Great Basin Naturalist
Volume 53 Number 2 Article 6
6-4-1993
Late Quaternary vegetation and climate in the Escalante River Basin on the central Colorado Plateau
Kim Withers Texas A&M University, College Station, Texas
Jim I. Mead Northern Arizona University, Flagstaff, Arizona
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Recommended Citation Withers, Kim and Mead, Jim I. (1993) "Late Quaternary vegetation and climate in the Escalante River Basin on the central Colorado Plateau," Great Basin Naturalist: Vol. 53 : No. 2 , Article 6. Available at: https://scholarsarchive.byu.edu/gbn/vol53/iss2/6
This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 53(2), pp. 145-161
LATE QUATERNARY VEGETATION AND CLIMATE IN THE ESCALANTE RIVER BASIN ON THE CENTRAL COLORADO PLATEAU
Kim Withersl and Jim I. Mead2
ABSTRA<.-"T.-Five alcoves (rock shelters) in the FOlty-Mile Canyon-Willow Gulch area of the Escalante River Basin in southeastern Utah yielded rich depOSits of late Quaternary macrobotanical remains. The depOSits were sampled and the contents identified in order to construct a chronology ofvegetational change. Fourteen radiocaroon dates indicate that the fossils were deposited between 12,690 and 7510 yr B.P. (years before present). Ninety-one plant taxa were identified, 62 to species. Six species were common to all alcoves: Gambel oak (Quercus gamhelii), box-elder (Acer negundo), prickly pear (Opuniia subgenus Platyopuntia), skunkbush (Rhus arorTUliUxl var. trilobata), serviceberry (Amelanchier utahensis), and Indian ricegrass (OnJwpsis hymerwides). Late Pleistocene samples (>11,000 yr B.P.) contain extralocal, elevationally depressed species such as Douglas fir (Pseudotsuga menziesii), spruce (Picea sr.), and mountain mahogany (Cercocarpus [edi/altus), and mesophytic species such as rose (Rosa tIJOodsii) and water birch (Betula occidentak). Early Holocene samples (11,000-8000 yr B.P.) contain no elevationally depressed conifers, and the remaining mesophytic species decrease in relative abundance. Reticulated hackherry (Celtisreticulata) becomes common. Theterminal Early Holocene sample (8000-7000yr B.P.) contains abundant Gambel oak and prickly pear, but little else. Paleoclimatic interpretations for the Late Pleistocene correspond well to those of most other workers on the Colorado Plateau. Climates that were wetter and at least seasonally cooler than they are today are inferred from the macrobotanical assemblage. However, the increased moisture is attributed to higher stream base levels and increased groundwater rather than directly to increased precipitation. Early Holocene climates are interpreted as warmer and drier than those ofthe Late Pleistocene but still wetter than the present climate. Groundwater levels appear to be decreasing due to stream entrenchment. Terminal Early Holocene climates were much warmer and at least seasonally drier. By the endofthe period, groundwater levels had decreased so much that the alcoves were unable to sustain plant communities; stream base level was probably near the present level.
Key words: Quaternary, Colorado Plateau, plants, oak, Quercus, Pleistocene.
The Colorado Plateau has been the focus of The macrofossils in this study (Withers 1989) late Quaternary paleoecolOgic work in the last were excavated from sandstone alcove (rock decade; however, the late Quaternary plant shelter) alluvium and thus can represent only communities of the central plateau, southeast the community of the microhabitat where the ern Utah, are still inadequately known because depOSit was found. The objectives of this re ofthe few case studies conducted. Late Pleisto search were to describe and to explore the pa cene-Early Holocene plant communities ofthe leoenvironmental implications of changes in central plateau have been described using plant communities through time. macrofossils found in the alluvial deposits from Cowboy Cave (Spaulding and Peterson STUDY AREA 1980, Spaulding and Van Devender 1980), from scattered packrat (Neotorna spp.) mid From 1986 to 1988 field crews from Quater dens from the Paradox Basin (Betancourt nary Studies Program, Northern Arizona Uni 1984, 1990), and from the megaherbivore versity, explored several alcoves in the Forty dung blankets from Bechan Cave (Davis et a!. Mile Canyon and Willow Gulch areas of the 1984, Mead et a!. 1986), Cowboy Cave (Han Escalante River Basin (Glen Canyon National sen 1980), and various alcoves (Mead and Recreation Area; Fig. 1), which is the eastern Agenbroad 1989, 1992). boundary of the Kaiparowits Basin. Because
1Wildlife lind Fi~herie~ Sciences. Tex.us A&M Univer:;ity, College Station. Texas n843. 2Quatem,lry St1Jdi~ l'rognl1l1 and Department ofGeology, Northern Arizona Univer:;ity, B"x. 5544, Fh'gstuff, Ari'l.On" 80011. Addre% for ,,11 '-"Orres]}onden'-' 145 146 GREAT BASiN NATURALIST [Volume 53 ,'- -'- -- --, I .J'- I '..:. ;,.".._--,'-.-~-.-.:------.:'.'. '.,: -' - '. I I I _ .... - I I I I '- I " ' I I , I ...... ,_._._'\__ J " ... '- ,~ .:--. '- '- I " I '- , \ • " -;--- I " ) I I I I I , I I I I - - I r._ .. -' -oj ~ Colorado Plateau -... .~ .. _ .. _I.. - "'. '- \ \ I ,, " .' ...... J, Fig. 1. Map of the Southwest locating the Colorado Plateau (delineated hy stipples). F, Forty-Mile Canyon; W, Willow Glllc;h; B, Bechan Cave; C, Cowboy Cave; M, midden :'>tudies of Betancourt (1984, 1990). our study is on lands administered by the Na (Agenbroad and Mead manuscript), Stratified tional Park Selvice, we were requested not to fluvial sediments arc usually found on either locate the ak'Oves (on a map or in a description) side of steep talus slopes that are composed of with any precision. Somewhat more detailed sandstone blocks and boulders overlain hyeo descriptions of the alcoves are in Mead and lian sand. These talus slopes often contain Agenbroad (1992). Anyone needing furtber de prominent layers of macrobotanical fossils and scriptions of locations may contact the regional other remains such as bones, hair, and dung. scientist at the Rocky Mountain Regional Of Five alcoves were chosen for study based on the flee, Denver, Colorado. At least 851 plant taxa abundance of plant remains evident in the de presentlylive in the Kaiparowits Basin, resulting posits, in many diverse local plant associations (Welsh ct al, 1978). Taxonomic terminology follows Welsh et al, (1987). MODERN ENVIRONMENTS Tbe canyons cut tbrough the Navajo Sand stone and the upper units ofthe more resistant Climate of the Escalante Basin is warm and Kayenta Formation. The alcoves appear to have semiarid with highlyvariable amounts ofannual been formed in the cross-bedded sandstone by precipitation. Strong orographic gradients are spalling (w,JI exfoliation), facilitated hy the ac exhibited by both temperature and precipita tiOll of moisture infiltration through joint-con tion, Mean annual temperature oflow-elevation trolled surface drainage. All alcoves in this sites along the river is 12°C, with 250 mm orless drainage area appear unique because they con mean annual precipitation. At high-elevation tain perched terraces, which are remnants of sites in the nearby Aquarius Plateau, met:m an ancient fluvial deposits. It is estimated that the nual temperature is 2°C, with 575 mm ofmean age oftbe fluvial deposits exceeds 30,000 years annual precipitation (Webb 1985), 1993] QUATERNARY VECIITATION ON COWRADO PLATEAU 147 Vegetation in the basin ranges from Great widespread but uncommon, usually occurring Basin desertscrub to spruce-fir (Picea spp. in sheltered, shady areas. Gambel oak (Quercus gambelii) L, common • Abies spp.) forest. Low areas between 1400 m and 1700 m elevation are a mixture of in wet, shady areas and hanging gardens in Wil shrublands and grassland (Tanner 1940). Pin low Gulch but is restricted to a relictual popula yon-juniper (Pinus spp.