026  55

–24.9 –25.1

Locality B, profile 02-3 Substratum 22s: ABtkb2, 90–97 cm, soil II Substratum 22s: ABtkb2, 90–97 cm, soil II Substratum 21c: Ck, 173–188 cm Substratum 21c: Ck, 173–188 cm

Fraction

Age (1) 14 C yr B.P.

Location

Bulk soil —

— — — Acid insoluble

— —

a

Base insoluble fraction. Base soluble fraction, reprecipitated via a second acid treatment. c Acid insoluble fraction (e.g., combined humates and residue fractions). b

Formation sediments. At Locality B, a total of five stratigraphic units (strata) are numbered 1–5, from oldest to youngest, and, in some cases, are divided into substrata. Our stratigraphic nomenclature is similar to that used at the Lubbock Lake site (Holliday, 1985), with vertical subdivisions indicated by numerical subscript GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL

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modifiers and lateral subdivisions (i.e., facies) indicated by lowercase abbreviations differentiating modes of deposition (e.g., a  alluvial, s  sheetwash, c  colluvial) (Table II, Figure 3). The same nomenclature is applied to a stratigraphic section in the main axis of Barger Gulch, referred to as Locality F (Figure 4). All stratigraphic units in the excavation area at Locality B are relatively fine-grained sheetwash and eolian deposits, with silt and very fine sand dominating the particle-size distribution. The sequence varies across the site area due to discontinuous stratigraphic units. In addition, lithological variation is subtle because most units are probably derived primarily from reworking of Troublesome Formation sediments and more recent upland eolian deposits. Nevertheless, based primarily on soil stratigraphy, it was possible to correlate the stratigraphy in the excavation block (profile 02-4) with a section of valley fill in a gully to the northwest (profile 02-3), providing a relatively complete stratigraphic framework spanning the last ~11,000 14C yr B.P. (Figure 3). In the main axis of Barger Gulch, at least three continuous alluvial terraces occur (Figure 4, Table III), all comprised of late-Quaternary alluvium. At Locality F (Figure 2), three stratigraphic units were described from the highest of the three terraces and correlated with the stratigraphy of Locality B (Table III), based primarily on radiocarbon ages. Locality B Stratigraphy The following section summarizes the stratigraphy of specific locations in the Locality B area. However, only the strata pertinent to understanding the Folsom occupation and latest Pleistocene/early Holocene paleoenvironments are discussed in detail. Generalized descriptions of all stratigraphic units in Barger Gulch are presented in Tables II and III. In the excavation area, approximately 0.75 m of unconsolidated late-Quaternary sediments overlie weathered Miocene Troublesome Formation. However, in a tributary gully to the north and northwest, valley fill is over 2 m thick (Figures 2 and 3). Excavation Area Profile 02-4 Throughout most of the Locality B area, stratum 1 contains soil I, typically represented by a truncated Btk horizon (Figure 3, Tables II and IV). However, in other areas of Barger Gulch, soil I contains a relatively thick (~25–30 cm) and dark (~10YR 4/2 to 3/2) A horizon, and probably represents a Cryoll, a cool or cold freely drained Mollisol (Soil Survey Staff, 1999). Stratum 2 is not represented in the excavation block, probably lost due to erosion (see below), and thus, stratum 3 directly overlies stratum 1. A buried soil showing Ab/ABtb/Btkb horizonation is formed in both units (Table IV) in the excavation area. The ABtb/Btkb horizons in stratum 1, as well as in residuum of the Troublesome Formation, represent soil I. The Ab horizon, however, referred to as soil III, is a mollic epipedon with irregular upper and lower boundaries formed in eolian silt of stratum 3. The reasons for considering the horizon sequence as two separate soils are discussed below. Although many of the Folsom artifacts at Locality B occur at the Ab/ABtb boundary (i.e., the soil III/soil I contact), a significant portion of the assemblage occurs within both strata 1 and 3, probably due to mixing by pedoturbation (Surovell et al., 2005). 606

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Modern: Cryept

III: expression ranges from mollic epipedon to Cryalf

II: Cryalf

I: Cryoll (?)

5

4

3

2

1

Late Pleistocene ( 10,500 yr B.P.)

Latest PleistoceneEarly Holocene ( 10,200–10,000 yr B.P.)

Substratumb 21c: 0–20 cm thick, dark brown to very dark brown (7.5YR 3/2 to 2/2), moderately to moderately well sorted, silt loam, contains fragments of brown (7.5YR 5/4) silt loam, abundant charcoal fragments, few (5–10%) angular to subrounded caliche fragments ranging in size from small pebble to large cobble, abrupt contact 10–40 cm thick, brown to dark brown (7.5YR 5/4 to 3/4), moderately well to well sorted, silt loam

Early Holocene (~10,000–9600 yr B.P.)

Substratumb 22s: 0–80 cm thick, brown to very dark grayish brown (7.5YR 5/2 to 2.5Y 3/2), massive, well sorted, silt loam, gradational contact

Mixed loess and sheetwash

Late Holocenea (~5000–2500 yr B.P.)

Reworked loess

Mixed colluvium and sheetwash

Sheetwash

Sheetwash

Sheetwash

Latest Holocenea ( 1000 yr B.P.)

Middle Holocene (~7500–6900 yr B.P.)

