Periglacial Steppe Research Articles

Late Glacial vegetation changes in the Kostroma Volga region based on pollen study of Lake Chukhlomskoye bottom sediments

The article presents the results of a palynological study of Lake Chukhlomskoye bottom sediments. Pollen analysis in combination with the results from 14C and 137Cs dating made it possible to reconstruct the main stages of changes in plant communities in the Kostroma Volga region at the end of the Pleniglacial and during the Late Glacial: 19.5–16.9 cal. ka BP – periglacial steppe, 16.9–14.7 cal. ka BP – birch periglacial forest-steppe, 14.7–12.7 cal. ka BP – spruce and birch-spruce forests and open woods with periglacial-steppe and meadow communities, 12.7–11.7 cal. ka BP – birch periglacial forest-steppe with rare participation of spruce.

Environmental conditions of the maximum phase of the Last Glaciation: reconstruction based on the studies of sediments of lakes Chukhlomskoye and Galichskoye

The paper discusses new palynological data obtained from the bottom sediments of Lake Chukhlomskoye (Kostroma region). The location of the lake beyond the limits of the Late Valdaian ice sheet made it possible to reconstruct the vegetation changes during the maximum phase of the glaciation and compare them with previously obtained data from sediments of Lake Galichskoye. 25-21 thousand calibrated years BP, the region was occupied by open periglacial steppe with limited participation of birch and spruce open forests in the most protected habitats, with continuous spread of permafrost. Climatic reconstructions based on the modern geographical analogue of the fossil pollen flora indicate that mean annual temperature in the region was about ˗4 °C (7 °C below modern), and annual precipitation was at near-present level.

Northern Urals (Russia) quaternary deposits and biostratigraphical record caves and grottoes

Investigations of the unconsolidated Upper Pleistocene – Holocene cave deposits of the Northern Urals resulted in detailed biostratigraphical and palaeoenvironmental data. The article summarizes data from important stratigraphical levels exposed in 19 cave localities, most part of which represent several layers dated to different periods. These data cover the period from the beginning of the Upper Pleistocene (MIS 5-4) to the end of the Upper Holocene. At the beginning of Marine Isotope Stage 3 (MIS 3), open woodlands were distributed, which at the end of MIS 3 were replaced by Forest–steppe landscapes. At the beginning of MIS 2 (LGM), Periglacial steppe landscape was distributed. At the end of MIS 2 and at the beginning of the Holocene (MIS 1), semi-open landscapes were common. At the beginning of the Middle Holocene, they were replaced by taiga landscapes. Fish were present in the fauna of the Northern Urals in the Upper Pleistocene and Holocene. Amphibians in the Upper Pleistocene as part of the fauna of the region appear only at its beginning (MIS 5-4) and end (Bølling – Allerød period). Since the beginning of the Holocene (MIS 1), they have been constantly present in the fauna of the Northern Urals. Reptiles appear in the fauna of the region only in the Middle Holocene. During the Upper Pleistocene, the avifauna and fauna of small mammals were dominated by species of open landscapes, and the fauna of large mammals was dominated by species of open and semi-open landscapes. At the beginning of the Holocene, the avifauna and theriofauna increased the number of species associated with tree and shrub vegetation and near-aquatic species. Since the beginning of the Middle Holocene, the avifauna and theriofauna have been dominated by species of taiga landscapes. Palynological data and data on the fauna of small and large mammals allow us to separate the deposits of the Upper Pleistocene and Holocene. These data also allow us to separate the deposits of the beginning (MIS 5-4), middle (MIS 3), end (MIS 2)) of the Upper Pleistocene, Lower, Middle and Upper Holocene (MIS 1).