-Juniperus spp.) wood tion in a valleylike area in the upper part of land occurs between 1800 m and 2300 m, Forty-Mile Canyon. There it is found only on followed bya yellow pine-oak-manzanita (Pinus the shady, north-facing side of the canyon pomlerosa-Quercus spp.-Arctostaphylos pun against the rocks and on the sides and bottoms gens) association between 2300 m and 2700 m. of deep, sandy washes. Shrub live oak (Q. tur Above 2700 m, the southern face and top ofthe binella) is common in the upper reaches ofboth Aquarius Plateau support a spruce-fir forest canyons but becomes rare as the alluvium be (Webb 1985). comes wetter. Shinnery oak (Q. havardii) is re stricted to the driest, uppermost poltions ofthe Topography in the immediate area ofForty canyons. Mile Canyon and Willow Gulch is diverse. Ele In the wettest parts of the canyons, near vation ranges from 2300 m at the top of the seeps and at the edge of the creek, important Kaiparowits Plateau, where the perennial species include horsetail (Equisetum spp.), streams that carve the canyons begin, to about common reed (Phragmites australis), sedges 1100 m at the confluence of the two canyons at (Carex spp., Sci''P''s spp.), birchleaf buckthorn the Escalante River. (Rha",,,,.., betulaifolia), and saltgrass (Distichlis Pinyon-juniper woodland dominates be spicata). Box-elder is found scattered along the tween 2300 m and 1500 m. Below the woodLmd canyon bottom and occasionally in hanging gar on benchesandinslick-rock areas is desertscrub dens. Common hanging garden species include dominated by blackbrush (Coleogyll£ ramosis various ferns and mosses as well as poison ivy sima). Within the canyons, habitats range from (Toxicodendron rydbergii) , cardinal flower nearly vertical rock faces to valleylike areas; (Lobelia cardinalis), watercress (Nasturtulm of from dry, rocky slopes and creekbeds with ac ficinale), cardinal monkeyflower (Mimulus tive, steep sanddunes to riparianwoodlands and cardinalis), andcolumbine (Aquilegia micrantha). hanging gardens. Valley areas are dissected by sandywashes. Manyplantspecies such as skunk FOSSIL LoCALITIES bush (Rims aromatica var. trllobata), single-leaf ash (Fran""s anomola), barbeny (Malumia fre Hooper's Hollow, Grobot Grotto, and BF montii), and serviceberry (Amelanchier utahen Alcove are in Forty-Mile Canyon; Shrub-Ox Al sis) are found throughout the creek system in a cove and Oak Haven are in Willow Gulch (Fig. variety of habitats, while others such as willow 1). All are located between 1100 and 1300 m in (Salix spp." tamarisk (Tamarix ramosissima, in elevation. troduced), box-elder (Acer negundo), and seep willow (Baccaris emoryi) are found only near Forty-Mile Canyon permanent running water. Cottonwood (Popu The alcoves in Forty-Mile Canyon are diffi lus fremontii) is common along permanent cult to reacl,. Approximately 10m of vertic>~ or watercourses as well as areas where ground nearly vertical sandstone rises from the deeply water is near the surface. entrenched streambed to the base of the al On dry slopes above the streambed, impor coves. Streamside vegetation below BF Alcove tant species include skunkbush, sagebrush and Grobot Grotto is dominated by willow and (Artemisia spp.), blackbrush, snakeweed (Gu tamarisk, but there is little streamside vegeta tierrezia spp.), prickly pear, brickellbush tion below Hooper's Hollow. (Brickellia califomica, B. grandiflora), and In Hooper's Hollow and Grobot Grotto are dian ricegrass (Oryzopsis hymenoides). In val large (100-200 m wide), southwest-facing al leys many of these same species are common coves (Fig. 2). Tops of the deposits in both are along with winter fat (Ceratoides lanata) and at approximately 1200 m elevation. In both al scattered individuals ofUtah juniper, single-leaf coves modem vegetation is chara<.terized by ash, netleaf hackbeny (Cdtis reticlJ.ata), and desert grassland species, ,vith a few scattered cliff-rose (Purshia mexicana). Hackberry is shrubs. Active and grass-stabilized sand dunes, 148 GREAT BASIN NATURALIST [Volume 53 Fig. 2 (above and on facing page), Photographs ofalcoves mentioned in text: A, Hooper's Hollow; 13, Grohot Grotto; C, BF Alcove; and D, Oak I-Iaven. Note people for scale in center of B; units containing macrobotanical remains occur about the rockfall "smile" in the center ofthe deposit. Dq-presclVcd botanical remains occur stratigraphically above the layered, fluvial and lacustrine units that oc'Cupy the basal units at all alcoves; sediments are of Sangamonian age. 1993] QUATERNARY VEGETATION ON COWRADO PLATEAU 149 '. ._- L50 GREAT BASIN NATURALIST [Volume 53 as well as large areas ofslick-rock sandstone, are 30,000 yr B.P. They were depoSited when the also found. Indian Iicegrass is the dominant canyon stream was dammed dO\vnstream, pos species. Other species noted in the area include sibly by either a sand dune or canyon wall col indigobush (Psorothamnusjrenwntii), MOlmon lapse. At any point duIing deposition behind the tea (Ephedra viridis), snakeweed, porcupine dam, the canyon would have contained fluvial prickly pear (Opuntia erinacea), buckwheat (and possibly eolian) units in the stream channel (Eriogonum inflatum), and dogweed (Dyssodia area, but there would have been lateral facies of pentachaeta). colluvial deposition in the alcoves. Once the BF Alcove is relatively narrow and faces sediment depth had reached the threshold of northeast (Fig. 2). It is the highest alcove in this the dam height, fluvial deposition would have study at about 1250 m elevation. The area just ceased; however, lateral colluvial deposition outside the alcove is a steep, grass-covered sand would have continued. "\\Te are recovering fossils dune. Species present include Indian ricegrass, in the lateral colluvial facies. dropseed (Sporobolus sp.), and occasionally \\118n the dam was breached, sediments Mormon tea and snakeweed. were probably downcut rapidly in at least the main streambed. Remaining sediments were Willow Gulch eroded from behind and beneath by groundwa ter flowing between the sediments and sand The alcoves in Willow Gulch are presently stone (seeps and springs). They were also fairly easy toenter, usually requiring onlya short eroded by channeling and water flowing over climb up a steep embankment or sand dune. the surface, espeCially at the alcove dIipline. SllIlIb-Ox Alcove is located in a wide, valleylike Remnants were left behind as predominantly area at 1190 m elevation and faces northwest. the lateral facies in the alcoves. The oldest dated Gambel oak and shrub live oak grow near the layer from our study, ca. 23,000 yr B.P. (SOA4), stream below the alcove. Skunkbush, barberry, as well as mammoth (Mammuthus) dung from and bIickellbush are common. Cottonwood is below the profile in Grobot Grotto, which dates abundant near the creekbed and Indian