Interpretation

Age

20–75 cm thick, brown to very dark grayish-brown (7.5YR 5/2 to 10YR 3/2), massive, moderately well to well sorted, silt loam, gradational contact

0–25 cm thick, dark brown to brown (7.5YR 3/2 to 4/2), massive to weakly laminated, moderately well sorted, loam to silt loam, trace ( 5%) angular to subrounded fine caliche gravel, abrupt to gradational contact

0–15 cm thick, dark brown to brown (7.5YR 3/2 to 10YR 4/3), loose single-grained to laminated, poorly to moderately sorted, gravelly loam to silt loam, few (~5–10%) angular to subrounded fine caliche gravel, gradational contact

Stratigraphic Description

b

Age assignment based on correlation with stratigraphy of Miller (1998). Vertical subdivisions are indicated by numerical subscript modifiers, and lateral subdivisions (i.e., facies) are indicated by lowercase abbreviations to differentiate modes of deposition (a  alluvial, s  slopewash, c  colluvial).

a

Soil

Stratum

Table II. Generalized descriptions and interpretations of stratigraphic units in the Barger Gulch Locality B area.

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Figure 3. Generalized geologic cross section at Barger Gulch Locality B showing soil- and litho-stratigraphic relationships of late-Quaternary deposits. Soils indicated by vertical lines; stratigraphic units are numbered. Also shown are relative positions of radiocarbon ages. Ages in bold are based on soil organic matter. All other ages are based on charcoal. See Figure 3 for location of profiles and Table II for stratigraphic descriptions. Laboratory numbers of radiocarbon ages are presented in Table I.

Figure 4. Generalized geologic cross section at Barger Gulch Locality F showing soil- and litho-stratigraphic relationships of late-Quaternary alluvial terraces. Soils indicated by vertical lines; stratigraphic units are numbered. Also shown are relative positions of radiocarbon ages based on charcoal recovered from the section. Note: position of oldest two dates based on correlation with profile 03-1. See Figure 3 for locations of profiles and Table VII for descriptions. Laboratory numbers for radiocarbon ages are presented in Table I.

Radiocarbon ages from charcoal associated with the Folsom assemblage range from ~10,770 to ~7510 14C yr B.P. (Table I, Figure 3). While the two oldest charcoal ages of ~10,770 and ~10,470 14C yr B.P. are consistent with the radiocarbon age range of Folsom occupation in the central U.S. (Frison, 1991; Haynes, 1992, 1993; Hofman, 608

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2.0–3.0 m

5.5–6.5 m

Qt2

Qt3

b

50–150 cm thick, dark brown to brown (7.5YR 4/2 to 5/4), massive to weakly laminated, moderately to moderately well sorted, loam to silt loam, few (5–10%) subangular to subrounded pebble- and gravel-sized caliche fragments, series of A/Bk soils formed in sheetwash alluvium(?), gradational contact 100–150 cm thick, brown to very dark grayish-brown (7.5YR 5/2 to 2.5Y 3/2), well sorted, silt loam, series of A/B soils formed in thin (25–35 cm) overbank sediments, charcoal flecks common, gradational contact 200–250 cm thick, light brown to pink (7.5YR 6/4 to 7/4), moderately to moderately well sorted, thinly to thickly bedded calcareous gravel and sand, capped by ~15 cm of laminated silts, gravels are imbricated and subangular to subrounded, gully fill 50 cm thick, light brown to strong brown (7.5YR 6/4 to 5/6), moderately well sorted massive silt loam, contains at least 1 soil, total thickness unknown, loess-derived valley fill?

22a

21a

1

Light brown to pink (7.5YR 6/4 to 7/4), moderately to moderately well sorted, thinly to thickly bedded calcareous gravel and sand, gravels are imbricated and subrounded to rounded, locally a strath terrace cut on 21a deposits, gully fill

Reddish-yellow to light reddish-brown (7.5YR 6/6 to 5YR 6/4), massive to laminated, fine calcareous sandy loam with lenses of gravelly sand and sandy gravel, no soils, gully fill

Description and Interpretation

3

nd

nd

b

Stratuma

Tentative stratigraphic correlation with Locality B. nd  no stratigraphic designation.

1.25–1.75 m

Qt1

a

Height above Floodplain

Terrace

Table III. Generalized descriptions of alluvial terraces and stratigraphy in main axis of Barger Gulch.

Latest Pleistocene ( 10,000 yr B.P.)

Early Holocene (10,000–9500 yr B.P.)

Early Holocene (9500–8000 yr B.P.)

Holocene ( 8000 yr B.P.)

Holocene ( 8000 yr B.P.)

Late Holocene (?) ( 2000 yr B.P.?)