The Large Rivers of the Past in West Siberia: Unknown Hydrological Regimen

The hydrological regime of large meandering rivers of the West Siberian Plain in the Late Pleniglacial/Late Glacial was reconstructed from the hydraulic geometry of palaeochannels. The main tools for the reconstruction were the power law relationship between channel bankfull width and mean maximum discharge, taken in the downstream direction, and relationships between peak flood discharge and the contributing basin area. Reconstructed values of daily maximum surface runoff depth during the snow thaw period in the Late Pleniglacial/Late Glacial were 60–75 mm/day in the north of the plain with tundra and sparse forest and 20–40 mm/day in the south with periglacial steppe. The mean daily maximum surface runoff depth for the entirety of West Siberia was about 46 mm, which is more than five times greater than the modern value. Annual river runoff was calculated with the ratio between mean annual and mean maximum runoff depths, estimated for the modern region’s analogues of ancient periglacial landscapes and climates. Total annual flow of the Ob into the ocean was about 1000 km3. This is three times the current flow from the same basin, so the river was a significant source of fresh water to the Arctic Ocean during the last deglaciation.

Palaeoecological and genetic analyses of Late Pleistocene bears in Asiatic Russia

Brown bears are one of the few large carnivore species that survived the final Pleistocene wave of extinctions, perhaps in part owing to their wide ecological plasticity, variety of forms and polyphagia. Although the brown bear has become a well‐studied system, many questions remain regarding the ecological, trophic and genetic diversity throughout their distribution. For example, knowledge about Asiatic Russian brown bears from the Late Pleistocene arctic tundra steppe, an ecosystem with no analogue in modern times, is sparse. Here we compared diets, morphometry and genetic affinities of Late Pleistocene bears based on broadly sampled subfossil remains from Asiatic Russia. Collecting sites included the Ural Mountains, the lower reaches of the Irtysh River, the upper reaches of the Ob River, the Altai Mountains of western Siberia, the Indigirka–Kolyma Lowlands and northwestern Chukotka. An extremely large bear specimen from the middle Indigirka (41 090 14C a BP) that lived in landscapes of treeless shrubs and wet meadows had a diet composed principally of large herbivorous mammals. A bear from western Chukotka (25 880 14C a BP), much smaller in size, had a diet close to that of modern brown bears. These two Late Pleistocene NE Russian brown bears may comprise a previously undiscovered, but extinct, genetic lineage. At the end of the Pleistocene (MIS 3 and MIS 2), the brown bears from the Ob River Valley and Urals lived in periglacial forest‐steppes and those from the southern Urals in conditions of periglacial steppe. Brown bears from the Ob River valley and Urals, as well as ancient Altai bears, were characterized by a varied diet, from polyphagia to vegetarianism. In living brown bears, the proportions of different dietary foods are primarily related to food availability, which depends on the geographical zone and climatic conditions. We conclude that the same was true for Late Pleistocene brown bears of NE Siberia.

Evolutionary history of the Eurasian steppe plant Schivereckia podolica (Brassicaceae) and its close relatives

The genus Schivereckia (Brassicaceae) – presently included in the genus Draba – comprises two species, S. podolica and S. doerfleri. Schivereckia podolica is an eastern European species with disjunct distribution in Russia, Ukraine, and Podolian-Galician outposts. Schivereckia doerfleri is reported from alpine fields of the Balkan Peninsula and from mountain chains of northern Anatolia, Turkey. Schivereckia podolica shares its ecological and biogeographic characteristics with a number of other steppe plants. The ‘relic hypothesis’ interprets the present locations of Schivereckia and its associates as relics of a formerly continuous distribution belt of a cold periglacial steppe vegetation.We employed nuclear ITS and chloroplast trnL-trnF region, and DNA fingerprinting (RAPD) to elucidate the evolutionary history and historical biogeography of Schivereckia. RAPD data prove isolation by distance and mirror the present disjunct geographical distribution pattern of Schivereckia podolica. Phylogenetic analyses revealed that Schivereckia might be polyphyletic and nested within a highly supported clade of central Asian, Caucasian and North American Draba species. Time divergence estimation and ancestral area reconstruction support the interpretation of species of the Schivereckia subclade as periglacial relic. Our results argue for a continuous persistence of certain steppe species since the Pleistocene, and support the view that the modern steppes of Europe have derived partly from Pleistocene glacial steppes.