Iice ca. 26,000 yr B.P. (Mead and Agenbroad 1989), grass is Widespread, Juniper trees are scattered is evidence that the valley sediments hadalready across the valley. Porcupine pIickly pear and hedge hog cactus (EchiruJcereus sp.) are found been extensively eroded by the last full glacial (ca. 21,000 yr B.P.). growing in cracks in the sandstone. Oak Haven is the lowest alcove in this study DepOSitional Environments at 1140 m elevation and faces nearly due west and Taphonomy (Fig. 2). It is narrow and shelflike, with a fairly large, rubble-filled shelter on the north end. It The taphonomy (what happens to organic is a wet, "active" alcove, although not so wet as remains after death) of the localities is impor to destroy the fossils, about 50 m above the tant to understand. Organic remains are pre present streambed. served in the alcoves behind the dIipline, that This is the only alcove in which species found is, inside the shelterandaway from direct effects in f(lSsil samples still occur alive today. Gambel of precipitation. Fossil remains were incorpo oak is abundant throughout the alcove. Other rated into the alcove sediment layers conceiv arboreal species found in or near the alcove are ably by vmious methods. hackberry and box-elde" Common shrubs in (1) A plant taxon could have washed into the clude seep willow, sacred datura (Datura me alcove, meaning that the plant actually grew at teloides), serviceben-y, skunkbush, roundleaf a different location (possibly at a much higher buffaloberry (Shepherdia rotundifolia), and elevation and in a different community) andwas sagebJUsh. A plunge pool is located to the south canied by stream action to a lower elevation, to of the alcove and contains common hanging finally come to depOSition in one ofour alcoves. garden species. Had this been the scenaIio, the matrix immedi The alcoves contain hvo major sedimentary ately around the plant remains would be fluvial units: (1) lower fluvial layers without organic in nature. The sediments would show some sort remains and (2) upper colluvial layers with fos of stream action or deposition. Even the plant sils. The multistratified, fluvial sediments, of remains would show some sort of transport which there are remnants remaining, filled the damage-which they do not show. This is not aI<.;oves and stream valley sometime prior to the case. 1993] QUATERNARY VEGETATION ON COLORADO PLATEAU 151 (2) A plant taxon could have been earned were dry-screened through I-mm mesh at the into the alcove by wind action. For this scenario site. Because the sand was so loose, it was nearly to realistically happen, the taxon in question impoSSible to avoid mixing the layers along the would have had to grow nearby enough to be edges ofthe pit. When sterile sand was found at carried into the alcove by wind. Long-distance 40 em, the excavation was terntinated and the transport ofso many fossils is not likely given the pit backfilled. A similar pit was excavated at the present physiographic situation. Certainly the top ofthe deposit (Holocene) at Grobot Grotto; taxon could have lived on the land above the however, the same slumpage problems were alcove-which is now mostly narrow slick-rock. encountered. Since recoverywas minimal, exca Having any of the fossil plants recovered in the vation was terminated at 30 em. alcove actually growing above the alcove, in In the Laboratory of Quaternary Paleontol stead ofin the canyon, does not really alter our ogy (Northern Arizona University), bulk sam overall conclusions. ples were dry-screened through a 20-mesh And fmally, (3) a plant taxon could have been (O.84-mm) soil screen. They were identified by growing in or immediately outside the alcove. using the mpdern collection housed in the Mu Plants in this scenario would have pristine seum of Northern Arizona Herbarium, consult macrofragments, except for situations where ing with various members ofthe biology faculty spalling wall rock damaged the specimens. The at Northern Arizona University, and examining encasing sediments imply that only a colluvial regional literature (Gould 1951, Martin and (spalling) depositional environment is present. Barkley 1961, Morris et aI. 1962, Delorit 1970, It is our opinion that given the three possible Elmore 1976, Albee 1979, Welsh 1986, Welsh depoSitional scenarios outlined above, the al et aI. 1987; Table 2). Plants were identified from cove and the fossils discussed here are the result twigs, leaves, seeds, flowers, flower parts (espe of, at least predominantly, in situ deposition; cially involucres), and fruits. Since the original that is, species actually grew inside or in the mass and the number of unidentifiable frag immediate vicinity of the alcove. ments varied from sample to sample, all identi fled pOlts were counted and aSSigned a relative METHODS abundance according to the following scale: 1 = rare (1-2 fragments), 2 = uncommon (3-10 Macrobotanical Fossil Collection fragments),3 = common (11-50 fragments), 4 and Analysis = very common (51-100 fragments), 5 = abun dant (>100 fragments) (following Van Deven Macrofossils were found in unconsolidated der 1973). layers Oeafmats) in the deposits or mixed with spall sands andblocks. Samples were difficult to Radiocarbon Dating obtain because the truncated deposits werevery When possible, radiocarbon dates were ob steep and loose (at angle of repose). Exposed tained on a single species from a single layer. In surfaces were cleaned to remove loose, con cases where no single species was abundant, a taminating slumpage. The in situ leafmats were composite sample of a single species from two sampled by removing the mass ofleaves, twigs, layers (BF Alcove) or a plant species and some and other plant materials by hand or with a fecal material from a Single layer (Oak Haven) trowel. In between the leafmats orwhere plant were used. Multiple dating procedures were remains were found mixed with the sand, bulk samples of sediment were collected. performed in each alcove to establish a chronol ogy. All radiocarbon dating was done by Beta Samples were taken from various locations Analytic Incorporated, Coral Gables, Florida within each site so tbat an accurate description (Table 1). of the plant community and a chronology ofthe site could be constructed. Generally, layers were sampled along a vertical line (profile) from the RESULTS top of the deposit. At Oak Haven, Grobot Grotto, and Shrub-Ox Alcove, sampleswere also Ninety-one fossil taxa were identified, 62 to taken to the side of the main prome to test as species (Withers 1989). Ofthose, only six were much ofthe variability ofeach layer as possible. common to all alcoves: Gambel oak, box-elder, At BF Alcove, a O.25-m pit was excavated and serviceberry, prickly pear, skunkbush, and In bulk-sampled at IO-cm intervals. These samples dian ricegrass. Fourteen radiocarbondateswere 152 GREAT BASIN NATURALIST [Volume 53 TABLE 1. Radiocarbon dates (all from Beta lab), in stratigraphic order, for Forty-Mile Canyon and Willow Gulch samples. GG, Grobot Grotto; BF, BF Alcove; HH, Hooper's Hollow; SOA, Shrub-Ox Alcove; OH, Oak Haven. Radiocarbon dute Sample (y, R.E) Lab no. Material dated Grobot Grotto GGO none GGI none GG2 7510 == 160 20999 Quercus gambelii branch GG3 9730 ± 170 20998 Quercus gamhelii branch GG4 none GG.5 9920 ± 100 20999 Quercus gambelii trunk BF Alcove BF 11,790 + 190 14727 Dung 12,130 ± 170 2099.5 Pseudotsuga menziesii needles Hooper's Hollow HHI 10,630 ± llO 25412 Quercus gambelii twigs HH2 12,010 ± 110 25411 Quercus gambelii twigs Shrub~Ox Alcove SOAI 8520 ± 80 25415 Quercus gamhelii twigs SOA2 8830 ± 190 25656 Quercus gamhelii twigs SOA3 12.690 ± 180 25416 Quenus gamhelii twigs SOM 23,100 ± 660 25413 Quercus sp. limb fragment Oak Haven OHI 9180 ± 100 28929 Quef'(,'Us gamhelii branch OH2 11,690 ± 120 25418 Rosa woodsii twigs OH3 ~9470 ::: 150 25657 Picea sp. twigs and dung OH4 none °Dat« not a~L~'pted, See t«xt, obtained to determine the age of the deposits. Thirty-five plant taxa were identified (Table Materials dated included Gambel oak wood, 2). Prickly pear and Gambel oak were the most rose (Rosa woodsii) branches, Douglas fir nee abundant species in the oldest layer; blackbrush dles, spruce twigs, and dung (Table 1). and Mannon tea were common. More species ofgrass (four) were identified from this sample Forty-Mile Canyon than from any other. Indian ricegrass was com mon. GROBOT GROTTO.