Age

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609

610

A Bw Ab1 ABtb2 Btkb2 2Btkb2

Horizon

0–5 5–23 23–42 42–50 50–57 57–63

Depth (cm) 7.5YR 3/2 7.5YR 3/4 10YR 3/2 7.5YR 4/2 7.5YR 4/4 7.5YR 4/4

Color (Moist) SiL SiL SiL SiL SiL SiL

Texture

a

2 f gr 2 f sb 2 f sb 2 f sb 3 f ab 3 f ab

Structure

b

sh/fr sh/fr sh/fr sh/fr h/fr h/fr

— — — — f f th f f th

Consistencec CaCO3 (Dry/moist) Concentrationsd n n n n e e

Reactione (10% HCl)

— f f p pf — f f d pf c d c pf c d c pf

Clay Filmsf

c, w c, w c, w c, w c, w —

Boundaryg

b

a

SiL  silt loam. Grade: 0  structureless, 1  weak, 2  moderate, 3  strong; class: f  fine, m  medium, c  coarse; type: sb  subangular blocky, ab  angular blocky, gr  granular, pl  platy, sg  single grained, ma  massive; → parting to. c Dry: so  soft, sh  slightly hard, h  hard; moist: fr  friable, fi  firm. d Amount: f  few, c  common; size: f  fine, m  medium; shape: th  threads, pa  patches. e ev  violent effervescence, es  strong effervescence, e  weak effervescence, n  no effervescence. f Amount: f  few, c  common; distinctness: f  faint, d  distinct; continuity: p  patchy, d  discontinuous, c  continuous; location: pf  ped faces. g Distinctness: a  abrupt, c  clear, g  gradual; topography: s  smooth, w  wavy. h Tr Fm  Troublesome Formation.

Tr Fmh

3 (III) 1 (I)

4

Stratum (Soil)

Table IV. Soil description from Barger Gulch Locality B Profile 02-4.

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1995; Holliday, 2000a), the younger ages, in the absence of any post-Folsom occupation (Surovell et al., 2001, 2005), are probably derived from natural burns. Charcoal in upper stratum 3 yielded an age of ~6880 14C yr B.P. Humate and residue fractions from the base of soil III produced ages of ~6000 and ~5890 14C yr B.P., respectively, while humates and residue from the upper portion produced ages of ~5440 and ~5180 14 C yr B.P., respectively. Both sets of paired ages from SOM are in relatively good agreement with each other. The radiocarbon ages clearly indicate a complex history of the current soil- and litho-stratigraphic setting of the excavation area. The broad range of charcoal ages, the relatively young age of soil III, and the stratigraphic position of the artifacts suggest that the soil formation during Folsom occupation was truncated, and the younger mollic epipedon is, in part, welded onto soil I (Ruhe and Olson, 1980). In other words, following truncation of soil I, the deposition of stratum-3 eolian silt began sometime between ~9450 and 7510 14C yr B.P. and lasted until ~6880 14C yr B.P. Formation of soil III was underway by at least 6000 14C yr B.P., continuing until at least 5200 14C yr B.P. Results of particle-size analysis and vertical artifact profiles from excavation units also tend to support this scenario of site-formation history (Surovell et al., 2005). The exact timing of erosion and truncation of soil I is uncertain. However, the radiocarbon ages from the excavation block indicate it probably occurred shortly after Folsom occupation, but prior to ~7880 14C yr B.P. and perhaps before ~9640 14 C yr B.P. (see below). Also uncertain is why stratum 2 is absent in the excavation block, but present in valley fill to the northwest (Figure 3), where it is capped by the relatively well-developed soil II. However, numerous charcoal lumps associated with the Folsom assemblage did produce early Holocene dates ranging from~9450–7510 14C yr B.P. (Figure 3 and Surovell et al., 2005). The absence of stratum 2 in the excavation area suggests that subsequent to erosion of soil I, stratum 1 and the artifact assemblage in the excavation area remained near the surface due to continued erosion during the early Holocene until deposition of stratum 3 shortly after ~8000 14C yr B.P. Gully Profile 02-3 Between profiles 02-4 and 02-3, located in a gully approximately 50 m northwest of the excavation block (Figure 2), late-Quaternary fill thickens to over 2.0 m, and 15–20 cm of substratum 21c overlies truncated soil I (Figure 3, Table V). A large (~15  15 cm) subangular cobble of caliche occurs in the upper portion of substratum 21c, with smaller fragments scattered throughout. The abrupt 1–21c contact, as well as the mixed and poorly sorted nature of substratum 21c, indicates that it is probably derived from a combination of sheetwash and colluvial processes. Augering in and around profile 02-3 indicated that substratum 21c is limited in extent to the axial portion of the tributary gully, where its distribution appears to be discontinuous either due to a patchy initial distribution or post-depositional erosion. As discussed above, the timing of truncation of soil I in the excavation block is uncertain, but is considered to have occurred shortly after Folsom occupation, dated to ~10,770–10,470 14C yr B.P. At Locality B, substratum 21c is interpreted to represent the reworked portion of the soil-I A horizon. Charcoal from substratum 21c produced GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL

611

612

C ABtkb1 Btk1b1 Btk2b1 ABtkb2 Btk1b2 Bwkb2 2Ck2 3Bwkb3 4C

Horizon

0–15 15–39 39–70 70–90 90–97 97–133 133–173 173–188 188–225 225

Depth (cm)

7.5YR 3/2 10YR 3/2 7.5YR 3/4 7.5YR 3/4 7.5YR 3/2 7.5YR 4/4 7.5YR 4/4 7.5YR 3/2 7.5YR 5/4 not described

Color (Moist) SiL SiL SiL SiL SiL SiL SiL SiL SiL

Texturea 0 sg 2 c sb → 3 m ab 3 f ab 3 m ab 2 f sb 2 m ab → 2 f ab 2 m sb 1 m sb 2 m sb