Images of camels on a mammoth tusk from West Siberia

This study introduces the engravings on a mammoth tusk from the lower reaches of the Tom River in West Siberia. The tusk was found in 1988 during construction works and was later transferred to the Tomsk State University, but it remained almost unknown to specialists until now. Radiocarbon dating by AMS reported the age of the tusk as 13,100–13,005 Сal BP (95% confidence level). The surface of the tusk is engraved with images of two-humped camels arranged in two pairs. In addition, arrows and wounds within the contours of the animals can be seen. The comparative analysis of the stylistic features of the camel figures shows that they correspond to the age of the tusk itself, making them, at present, the oldest camel images in Asia. The discovery of the engravings in this region is consistent with the theory of mobile population groups moving to western Siberia from the periglacial steppe to the south in the Late Upper Paleolithic.

The Early Khvalynian stage in the Caspian Sea evolution: Pollen records, palynofloras and reconstructions of paleoenvironments

In addressing the controversial issues of the Late Pleistocene paleogeography of the Caspian basin, an important role is given to the results of the spore-pollen analysis and based on them reconstructions of climate and vegetation changes in the Northern Caspian region during the development of the Early Khvalynian transgression. In this paper we present the main results of the palynological study of deposits (chocolate clays and the overlying and underlying layers from the Srednyaya Akhtuba section) of maximum stage of the Early Khvalynian transgression; the materials are illustrated by pollen diagram with the data of spore-pollen analysis and the detailed list of palynoflora recovered from the studied Lower Khvalynian sequences, and complemented with photographs of pollen belonging to the principal autochtonous taxa and, for comparison, to some redeposited palynomorphs. Palynological materials indicate subaqual (brackish marine and freshwater) sedimentation of the sediments studied in periglacial landscapes and, for the most part, under very harsh climatic conditions. The performed climate-stratigraphic reconstructions do not contradict the data of absolute dating on the accumulation of the studied deposits during the Late Valdai (Ostashkov) late Glacial period. During this interval in the territory of the study area, plant communities of the glacial climate — tundra-steppe, periglacial forest-steppe, periglacial steppe, periglacial parklands and periglacial forests — were developed. The widespread occurrence in the composition of the periglacial vegetation cover of the studied region of the yernik formations from Betula nana and shrub communities from Betula fruticosa, B. nana, Alnaster fruticosus, Juniperus, etc. testifies to the severe climatic conditions and, possibly, the existence of sporadic permafrost in the cold stages (stadials) of the Ostashkov Late Glacial period.

Stratigraphic interpretation of loess in the marginal zone of the Dnieper I ice sheet and the evolution of its landscape after deglaciation (Dnieper Upland, Ukraine)

Litho-, pedo- and palynological analyses constrained by radiometric dating of two loess–palaeosol sequences, Nahirne and Velyka Andrusivka, exposed in a cliff at the eastern edge of the Dnieper Upland, Ukraine, document regional environmental changes in the western marginal zone of the Dnieper lobe. The postglacial loess sedimentation cycle was initiated during MIS 8, immediately after the ice sheet recession. On the basis of palaeorelief analysis, subsequent morphogenetic stages of the original postglacial relief, associated with the modifying and masking role of the loess, and destructive slope processes were reconstructed. Periglacial steppe with consistently present scattered trees formed the Pleistocene landscape in the Middle Dniester area during the last three glacial periods. In such an environment, the following loess beds, correlated with marine isotope stages (MIS), were deposited: Dnieper ( dn ) – MIS 8, Tyasmyn ( ts ) – MIS 6, Uday ( ud ) – MIS 4 and Bug ( bg ) – MIS 2. During the last two warm periods: Kaydaky ( kd ) – MIS 7 and Pryluky ( pl ) – MIS 5, the landscape was not fully forested. As a result, the individual palynological features of these soils show a diverse character. The TL and OSL dates form a sequence with numerous inversions that are difficult to interpret. Although these data do not significantly influence the interpretation, they show that: 1) there are loesses that undoubtedly formed after the maximum extent of the ice sheet by short-distance transport of dust material from local fresh glacial deposits and the underlying Paleogene rocks; 2) date distortions result from the activities of an exceptionally rich pedofauna that has contaminated the material not only in the soil sections of the profile but also in the adjacent loess. It is possible to delimit a few stages of pedofaunal activity in each of the soil units.