-Excavation into Holo The remaining samples reveal no discernible cene sediments at the top ofthe deposit yielded change until all plant species disappeared from few macrofossils; and since there was no obvious the alcove sometime after 7500 yr B.P. Gambel stratigraphy, they were treated as one sample, oakwas the mostabundant fossil species. Prickly GGO (not dated). This sample is identical to the pear was common in an undated sample from modem flora as observed elsewhere in the can the bottom of the profile (GG4) and in the yon. youngest sample (GG1). Other species present Four samples were collected from the 275 in small numbers include box-elder, hackberry, cm-thick Gambel oak leaf-roof spall layer that fishhook cactus (Sclerocactus sp.), Solomon's began 157cm downslope from the datum at the seal (Smuacina sp.), andIndian ricegrass. Indian top ofthe deposit. Two ofthesesamplescontain ricegrass was the only species present in all ing Gambel oakwere submittedfor radiocarbon samples. dating. Twigs from the top and near the bottom BF ALcOYE.-The four samples from this ofthe oak leaflayer dated their deposition from alcove showed little variation except for a de ca. 9700 to 7500 yr B.P. (Table 1). A piece ofoak crease in overall macrofossil abundance as the stump, located in situ approximately 17 m west sterile horizon was approached. The lack ofdis ofthe profile, dated ca. 9900 yr B.P. cernible stratigraphy in the test pit and low 1993J QUATERNARY VEGETATION ON COLORADO P, ATEAU 153 weights ofanyone fossil species resulted in the to the youngest sample. Hackberry was com decision to pool the Douglas fir needles from all mon in the sample dated 8830 yr B.P. but be samples for radiocarbon dating. The two dates came uncommon by 8520 yr B.P. Prickly pear merge around ca. 12,000 yr. B.P. (Table 1). was never abundant as in the other alcoves, but ThiIty-nine taxa were identiHed (Table 2). wa<; only rare or uncommon in the youngest Most abundant were Douglas fir, bigtootb ma samples. An increased number of grass taxa ple (Acer gral1didel1tatum), box-elder, prickly (five) was observed in the youngest sample. pear, mountain mahogany (Cercocarpus ledifo Mountain mahogany was rare or uncommon in lius), and indigobush. Other common species all samples. Sagebrush and saltbush were rare included Gambel oak, Clilyant (Ribes cf. or uncommon in the youngest samples as were cereum), skunkbush, hackberry and Single-leaf serviceben'y, rose, and brickellbush. ash. Spruce and Hr were present, represented Several species were recorded only from this respectively by one needle and one cone scale. alcove~willow in the Late Pleistocene sample, HOOPER'S HOLLOw~The oak layer at and Utah fEmdlerella (Fendlerella utahemi,,), Hooper's Hollow begins 190 cm from the da two sedges (Carex bella aud Cladium califomi tum, is up to 100 em thick, and has been burned cum), and cottonwood in the youngest sample; extensively. Two samples record plant commu all were rare. nities ofca. 10,600 and 12,000 yr B.P. (Table 1). OAK HAVEN .-Oak Haven has both an oak Only 13taxa were identifiedfrom this alcove, layer and a rose layer. The stratum containing the low number reflecting the lack ofpreserva the rose layer was chosen for profiling. The tion because of buming (Table 2). Gambeloak deposit began 20 em from the datum and was was very common in the Late Pleistocene sam about J55 em thick. The oak layer is located to ple. Uncommon or fare species identified from the north of the profile in a pile of wall fall this sample were prickly pear, rose, box-elder, overlain by eolian sand. water birch (Betula occidentalis), spruce, and Three samples were submitted for radiocar Indian ricegrass. bon dating (Table 1). The Gambel oak sample The Early Holocene sample records changes from the oak layer dated ca. 9200 yr B.P. The in the vegetation during the Pleistocene-Holo two lower radiocarbon dates from the proBle are cene transition. Rose, water birch, box-elder, reversed relative to the stratigraphy. Rose from spruce, and mountain mahoganywere no longer the top ofthe unit dates ca. 11,700 yr B.P., while at the site. Gambel oak and prickly pear were a sample of spruce twigs combined with ungu common. Unc'ommon or fare species included late dung hum below the rose layer dates ca. poison ivy, Solomon's seal, skunkbush, and In 9500 yr B.P. We feel that the spruce date (Beta dian ricegrass. 25657) is equivocal, as it is a combination oftwo entirely different species and should not be as Willow Gulch signed to spruce or the stratigraphic position SHRUB-OX ALCOVE.~The oak layer in until confirmation. Shrub-OxAlcove began 120 cm from the datum Forty taxa were identified from all samples and was about 350 cm thick. Gambel oak was (Table 2). The youngest sample contained abun dated from four samples. The oldest date dant fossils of Gambel oak. Water birch and (23,100 + 660 yr B.P.) was on what appeared to bigtooth maple were common. Rose is the most be oak wood from a highly degraded part ofthe abundant species from the dated rose layer. macrofossil layer. Only three taxa (oak, hack Common species hom the layer include Gam berry, and mountain mahogany) were recovered bel oak, bigtooth maple, and Douglas fh: Rare from this sample, but the community appears to or uncommon species include sagebnlsh, box have been similar to those of the latest Pleisto elder, water birch, prickly pear, and spruc'C. The cene in this and other alcoves. The remaining two lower, undatedlayers contain abundant rose samples record the commUluties from the Late and spruce; water birch and box-elder were Pleistocene and the Early Holocene, ca. J2,700 common. to 8500 yr B.P. (Table 1). The Forty-Mile Canyon-Willow Gulch Thirty-eight plant taxa were identified (Ta Sequenc'C ble 2). Gambel oak was abundant or very com mon in all samples. Box-elder and bigtooth The fossil abundances of selected species maple decreased in abundance fl'om the oldest from both canyons were grouped into a sedated 154 GREAT BASIN NATLJRAUST [Volume 53 1'AIHJ'; 2. Plants rt::oovered from Late Pleistocene and Holocene deposits in BF Alcove. Grobot Grotto. Hooper's Hollow, Quk II,wen, and Shrub-Ox Alcove, southeastern Utah. ceo eel GG2 GGJ GG4 81" HHI HHZ $OAl SOA2 SOA3 SOM aHI OH2 OID OH4 Graze AlulnJ/lOgnn {!,erardii I ArulrutJO{!,Qtl cfl!.ltnnerattts I Boutdoua harbato I lJoute1mUl eri0l)oda I BOlltdoua ttifitlll I Carex ~p. I I Cladium cnli.fornicum I De.9C;hanl.IJsia caeJqnulsa I JJLvticJdis S1Jicata I I ELymu.s cf. virginicus I EUf'TlUS sp. {includes A{!,mpyron} 1 I Hilaria fi;!,irbt 2 MuiJenbe'];ia cf. wri{;htu I Oryzopsis lY'TJ(.