Structureb — sh/fr h/fi h/fi sh/fr sh/fr sh/fr so/fr sh/fr

— f f th & p f f th & p f f th & p f f th & p f f th & p f f th & p f f th f f th

Consistencec CaCO3 (Dry/moist) Concentrationsd n es es es e e e e e

Reactione (10% HCl)

— f f d pf c d d pf c d d pf f f d pf c d c pf — — —

Clay Filmsf

c, s c, w c, w c, s c, w c, w c, s a, w —

Boundaryg

b

a

SiL  silt loam. Grade: 0  structureless, 1  weak, 2  moderate, 3  strong; class: f  fine, m  medium, c  coarse; type: sb  subangular blocky, ab  angular blocky, gr  granular, pl  platy, sg  single grained, ma  massive; → parting to. c Dry: so  soft, sh  slightly hard, h  hard; moist: fr  friable, fi  firm. d Amount: f  few, c  common; size  f  fine, m  medium; shape: th  threads, pa  patches. e ev  violent effervescence, es  strong effervescence, e  weak effervescence, n  no effervescence. f Amount: f  few, c  common; distinctness: f  faint, d  distinct; continuity: p  patchy, d  discontinuous, c  continuous; location: pf  ped faces. g Distinctness: a  abrupt, c  clear, g  gradual; topography: s  smooth, w  wavy. h Tr Fm  Troublesome Formation.

21c 1 (I) Tr Fmh

22s (II)

5 3 (III)

Stratum (Soil)

Table V. Soil description from Barger Gulch Locality B Profile 02-3.

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a radiocarbon age of ~10,680 14C yr B.P., and may have been redeposited from the occupation area. A SOM age of ~10,170 14C yr B.P. from the upper part of 21c is difficult to interpret, but is considered to be derived from soil I rather than soil formation following deposition. Charcoal from the upper part of substratum 22s produced a radiocarbon age of ~9640 14C yr B.P., suggestive of fairly rapid accumulation of valley fill following deposition of 21c. Radiocarbon ages from stratum 2 at profile 02-3 are considered to indicate that erosion of the Folsom landscape and subsequent deposition occurred between ~10,170 and 9640 14C yr B.P. Soil II, a relatively well-developed soil formed throughout substratum 22s (Table III), is classified as a Cryalf, a freely drained Alfisol of cold regions (Soil Survey Staff, 1999). SOM from a thin (~7 cm) ABt horizon indicates that deposition of substratum 22s slowed enough for pedogenesis to dominate by at least ~8790 14C yr B.P. (Figure 3). This soil is considered a correlate of a Cryalf formed in sheetwash at site 5GA3012 in Horse Gulch (Figure 1, Table VI), which yielded a Cody point (~9500–8500 14C yr B.P.). Thus, while uplands in the site area appear to have been eroding during the early Holocene, the tributary gully was undergoing sediment accumulation perhaps coeval with soil formation during much of the same interval. While stratum 3 remains undated in the tributary, it may indicate that after a brief episode of slowed aggradation and formation of the soil-II ABt horizon, sediment accumulation continued into the middle Holocene, with soil III forming in the gully either concomitant with aggradation, or during a subsequent episode of landscape stability in the Locality B area. Barger Gulch Locality F Stratigraphy The main branch of Barger Gulch contains at least three alluvial terraces (Table III), the highest of which was aggrading prior to 10,000 14C yr B.P., based on a radiocarbon age of charcoal from Qt3 alluvium (Figure 4). At present, all that can be said about Qt1 and Qt2 is that they date to  8000 14C yr B.P., based on upper ages of Qt3 (see below). Miller (1998) described terraces in Middle Park using terminology developed by Leopold and Miller (1954) for alluvial deposits in eastern Wyoming. Considering the difficulties in correlating terraces from drainage basins within the same region (e.g., Mandel, 1995), and even within the same drainage (e.g., Albanese and Wilson, 1974), Miller’s (1998) approach is tenuous and not followed here. A 5.5 m section of Qt3 alluvium is exposed in profile 02-2 at Locality F, a perennial spring-fed reach of Barger Gulch approximately 0.75 km upstream from Locality B (Figures 2 and 4, Table VII). The oldest late-Quaternary alluvium exposed at Locality F is stratum 1, a massive silt containing at least one buried soil. Stratum 1 is overlain by approximately 3.5 m of bedded alluvial silts, sands, and imbricated gravel of substratum 21a, which is, in turn, overlain by ~1.5 m of laminated to massive silts (substratum 22a). Approximately 200 m upstream from Locality F, at profile 03-1 (Figure 2), stratum 1 is partially eroded, and charcoal from a truncated soil at the stratum 1/substratum 21a contact produced an age of ~10,030 14C yr B.P. Also at profile 03-1, a layer of charcoal in a massive silt lens of 21a yielded an age of ~9710 14C yr B.P. Thus, an episode of erosion in the main axis of Barger Gulch is constrained between ~10,030 and 9710 14C yr B.P., probably corresponding with the erosion recognized at Locality GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL

613

614

0–15 15–22 25–40 40–53 53–66 66–84 84–93 93–125

A1 A2 C Ab1 ABtb1 Bt1b1 Bt2b1 2Btb2

10YR 4/3 10YR 5/2 10YR 6/2 10YR 5/2 10YR 5/3 10YR 6/3 10YR 5/3 7.5YR 5/4

Color (Moist) L SiL SiL SiL SiL SiCL SiL SiCL

Texturea laminated laminated 0 ma 2 f sbk 1 vf pr→3 f ab 1 f pr→3 f ab 2 m sb 1 f sb