~les ('" Stipa) 3 2 2 I 2 II 2 II 2 I PlInil,.lml hulhosm1l 2 Po(/. cf. bi{!.e1aoii 1 Set(UitL L~ I Sporoho <..f. jleXflQiNL':; I ~I}Orohol(t'i::k' - I Trl(/{ms I'" l(;IUu~' (=Erilmeuron) I 1 Brow.'ic Abies sr. I Acer grand.identatwn 5 I I 3 3 3 Acer negundJ) III 5 I 3 3 4 I 2 2 Alltrtrt'mthus sp. I I Ambro,\'ia sr. I 2 Agaw sp. I Anwkmcltier llt'{lhem~' 3 2 II I I I Arctostaphylu., pur'1!:ens I Artelrli.~i1, <.:1'. c(trmtllii 2 I 2 ArtemUut c1'. dr(/.(,;wv:.;u.bL~ I Artem~~i.(1 tritl.ent(lt(, I II I A\ter d. 8pin()SU~' I 2 2 I AtrilJlcx ca1L(iSCt'fI..V I II I 2 Atrlpk'X d. confertifilf.W I - Baccharis sknt I &'ttJa occil ali:.. II 3 2 I 3 Bricke1litl J!.mndifWm I 2 I I 1 J1rlcJwliia sp. 2 2 1 CCtuwthu.<; ferulleri I Celtts ,-eti.cultda I 1 I 2 3 2 3 I 1 Cer{l!OUles umata I I Cera:JCtlrpu<; kdijoJi.u..v 3 2 2 I III 2 2 CfKmol)(){lium atmvtl1m$ I Chena'J()(lium d. murale 1 ChcnalW(lium sp. 2 Chrysotha711,TU.IS sp. I Coleogyne rrmulrissima 3 2 I Comarulra paliida 2 CorydaJ1s aurea I Cnn)tautha lJterocarya II CnJptantha seuJ$issimn I C'lJptanth(( sp. 2 I I Datum meteloide.... (:::D. wrightii.) I I DithlJrea calijiJfTIi TABLE 2. Continued. CGO GGl GG2 GG3 GG4 BF HHl HH2 SOAI SOA2 SOA3 SOA4 OHl OH2 om OH4 Gutierrezw sp. I HapZopappus hetetpogon I Haplopappus cf. drwnrrumdii 1 Haplopappus sp. I 2 I 2 I I Heterotheca sp. I H ymenoxys sp. I Junipe1l1s scopukJrum I Juniperus sp. I Lycium pallidum I Mentzelia sp. I Opuntia sp. 3 3 3 4 3 2 2 I I 4 2 PhlJJx sp. 2 2 Picea sp. II I .5 3 Polygonum sr. I Populus cf. frenwntii I Pseudotsuga menziesii .5 3 II Purshia tndentata I 2 2 Quercus gamhelii I 3 5 5 3 3 3 4 5 4 5 5 3 2 I Quercus sp. I Rhus aromatica var. tlilobata I II 3 I III 2 3 3 Ribes cf. cereum 2 Rosa woodsii I 2 2 I I 5 .5 5 Salix sp. I Sclerocactus sp. 2 Shepherdia rotundifolia I I Smilacina sp. 2 2 I 2 2 I Sphaeralcea sp. I I Toxicodendron rydbergii 2 Trifolium sp. I Wyethia sp. I I chronosequence (Fig. 3). Some ofthe variability though the species persist in other cases nearby seen in the sequence is probably an artifact of today. samplingand site-to-site differences rather than The more xerophytic species in the sequence a real changeinvegetation; however, the general are prickly pear, sagebrush, saltbush, sknnk trend toward decreasing abundance of meso bush, blackbrush, and Indian ricegrass. Skunk phytic species is obvious. Douglas fir, spruce, bush persists in the record until 9180 yr B.P and cUlTant, andfir do not appearin the fossil record was most abundant in samples that date prior to after 11,690 yr B.P Willow is also absent from 12,000 yr B.P It is common and abnndant in the the record after 11,690 yr B.P, but it is common canyons today. Prickly pear was most abundant in areas with permanent water in the canyons in the sample from ca. 12,100 yr B.P and is very today. Rose, bigtooth maple, single-leafash, and common in the canyons today. Indian ricegrass, mountain mahogany persist until 8520 yr B.P, sagebrush, saltbush, and blackbrush show little while box-elder persists until 7510yr B.P These variability in abundance throughout the time species were not as abundant in samples from spanned by the record although they are not after 11,000yr B.P as theywere in samples prior fonnd in evelY sample. Today, Indian ricegrass to that time. Hackbenyandwater birchbecome is the most widespread species in the canyons, more abundant in the samples after 11,000 yr while the others are freqnently enconntered in B.P Hackberry, box-elder, and single-leaf ash drier habitats. are found restricted to the more mesic habitats in the canyons today. Oak shows little variability DISCUSSION in abundance throughout the sequence. The lack ofdeposits ofany kind dating after 7510 yr Plant communities that can be described B.P suggests thatthe disappearance ofoakfrom from fossils recovered from the alcoves are the alcoves bv, that time was a real event.. al- much like those fonnd in the canyons today. 156 GREAT BASIN NATURALIST [Volume 53 YEARS B.P. , • ,, , ,, , , , , , ,• , , , ,, , ,;' ,, ,, , ,, ,, , ,, , ,, , , , 7510 t 160 8520! 00 f- 8830:1"190 f- l- ,= 1/ 9180± 100 i- 9130± 110 1/ 10,000 10,630±1I0 - 11,000 1I,690±120 - 12,010 ±110 V 12,130:1"140 ""\ , ,, ,, 12,690± IBO , 1/ ,, •• , 1',000 f\-e, o 4030 50 40 4030 50402.030 50302.0302.0202.0202 Fig. 3. Fossil abundances (rating 0 to 5) ofselected plant species from all sites. See text for discussion. Dotted lines refer to inferred previous or known presence today ofsome species. Major differences lie in the abundance ofmeso were located in its present position since at best phytic, relictual species and elevationally de a steep embankment must be climbed and at pressed, extralocal species dating from the Late worst 10 m ofvertical sandstone must be scaled. Pleistocene and Early Holocene. The most (2) The existence of Oak Haven. This "active" striking similarity is the abundance of Gambel alcove is wet and is the only alcove currently oak in the fossil record and its abundance in supporting Gambel oak. We believe that condi some areas of the canyons today. While more tions at Oak Haven are analogous to conditions xerophytic species such as Indian ricegrass and in otheralcoves during the Late Pleistocene. (3) prickly pear bave shown changes in abundance, All fossil deposits are located behind the their distributions appear to have changed little dripline, away from direct effects of precipita in the last 13,000 years. tion, in dry section ofthe alcoves. This suggests Inferences regarding changing precipitation that precipitation was neverdirectly responsible regimes based on paleovegetational changes for maintenance of plant communities within seen in the fossil record are complicated by the the alcoves but that plants were dependent on distinct possibility that the communities were availability ofgroundwater flowing from seeps, much nearer the stream in the past than the springs, and the intermittent canyon stream. fossil depOSits are today. Three facts suggest that Gambel oak is a definitive indicator of cli this was true. (1) Bison (Bison), camel (cf. mate because it is sensitive to narrow limits of Camelops), and mammoth frequented alcoves moisture and temperature (Grover et aJ. 1970, in the system as recently as ca. 12,000 yr B.P. Neilson and Wullstein 1983). Its presence (Mead and Agenbroad 1989, 1992). It seems throughout the timespannedby this assemblage highly unlikely that any large animal would be makes it suitable for making analogies, at least able to gain entrance to the alcoves ifthe stream for temperature. It is found only in areas with 199.3] QUATERNARY VECETATION ON COWRADO PL>lTEAU 157 mean annual temperatures of 7-100 C (Harper pIes, is rarely found in areas with mean annual et al. 1985). It is indicative ofat least 450 mm of temperatures helow 6'C (Betancourt 1984). annual rainfall and no less than 250 mm of Gambel oak, also velY common or abundant in winter precipitation, but its proximity to a the fossil <.L.'i'semblages, is found only in areas source ofgroundwatersuchas a stream modifies with mean annual temperatures of 7-10°C these requirements (Grover et al. 1970). The (Harper et al. 1985). Mean annual temperature extralocal species "vill also he used as important in the Escalante Basin today is 1O-12'C (Webb indicators of changing temperature and mois 1985). It is unlikely that cooling in the Escalante ture regimes. Basin during the Late Pleistocene exceeded the Based on the plant taxa identified from the 3-4'C postulated by Betancourt (1984) for samples (Tahle 2), we have designated three higher-elevation sites in the Paradox Basin. We stages: Late Pleistocene, which includes sam interpret that mean annual temperature ex ples dating>11,000 yr BY (including OH3); tremes in the Escalante River area may have Early Holocene, hehveen 11,000 and 8000 yr been little different from those of today. Cold B.P.; and Terminal Early Holocene, between air drainage from the Aquarius Plateau and in 8000 and 7000 yr B.P. creased local water availability may have been responsible for the persistence ofmontane spe Chronology of Paleoenvironmental Change cies in the canyons. U ..TE PLEISTOCENE ENVIRONMENT AJthough temperatures in the Escalante Ba (>II,OOO YR B.P.).-Elevationally depressed, sin may have been little different from what they montane species are the hallmark ofLate Pleis are today, it is apparent that the almves were tocene plant communities. In Forty-Mile Can much wetter. We hypothesize that this was at yon andWillow Gulch, spruce occurred as much tributable to increased groundwater rather than as 900 III !mver than todaywhile waterhirch \vas increased loeal rainfall. During the Late Pleis up to 80 m lower. Douglas fir, currant, and tocene these canyons contained much more val mountain mahogany exhibited depressions be ley fill than they do today, and stream base level tween 260 and 300 m, and bigtooth maple was was up to 50 m higher. as much as 140 m lower. All these species (with Stream entrenchment can take place dUri,ng the possible exception of bigtooth maple) are moist periods when plant cover is abundant or extralocal and probably occur no closer than the plentiful in the upper reaches of the stream Aquarius Plateau, about 70 km to the north. (Antevs 1955). If rainfall was greater at higher Relictual, mesophytic species were abun elevations dUring the Late Pleistocene, as is dant during this time. Species that appear to suggested by most workers (Phillips 1977, 1984, have been common in the Late Pleistocene, Spaulding and Van Devender 1980, Betancourt such as rose and box-elder, are rare or uncom 1984, Davis et al. 1984), then Forty-Mile and mon in the canyons today. '''''here they do occur, Willow creeks probably began to hecome en they are restdcted to streamside or other shady, trenched during the Late Pleistocene. Because wet situations where seasonal droughts do not of entrenchment, the erosion of valley fill con occur. While Gambel oak is common is some tinued and groundwater levels began to de areas ofthe canyons, it is also restricted to more crease. mesic habitats. The only impOltant xerophytiC EARLY HOLOCENE ENVIRONMENT (11.000 plant in the assemblages is prickly pear. The TO 8000 YR B.P.).-The major reorganization of plant community of the earlier time can be the plant communities dUring this time noted by inteIIJYeted as representing a somewhat cooler Betancomt (1984) and other workers on the environment with more available moisture than Colorado Plateau is apparent in this assemblage is found in the alcoves today. \Vhether these also. The only extralocal species found in the were just seasonal differences cannot be stated fossil assemblages were bigtooth maple and at this time. mountain mahogany. \Vhile still at lower eleva The Escalante River served as a route by tions than they are found today, elevational de which high-elevation species such as Douglas fir pressions decreased to 30 m and 200 m, and spruce were able to disperse into the lower respectively. Fossils ofthese and other relictual, canyons. Mean annual temperature in the mesophytic species such as rose persisted but Aquarius Plateau today is about 2'C. However, were less abundant than in the previous stage. prickly pear, which is common in the fossil sam- Even Gambel oak decreased slightly in some of 158 GREAT BASIN NATURALIST [Volume 53 the samples. There were no apparent increases less reliable or completely dried up. Coupled in the abundance of xerophytic plants, but de with warmer seasonal temperatures, this drying creasing abundances of mesophytic plants are resulted in the disappearance of most meso evidence of changes in both temperature and phytic species by the end ofour stage. moisture regimes. TERMINAL EARLY HOLOCENE ENVIRON Data for Forty-Mile Canyon and Willow MENT (8000 TO 7000 YR B.P.).-The one sample Gulch appear to indicate two shifts in the flora from this time period is from Grobot Grotto. that are similar to tbose seen in the Paradox The fact that there is omy one sample in itself Basin. Extralocal species except rose drop from suggests a major change in the environment the record prior to ca. 1O,600yr B.P. Bigtooth after ca. 8500 yr B.P. The sample contains only maple, rose, and mountain mahogany recover Gambel oak and pricklypearin any quantity No for a short time between ca. 10,000 and 8500 yr plant macrofossils are found above this layer in B.P. before dropping from the record. Gambel Grobot Grotto, and none above layers dating a oak increased in abundance and hackberry be thousand years earlierin Shrub-Ox Alcove. This came common during this time, indicating that sample appears to mark the beginning of ex summer moisture may have become more reli tremely dry environments in the alcoves and the able with the establishment of the monsoonal establishment of the present environmental boundary. regime. Shifts in the floras seen in the Escalante Because of its ability to reproduce vegeta Basin during the Early Holocene indicate that tively (Cottam et al. 1959), Gmnbel oak is able priorto 10,600 yr B.P. temperatures ortempera to persist in areas where seedlings are unable to ture extremes began to increase and moisture become established. The stand ofoaks thatlived availability was less reliable. This resulted in in Grobot Grotto until 7510 yr B.P.likely repre decreased abundances ofmesophytic plants and sents the last hold-outs before the local water the upslope retreat of Douglas fir and sprue-e. table became so low that even they died. The After 10,600 yr B.P., the recovery ofmesophytic disappearance of Gambel oak and all plants species such as bigtooth maple indicated that from the alcoves is interpreted as representing moisture availability became more reliable warmer, and at least seasonally drier, climates, again, although most species do not appear to andtherefore the entrenchingofthe streambed. have been'as abundant as in the latest Pleisto Stream base level was probably very near its cene. This moisture is still more attributable to presentposition by this time, leaving the alcoves increased groundwater due to higher stream high and dry, without groundwater resources to base level. Groundwater levels and rate of feed the seeps and springs that had previously stream entrenchment