Structureb — — — sh/fr h/fi h/fi sh/fr sh/fr

— — — — — — — cmt&p

n n n n n n n es

— — — — f d d pf c d c pf f d d pf f d d pf

c, w c, w c, w c, w c, w c, w c, w —

Consistencec CaCO3 Reactione (Dry/moist) Concentrationsd (10% HCl) Clay Filmsf Boundaryg

Gravel layer, 93–100 cm; weathered Troublesome

Cody point from 66 cm

0–40 cm: ~5%  2 mm Subangular clasts

Comments

b

a

SiL  silt loam; SiCL  silty, clay loam; L  loam. Grade: 0  structureless, 1  weak, 2  moderate, 3  strong; class: f  fine, m  medium, c  coarse; type: sb  subangular blocky, ab  angular blocky, gr  granular, pl  platy, sg  single grained, ma  massive; → parting to. c Dry: so  soft, sh  slightly hard, h  hard; moist: fr  friable, fi  firm. d Amount: f  few, c  common; size  f  fine, m  medium; shape: th  threads, pa  patches. e ev  violent effervescence, es  strong effervescence, e  weak effervescence, n  no effervescence. f Amount: f  few, c  common; distinctness: f  faint, d  distinct; continuity: p  patchy, d  discontinuous, c  continuous; location: pf  ped faces. g Distinctness: a  abrupt, c  clear, g  gradual; topography: s  smooth, w  wavy.

Depth (cm)

Horizon

Table VI. Soil description from 5GA3012, Horse Gulch Profile 03-1.

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A Bk Akb1 Bkb1 Akb2 Bkb2 Akb3 Bkb3 A1b4 A2b4 ABwb4 Ab5 ABwb5 Ab6 Bk1b6 Bk2b6 2C1 2C2 2C3

Horizon

0–35 35–52 52–70 70–79 79–90 90–95 95–120 120–137 137–145 145–165 165–180 180–190 190–203 203–213 213–249 249–284 284–400 400–450 450–475 475

Depth (cm) 7.5YR 4/3 7.5YR 4/2 10YR 4/2 10YR 4/3 7.5YR 4/2 7.5YR 5/2 10YR 5/4 7.5YR 4/4 10YR 3/2 2.5Y 3/2 2.5Y 3/2 10YR 3/2 7.5YR 4/2 10YR 4/2 7.5YR 5/4 7.5YR 5/2 7.5YR 7/4 7.5YR 7/4 7.5YR 7/4 covered

Color (Moist) Structureb

Consistencec CaCO3 (Dry/moist) Concentrationsd

Reactione (10% HCl) Clay Filmsf

Boundaryg

L 0 ma h/fr — e — c, s L 0 ma sh/fr ffd&t es — a, s SiL 0 ma → 2 f sb sh/fr fft&p es — c, w SiL laminated sh/fr fft&p es — c, w SiL 0 ma → 2 f sb sh/fr fft&p es — c, w SiL laminated sh/fr fft&p s — c, w SiL 0 ma → 2 f sb sh/fr fft&p es — c, w SiL laminated sh/fr fft&p es — g, s SiL 2 f sb sh/fr — e — c, w SiL 2 f sb → 2 f gr sh/fr — n — c, s SiCL 1 f pr → 2 f sb sh/fr — n f f p pf c, w SiL 2 f sb sh/fr — n — c, w SiCL 2 f pr → 3 f ab sh/fr — n f f p pf c, s SiL 1 f pr → 3 f ab sh/fr — n — c, s SiCL 2 m sb — fft&p e f f p pf c, w SiCL 2 m ab sh/fr fft&p es f f p pf g, s Thinly to thickly bedded calcareous sand and imbricated gravel capped by 15 cm of laminated silt Thickly bedded imbricated subrounded to rounded calcareous gravel Gravelly loam

Texturea

b

a

SiL  silt loam; SiCL  silty, clay loam; L  loam. Grade: 0  structureless, 1  weak, 2  moderate, 3  strong; class: f  fine, m  medium, c  coarse; type: sb  subangular blocky, ab  angular blocky, gr  granular, pl  platy, sg  single grained, ma  massive; → parting to. c Dry: so  soft, sh  slightly hard, h  hard; moist: fr  friable, fi  firm. d Amount: f  few, c  common; size  f  fine, m  medium; shape: th  threads, pa  patches. e ev  violent effervescence, es  strong effervescence, e  weak effervescence, n  no effervescence. f Amount: f  few, c  common; distinctness: f  faint, d  distinct; continuity: p  patchy, d  discontinuous, c  continuous; location: pf  ped faces. g Distinctness: a  abrupt, c  clear, g  gradual; topography: s  smooth, w  wavy.

21a

22a

3

Stratum (Soil)

Table VII. Soil description from Barger Gulch Locality F Profile 02-2.