fluctuated during this supported the plant communities. stage in response to fluctuating rainfall amounts as the summer monsoon moved to its present Biogeographie and Paleoclimatic position, Considerations A last date ofca. 8700 yr B.P. on Gambeloak The Escalante River appears to have served leaves at Cowboy Cave is interpretedas marking as a major migrational route for high-elevation the inception ofa more xeric climate (Spaulding and mesophytic species during the Late Pleisto andVan Devender 1980). The sample that dates cene. It is likely that stream base level was as ca. 8500 yr B.P. from Shrub-Ox Alcove contains much as 50 m higher than at present. At the abundant Gambel oale anthers. Gambel oak on lower elevationallimits ofa plant, high tempera extremely xeric sites often fails to produce ma tures and deficient soil moisture produce tran ture female flowers but produces an abundance spiration stress in established individuals and of m,Je catkins (Freeman et al. 1981). We be reduce the potential for germination and seed lieve this radiocarbon date marks the termina ling establishment. Relaxation ofthese controls tion of mesic environments in the alcoves, with can be accomplished by lowering summer tem the stream base level no more than 10 m above perature extremes, which would result in in its present position. The alcoves were still wetter creased effective moisture, or by increasing than they are today for most of the stage, but precipitation (Betancourt 1984). Since drought continued stream entrenchment culminated in wa." not a limiting factor, the elevational depres drier alcove environments by the end of the sion of montane species observed in this area Early Holocene. The seeps and springs became was probably primarily the resultofcooler summer 1993] QUATERNARY VEGETATION ON COLORADO PLATEAU 159 temperatures, possibly facilitated by cold air Sexual reproduction in oaks stopped by the end drainage from the Aquarius Plateau. of the stage, as evidenced by the abundance of Southerly displacement of the polar jet male catkins in samples from Shrub-Ox Alcove. stream during the Late Pleistocene has been The polar jet stream retracted to the north as proposed, based on paleovegetation (Van De the summer monsoonal boundary approached vender and Spaulding 1979) and modem plant its current position. This would result in higher distributions (Neilson and Wullstein 1983). In annual temperatures and summer extremes, southeastern Utah this would result in milder, less available moisture, more reliable summer wetter winters and cooler, drier summers, an precipitation, and a prolonged spring drought. equable climate (Betancourt 1984). This inter The senescence and death of oak trees and pretation corresponds well with the interpreta the disappearance of all vegetation from the tion of the assemblages from our study as well alcoves in the canyons by ca. 7500 y,r B.P are as with those of most other workers (except interpreted as the result of warmer tempera Cole) on the Colorado Plateau. Gambel oak was tures and the establishment ofstream base level at low-elevation sites in these canyons and at nearits present position. Most otherworkers on Cowboy Cave and Bechan Cave. Today, its the Colorado Plateau have interpreted their re northern limit coincides with the polar jet cords as refleCting greater effective moisture stream at 41 0 N latitude. The fossil distribution during this time. However, our record reflects ofGambel oak suggests that the polarjetstream more xeric conditions as a result of decreased was displaced into the central Plateau at about groundwater and possibly seasonally drier con 380 N latitude during the Late Pleistocene. ditions. The disappearance ofGambel oak from The terminal date for woodland communi low-elevation sites in the central Colorado Pla ties in the Southwest is conSistently younger teau may represent a northward retraction than the traditional date ofca. 11,000 yr B.P for andlor upslope retreat of the species as condi the end of the Pleistocene. However, in North tions at lower elevations becametoo hotanddry. America this boundary is generally believed to Conversely, the xerophytiC shrub live oak, whose be time-transgressive (Watson and Wright northern boundary currently coincides with the 1980) and appears to vary with latitude. Transi monsoonal boundary, and Pinus edulis ex tional communities with mesic characteristics panded northward in response to hotter, drier perSisted in many areas until the end of the conditions, similar to Antevs' Altithermal. liv Early Holocene, ca. 7800 yr B.P, when wood ing populations of relict hybrids between the land species disappeared from modem desert two oak species as well as between P edulis and areas (Van Devender 1977). The changes in P monophylla occur at the Middle Holocene vegetation during this time involved a gradual northern limit ofshrub oak and P edulis andthe decrease in the abundance and number of soutbernlimit ofGambel oak and P monophylla woodland species and a relative increase in the (Launer 1974, Neilson and Wullstein 1983). To importance of desert species, many of which day shrub live oak and P edulis reach their werealreadypresent(Phillips 1977). Intheeast northern limits far south of the relict hybrid ern Grand Canyon, peak values for vegetation populations. change (species flux) were recorded between Cole (1981) suggests that a northward shift 12,000 and 8000 y,r B.P Cole (1985) believes of the summer monsoon and polar jet stream that Wisconsin species tended to disappear would explain his xeric record for the eastern prior to the establishment of modern species. Grand Canyon, while the same scenario is in The major reorganization of vegetation zones voked to explain the occurrence of the hybrids during this time precludes the use ofelevational and is consistent with a recent study of modern analogs (Betancourt 1984). andfossil distributions ofapomictic and sexually In our study we postulate that warmer tem reprodUcing populations of muttongrass (Poa peratures and increased drought stress due to fendleriana) on the Colorado Plateau (Soreng fluctuating groundwater levels resulted in the and Van Devender 1989). It would also explain upslope retreat of montane conifers and de the dry record for the Escalante Basin. Those creased abundance of mesophytic species dur records from the central Plateau that are inter ing the Early Holocene. Mesophytic species preted as wet are from higher-elevation sites showed a briefrecovery late in our stage before where wetter, cooler situations could have disappearing from the alcoves ca. 8500 y,r B.P persisted. 160 GREAT BASIN NATURALIST [Volume 53 SUMMARY LITERATURE CITED The record from Forty-Mile Canyon and ALBF.E, B. J. 1979. A guide to the identification ofseeds used Willow Gulch reflects at least seasonally and/or by prehistoric Indians. Pages 283-321 in J. D. Jen nings, ed., Swallovv Shelter. University of Utah Anthro locallywetter, more equable climates in the Late pology Papers 103. Pleistocene. The macrofossils record gradual ANTEVS, E. 1955. Geologic-climatic dating in the West. warming and drying, which became extreme American Antiquity 20: 317-335. sometime after8500yrB.P. so that all vegetation BETANCOUH.T, J. L. 1984. Late Quaternary plant zonation and climate in southeastern Utah. Great Basin Natu disappeared from the' alcoves by 7500 yr B.P. ralist 44: 1--35. The increased dryness is directly attributable to _--0' 1990. Late Quaternary biogeogmphy of the Colorado falling streambase levels anddecreased ground Plateau. Pages 259--292 in J. L. Betancourt, T. R. Van water and only indirectly to changing amounts Devender, and P. S. Martin, eds., Packrat middens. The last 40,000 years ofbiotic change. University ofArizona and/or seasonality ofprecipitation. Press, Tucson. 