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B between ~10,170 and 9640 14C yr B.P. Radiocarbon-dated charcoal from the upper portion of substratum 21a at Locality F suggests that aggradation in Barger Gulch continued until at least ~9550 14C yr B.P. (Figure 4). Substratum 22a is ~1.5 m of silt containing a series of buried soils with A/Bw and A/Bk horizonation (Table VII). Charcoal samples from the lower and upper portions of 22a yielded ages of ~9460 and ~8130 14C yr B.P., respectively (Figure 4). Radiocarbon-dated charcoal associated with chipped stone in the middle of substratum 22a at Locality F yielded an age of ~8510 14C yr B.P. (Kornfeld, 1998; Figure 4). Substratum 22a is considered to represent relative stability of the valley-bottom surface, with minor episodes of overbank sedimentation separated by soil formation in Barger Gulch between ~9460 and 8130 14C yr B.P., probably followed shortly after by channel entrenchment and floodplain abandonment. DISCUSSION Few studies in western Middle Park have recognized late-Pleistocene deposits, and thus a paleoenvironmental picture prior to 10,000 14C yr B.P. is somewhat vague. Pollen and beetle assemblages from the Mary Jane site (Short and Elias, 1987), located along the southeastern margin of Middle Park at 2882 m in the upper Frasier River drainage, indicate open tundra under a cold, dry climate between 13,740 ± 160 14 C yr B.P. (DIC-671) and 12,380 ± 180 14C yr B.P. (DIC-516). Records from elsewhere in the Front Range (Short, 1985; Elias, 1996, 1997; Reasoner and Jodry, 2000) indicate rapid warming by 12,000 14C yr B.P. Stratum 1 at Locality B represents eolian sedimentation occurring during the latest Pleistocene. While this may be interpreted to indicate relatively dry conditions in lower Middle Park during the latest Pleistocene, it may also reflect greater availability of alluvial silts and sands for reworking by eolian processes, probably derived from sediments along the Colorado River and higher-order tributaries draining glaciated peaks. Nevertheless, much of stratum 1 is probably more accurately considered reworked by sheetwash rather than primary airfall loess. The association of Folsom artifacts at Locality B with soil I indicates relative landscape stability by at least ~10,500 14C yr B.P. (Figure 3). Similarly, stratum 1 along the main axis of Barger Gulch probably represents valley fill derived from reworking of upland eolian silt and fine sand during the latest Pleistocene. An abrupt climatic reversal to cooler conditions between 11,000 and 10,000 14C yr B.P. in the Front Range related to the Younger Dryas oscillation is represented in glacial (Benedict, 1973, 1985), pollen (Reasoner and Jodry, 2000), and lacustrine (Menounos and Reasoner, 1997) records. No fossil insect assemblages between ~11,000 and 10,000 14C yr B.P. are reported from the Southern Rocky Mountains, although assemblages from elsewhere in the Rocky Mountains do not show evidence of cooling during the Younger Dryas, but rather suggest winter and summer temperatures greater than present by ~10,000 14C yr B.P. (Elias, 1996, 1997). The Upper Twin Mountain (UTM) site is located approximately 20 km north of Barger Gulch (Figure 1), and occurs 230 m in elevation higher than Locality B. UTM contains a Goshen bison kill dating between ~10,470 and ~10,240 14C yr B.P. in a rotational slump on a steep-sided, east-facing hillslope (Kornfeld et al., 1999; Figure 5). The 616

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Figure 5. Generalized geologic cross section from the Upper Twin Mountain site, showing stratigraphic designations of Kornfeld et al. (1999). Stratum I, indicated by shading, contained a Goshen bison bone bed situated within a rotational slump. Relative positions of radiocarbon ages based on bone (b) from the bison kill and charcoal (c) from overlying stratum II are also shown. Horizontal lines within stratigraphic units indicate soil formation. No scale implied. Modified from Kornfeld et al. (1999).

kill is thought to have occurred shortly after formation of the slump (Kornfeld et al., 1999:658), and was buried soon thereafter by sheetwash alluvium. While the exact timing of the mass wasting at UTM is uncertain, it may be related to the abrupt onset of the Younger Dryas at 11,000 14C yr B.P., which resulted in slumping on a landscape out-of-phase with climate. The pollen assemblage from sediments encasing the bone bed is dominated by Pinus, with lesser amounts of Abies and Picea, suggesting conifer cover at or near the site (Kornfeld et al., 1999). However, Artemisia and Poaceae are also present, and together with the arboreal pollen, probably indicate a steppe understory separated by conifer stands at UTM by ~10,200 14C yr B.P., or shortly after (Kornfeld et al., 1999). A relatively complex sequence of events occurred during the latest Pleistocene and early Holocene in Barger Gulch. Between ~10,170 and 9640 14C yr B.P., erosion probably due to sheetwash in the excavation area at Locality B reworked portions of the landscape into the tributary gully to the northwest (Figure 3), while erosion along the main axis of Barger Gulch, occurring between ~10,030 and 9710 14C yr B.P., was followed by relatively rapid aggradation until ~9550 14C yr B.P. (Figure 4). We consider the overall landscape instability occurring at approximately 10,000 14C yr B.P. to reflect the onset of a change in precipitation regime, possibly corresponding with an intensified monsoon in the Southern Rocky Mountains during the early Holocene (e.g., Markgraf and Scott, 1981; Friedman et al., 1988; Carrara et al., 1991; Fall, 1997). The rapid onset of intense summer convective storms related to an early Holocene monsoon may have resulted in the erosion and transport to valley bottoms of sediments stored on hillslopes during the late Pleistocene (e.g., Bull, 1991). Erosion in upland settings and aggradation in tributary gullies, as well as along the main axis of Barger Gulch beginning at ~10,000 14C yr B.P., suggests a biogeomorphic response GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL

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similar to Knox’s (1972) model for alluvial systems in subhumid and humid environments, with an abrupt change in precipitation regime accompanied by erosion and increased sediment yields until vegetation response stabilizes hillslopes. Erosion occurring between ~10,000 and 9800 14C yr B.P. is also recognized at the Lindenmeier Folsom site on the High Plains of north-central Colorado (Haynes et al., 1992), as well as at the Folsom-type site in north-eastern New Mexico (Meltzer et al., 2002). Whether or not Middle Park was characterized by a “Folsom drought” similar to that invoked by Holliday (2000b) to explain drying on the Southern High Plains between 11,000 and 10,000 14C yr B.P. is currently unclear. However, the abrupt environmental change(s) at ~10,000 14C yr B.P. caused considerable erosion in Barger Gulch. Virtually all paleoclimatic records from the Southern Rocky Mountains in Colorado indicate summer temperatures and precipitation levels greater than at present between ~10,000 and 8000 14C yr B.P. (e.g., Andrews et al., 1975; Petersen and Mehringer, 1976; Markgraf and Scott, 1981; Elias, 1983, 1985, 1996; Carrara et al., 1984, 1991; Fall, 1985, 1997; Short, 1985; Figure 6), resulting in expansion of both upper and lower treelines during the early Holocene. Fossil beetle assemblages from north-central Colorado indicate that, while mean July temperatures may have been as much as 6.7°C greater than at present, mean January temperatures were probably well below present levels between 10,000 and 8000 14C yr B.P. (Elias, 1996; Figure 6). Landscape stability and soil formation appear to have dominated Barger Gulch and other areas of western Middle Park during much of the early Holocene (Figure 6). Soil II, formed in substratum 22s at Locality B, suggests that shortly after ~9640 14C yr B.P. deposition slowed sufficiently for pedogenesis to keep pace, continuing until at least ~8790 14C yr B.P. However, along the main axis of Barger Gulch, deposition of fine-grained alluvium separated by periods of soil formation between ~9460 and 8130 14C yr B.P. suggests relative stability compared to the rapid aggradation of over 3 m of alluvial silts, sands, and gravels between ~10,030 and 9550 14C yr B.P. At the UTM site, a buried soil displaying A/Bt horizonation formed sometime after Goshen occupation (Reider, 1998), but prior to ~6015 14C yr B.P. (Figure 5). As discussed above, a Cody point (9500–8500 14C yr B.P.) was associated with a Cryalf at 5GA3012 in Horse Gulch. At the Jerry Craig site ~12 km northwest of Locality B, a soil showing O/E/Bt horizonation encasing a Cody bison bone bed dated to ~9310 14C yr B.P. was probably forming until at least ~8490 14C yr B.P. (Figures 1 and 6; Reider, 1998). In summary, when recovered from a buried context, late-Paleoindian materials in Middle Park are typically associated with relatively well-developed soils, indicating landscape stability throughout much of the early Holocene, which is consistent with regional pedological data (Reider, 1990). Most soils formed during the early Holocene in Middle Park are probably best classified as Cryalfs, many of which display morphological features indicative of genesis under deciduous and coniferous vegetation, namely O and E horizons, as well as ochric epipedons associated with Bt horizons (Buol et al., 1997). Consistent with the morphology of early Holocene soils, increased frequencies of pine, spruce, and fir pollen in early Holocene deposits also signal the expansion of lower treeline into lower elevations of the park between ~10,000 and 8000 14C yr B.P. (Kornfeld et al., 1999). Furthermore, early Holocene charcoal samples from Locality B— all of which are probably noncultural—are identified as Pinus, predominantly Pinus 618

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Figure 6. Comparison of latest Pleistocene and early Holocene geomorphic records from Paleoindian sites in Middle Park with select proxy environmental records from north central Colorado. Vertical lines in Middle Park records represent soil formation, while black areas indicate episodes of erosion. UTM  Upper Twin Mountain.

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Figure 7. Frequencies of diagnostic Paleoindian components in Middle Park and their approximate temporal spans. The diagram is not intended to imply that there is no temporal overlap between components, only to demonstrate the abundance of Folsom and Goshen components relative to later Paleoindian components. Data from Kornfeld (1998, 2002) and Kornfeld and Frison (2000).

ponderosa (ponderosa pine) (Davis, 2003). The soil and pollen data from throughout Middle Park, as well as the identified charcoal at Locality B, at the very least indicate more widespread forest cover between ~10,000 and 8000 14C yr B.P. in areas currently supporting sagebrush steppe. Folsom and Goshen components comprise 45 (76%) of the 59 Paleoindian localities in Middle Park (Figure 7; Naze, 1986; Kornfeld and Frison, 2000; Surovell et al., 2001; Kornfeld, 2002). While this apparent abundance of early Paleoindian assemblages (~11,000–10,000 14C yr B.P.) relative to late Paleoindian assemblages (~10,000–8000 14C yr B.P.) may result from sampling or collector bias, it seems significant given that the temporal span of the latter is twice as long as the former in 14 C yr (~1.3 times cal yr), and may indicate something “real” in terms of Paleoindian use of Middle Park, as we discuss below. Data regarding seasonal use of Middle Park by Paleoindians are currently sparse. Nevertheless, preliminary observations and hypotheses can (and should) be made. Tooth-eruption data from Upper Twin Mountain indicate the Goshen bison kill (~10,400 14 C yr B.P.) occurred in late fall or early winter (Kornfeld et al., 1999), while the Jerry Craig Cody bison kill (~9310 14C yr B.P.) apparently occurred during the summer (Hill and Kornfeld, 1999). Stone tool assemblages from Upper Twin Mountain and Locality B are dominated by locally available Kremmling chert (Kornfeld et al., 1999; Surovell 620