467 pp. OUf record is in general agreement with CoLE, K. L. 1981. Late Quaternary environments in the other Colorado Plateau records except that it eastern Grand Canyon: vegetational gradients over the appears to reflect at least seasonally driercondi last 25,000 years. Unpublished doctoral dissertation, University ofArizona, Tucson. 170 pp. tions during the Middle Holocene. This is prob ___. 1985. Past rates of change, species richness, and a ably because our sites are at lower elevations model of vegetational inertia in the Grand Canyon, than the others. Further studies ofsimilar mac Arizona. American Naturalist 125: 289-303. robotanical deposits in the surrounding area, if COTTAM, \iV. P., J. M. TuCKEH, AND R. DHOBNICK. 1959. Some clues to Great Basin postpluvial climates pro they exist, are in order. Expansion ofthis record vided by oak distributions. Ecology 40: 361-377. could delineate the changing position of the DAVIS, O. K., L. D. ACENBHOAD, P. S. MAHTIN, AND J. I. northern limit ofGambel oak and, by extension, MEAD. 1984. The Pleistocene dung blanket of Bechan the geographic positions of both the monsoon Cave, Utah. Pages 267-282 in H. H. Genoways and andpolarjetstream throughout the late Quater M. Dawson, eds., Contributions in QuaternalY verte brate paleontology: a volume in memorial to John E. nary. Additional data from this area would also GuUday. Special Publications ofthe Carnegie Museum help refine the geographic and temporal of Natural History 8. boundnries of both wet and dry altithermals. DELOHIT, R. J. 1970. Illustrated taxonomy manual of weed Additional paleobotanical studies in the area are seeds. Agronomy Publication, River Falls, Wisconsin. 175 pp. necessary ifthe late Quaternarypaleoecology of ELMOH.E, F. H. 1976. Trees and shrubs of the Southwest the central Colorado Plateau is to be adequately uplands. Southwest Parks and Monuments Associa understood. tion, Tucson. 214 pp. FHEE!'>·fAN, D. G, E. D. McAHTHUH, K. T. RAHPEH, AND C. BL."UEH, 1981. The influence of environment on the floral sex ratio of monoecious plants. Evolution 35: ACKNOWLEDGMENTS 194-197. GOULD, F. W. 1951. Grasses ofsouthwestern United States, Univsity of Arizona Press, Tucson. 352 pp. We thank Louella Holter (Bilby Research GHOVEH, B. L., E. A. RICIIAHDSON, AND A. R. SoUTHAHD. 1970. Quercus gambelii as an indicator of climatic Center) for typing the manuscript. We appreci means. Proceedings of the Utah Academy of Science ate the follOwing people for their help in the 47,187-191. field: Larry Agenbroad, Liz Bamosky, John Ben HANSEN, R. 1980. Late Pleistocene plant fragments in the jamin, Brian and Karen Cluer, and Emilee dung ofherbivores at Cowboy Cave. Pages 179--189in Mead. Emilee provided all photography and J. D. Jennings, ed., Cowboy Cave. University of Utah Anthropology Papers 104. drafting. The Rocky Mountain Region, National HAHI'EH, K. T., F. J. WAGSTAFF. AND L. M. KUNZLEH. 1985. Park Service, Denver office, and Glen Canyon Biology and management of the Gambel oak vegeta National Recreation Area provided logistic and tive type: a literature review. USDA INT-179, Inter curation assistance. We appreciate the special mountain Forest and Range Experiment Station, Ogden, Utah. and continued help ofAdrienne Anderson and lANNEH, R. M. 1974. Natural hybridization behveen Pinus Bob Schiller (National Park Service). Funding edulis and Pinus monophylla in the American South for this project was provided by National Sci west. SUvae Genetica 23: 108-116. ence Foundntion grants EAR 8708287 and MAHTIN, A. C., AND W. D. BAHKLEY. 1961. Seed identifica 8845217 to Mead and L. D. Agenbroad, and tion manual. University ofCalifornia Press, Berkeley. MEAD, J. I., AND L. D. AGEKIHlOAD. 1989. Pleistocene National Park Service contract CS-1200-A-4062 dung and the extinct herbivores of the Colorado Pla to Agenbroad. teau, Southwestern USA. Cranium 6: 29-44. 1993] QUKfERNARY VEGETATION ON COWIlAOO PLATEAU 161 _,~. 1992. Isotope dating of Pleishx-eoe dung deposits Pages 159-J.62 in J. O. Jennings, ed., Cowboy CAlve. from the Colonulo Plateau, Arizona and Utah. Radio University of Utah Anthropology Papers 104. cadxHI 34: 1-19. 'fANNF.I\, V. M. 1940. A biotic study of the Kaiparowits MEAD, J. I., L. D. AGENBROAD. O. K. DAVIS, AND P. S. region of Utah. Grt::a! Basin Naturalist 1: 97-126. MAHTIN.1986. DungofMammutlws in the arid South VAN DF:VF:NOF.lt. T. R. 1973. Late l>leistocene plants and W'CSt, North America. Quaternary Research 25: 121 animals of thl:l Sonoran Desert: a survey of ancient 127. packmt middens i.n southwestern Ariwna. Unl'ub MOl\HIS. M.S., J. E. SCIIMAUTZ, AND P. F. SrrcKNRY. 1962. lisheddodoral dissertation, Uniycrsityof Arimna, Tuc Winter field key to the native shmbs of Montana. son. 179 pp. Montana Forf'~t and Conservation Experiment Sta _._. 1m. Holocene \lKKJCIlan(l~ in the south\W..stl~m tion, Montana St-d.tc University and Intermountain deserts. Science 198: JB9~I92. Forest and Range E."(periment Station, USFS, USDA. VAN DF.VRNOP'lI. T. R., AND W. G. SPAULDING. 1979. De NEIl.SON, H. P" AND L. H. WuI.I.STEJN. 1983. Biogeogra velopment of vegetation and climate in the south""''est· phy of two sOllthwe.~t American oaks in relation to ern United States. Scien(:e 204: 701-710. atmospheric dynamk:s. Joumal of Biogeography 10: WATSON, It A., AND H. E. WRIGHT, JR. 1980. TIle end of 275-2117. the Plci:.tocene: a general critique of chronostrati Pill LLIPS. A. M. Ill. 1971. Pad::mts. plants and the Pleisto graphic clasSification. Boreas 9: 153--163. cene in the lower Grand Canyon. Unpublished doc WK8B, It H.1985. Late Holoctme flooding on the Escalante toral diss~rtation, University of Arizona, Tucson. 123 River, SOtltll-ccntr.d Utah. Unpnhlished doctorul dis pp. sertation. University ofArizona, Tucson. 204 pp. _--c. 19M. Shasta ground sloth extinction: fossil packrat WELSH, S. L. 1986. Quercus (Fagaceae) in the Utah nora. 107-11 midden evidence from the western Grand Canyon. Gre<\t Basin Natur... list 46: L Pages 148-158 ilL E S. Martin and R. G. Klein, eds., WF.LSII. S. L., N. D. AlWOOD, AND J. It MUIWOCK. 11:I7S. Natura~st Quaternary c:dinctions: a prehistoric revolution. Uni K,lipllrowit:s flora. Great Basin 3S: 125-179. WF.I.SII, S. J..... N. D. A'lwoon, S. GoODIIlCll. AND L. C. versity ofArizona Press, Tucson. 892 pp. HIG(:INS. 19':17. A Utah llora. Grf"..at Basin Nahlralisl SC)B£NG, R. J., AND T. H. VAN OJ<:VENDgR. 1989. Late Qua Memoir9. Brigham Young University, Provo, Utah. 894 ternary fossils ofPoafendledana (nmttongrass): 11010 pp. (:ene expansions of apomicts. Southwestern Naturalist WITTll~HS. K. 1989. Late Quaternary vegetation and climate 34: 35-45. of Forty-Mile Canyon and Willow Gulch, in the centml SPAULDING, W. G., AND K. L. PETEHSEN. 1980. Late Pleis Cobrado Plateau. 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