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et al., 2001). We feel the data above, along with the higher frequencies of early Paleoindian sites relative to later Paleoindian sites, suggest more intensive use of Middle Park during the Younger Dryas, possibly because it was more attractive on an annual basis. During the early Holocene, however, winter temperatures were apparently below present values (Elias, 1996), perhaps rendering Middle Park inhospitable to late Paleoindians during the winter months. In addition, the soils and pollen data discussed above (see also Kornfeld et al., 1999) suggest expansion of lower treeline into the lower reaches of Middle Park between ~10,000–8000 14C yr B.P., possibly the result of increased summer precipitation. Environmental changes during the early Holocene apparently transformed Middle Park from mixed spruce-fir parkland to mixed montane-subalpine forest, perhaps creating a less-favorable setting for mammalian fauna typical of parklands in the Colorado Rocky Mountains, such as bison, elk, mule deer, and pronghorn antelope (Benedict, 1992). Late Paleoindian groups may have broadened their use of the higher elevations in north-central Colorado to include settlement along more productive ecotonal zones displaced to lower elevations during the early Holocene. Interestingly, intensive use of areas in the current alpine zone did not occur in the Front Range until shortly after ~10,000 14C yr B.P. (Benedict, 1992; Pitblado, 2003), approximately contemporaneous with the expansion of upper treeline to higher elevations (Short, 1985; Reasoner and Jodry, 2000), presumably the result of warmer summer temperatures during the early Holocene (Elias, 1996). Potential paleoenvironmental factors involved in variable use of Middle Park by Paleoindians deserve future investigation. CONCLUSIONS Soil and stratigraphic sequences described for the Barger Gulch area compare reasonably well with those described elsewhere for Middle Park (Reider, 1998; Kornfeld et al., 1999). The association of Folsom artifacts with a buried soil at Barger Gulch Locality B indicates relative surface stability during the latest Pleistocene. Shortly after Folsom occupation, however, rapid oscillations in climate resulted in erosion of at least portions of the Folsom landscape. A soil forming between approximately 9500 and 8000 14C yr B.P. at Localities B and F has the potential to produce later Paleoindian material, suggesting that landscape stability followed shortly after erosion during the early Holocene. However, some erosion may have continued in upland settings until ~8000 14C yr B.P. Nevertheless, both early and late Paleoindian sites occurring along the main axis of Barger Gulch should be relatively deeply buried, and may occur in relatively undisturbed contexts. However, similar to Locality B, some reworking of early Paleoindian sites occurring along the main axis of Barger Gulch by alluvial processes may have occurred between ~10,000 and 9700 14C yr B.P. Our work in Barger Gulch demonstrates the importance of reconstructing landscape evolution at multiple spatial scales in order to understand site-formation processes and assemblage integrity, especially in shallow contexts characterized by steep hillslopes and fine-grained sediments. In addition, we have demonstrated that the main axis of Barger Gulch holds good potential for buried Paleoindian sites. To GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL

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date, kill sites are absent from Barger Gulch. However, Paleoindian-age deposits are more or less continuous along the main axis, an appropriate setting for kill sites to occur, and the valley bottom can be a focus for future survey. Additional work in the area will concentrate on better constraining episodes of erosion and deposition during the late Quaternary. Specifically, it is important to determine the timing and extent of erosion evident at both Locality B and UTM during the latest Pleistocene and early Holocene. Finally, a more comprehensive study of the alluvial stratigraphy and landforms in other stream valleys is warranted for determining whether or not the scenario of late-Quaternary landscape evolution in Barger Gulch is representative of Middle Park as a whole, a fundamental step in approaching broader issues related to site distribution and preservation, and, ultimately, Paleoindian land-use and settlement technologies. Frank Rupp of the Bureau of Land Management, Kremmling Resource Office, provided logistical and financial support, and aided in permitting, all of which made much of this fieldwork possible. Thanks to the Bruchez family for allowing access to Locality B across their property. Partial funding came in the form of a Geological Society of America Research Grant and a ChevronTexaco Geology Fellowship (JHM). Additional support came from a grant from the Colorado State Historical Fund and the George C. Frison Institute, University of Wyoming (TAS, NMW, MC, and GCF). The University of Arizona NSF-AMS kindly provided two radiocarbon ages. Thanks to Beth Ann Camp and Matt Titre for help in the field. Beth Ann Camp, Vance T. Holliday, and Michael McFaul read earlier versions of this manuscript. The careful reading and insights of Eric T. Karlstrom, E. Arthur Bettis III, and an anonymous reviewer are greatly appreciated and resulted in significant improvements to the manuscript.

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Received February 24, 2004 Accepted for publication September 9, 2004

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