Divnogorie pedolithocomplex of the Russian Plain: Latest Pleistocene deposits and environments based on study of the Divnogorie 9 geoarchaeological site (middle reaches of the Don River)

Latest Pleistocene fossil soils and deposits have been described in the geoarchaeological site Divnogorie 9 (the Middle Don drainage basin) under the name of Divnogorie pedolithocomplex. For the first time, a soil-sediment series with 2–3 levels of soil formation has been described in the forest-steppe of the Russian Plain and dated to the Bølling–Allerød interval (BØ–AL). That was the last soil formation of Pre-Holocene time. The soils are weakly developed (immature), of meadow and burozem genesis. They formed for a few hundreds of years in periglacial steppe environments, under forest-steppe vegetation. The lower soil (attributed to Bølling) is dated at 13.5–14 ka cal BP and defined as a weakly developed Petrocalcic Skeletic Colluvic Siltic Chernozem. Under coniferous-broadleaf forests the middle soil is described as Umric Calcaric Colluvic Siltic Leptosol and dated at Dryas-2. The uppermost soil dated at Allerød is weakly developed Skeletic Calcaric Colluvic Siltic Cambisol formed under coniferous-broadleaf forest-steppe. The data obtained made possible reconstructions of the Late Glacial climates and landscapes. The underlying sediments are the deposits of Divnogorie paleo-lake. The deposits include seven bone-bearing horizons with Late Upper Palaeolithic lithic tools. They are dated to the interval 17–14 ka Cal BP.

Lateglacial and Holocene vegetational and climatic changes in the southern taiga zone of West Siberia according to pollen records from Zhukovskoye peat mire

Pollen analyses and radiocarbon dates from Zhukovskoye peat mire (56°20′N, 84°50′E), situated in the south-eastern part of the boreal forest (taiga) zone of West Siberia, suggest that climatic oscillations of the Lateglacial were well expressed in the region. These events are tentatively correlated with the Alleröd warming and the Younger Dryas cooling in Europe. In the Alleröd, complex vegetation combined larch copses with birch and spruce in wetter places with dry steppe communities dominated by Artemisia and Poaceae. The climate was cool and continental, with moderately warm summers. Due to the onset of colder and drier climate in the Younger Dryas the wooded areas were reduced, while xerophile herbaceous communities with periglacial steppe elements expanded. Following the warming in the Early Holocene, woody vegetation was established in the area. At the early stage the woods were dominated by birch, larch and Scots pine. Later an increase in moisture caused formation of Picea taiga forest with Abies and Pinus sibirica. In the relatively warm and humid climate the process of mire development spread over a major part of West Siberia. Comparison with the published data on other sites in West Siberian plain shows that the warmest conditions existed in the region approximately 6–5 ka BP (non-calibrated 14C age). The warming is indicated by spread of more heat-demanding forest communities in the taiga zone and by a shift of the forest/tundra boundary to the north. Cooling in the late Holocene caused a decline of the relatively thermophile species in the forests and a retreat of the northern tree line to the south. Development of the raised peat bogs in the southern taiga sub-zone of West Siberia reflected in pollen profiles shows that this cooling continued in Subatlantic time.

Episodic build-up of alluvial fan deposits during the Weichselian Pleniglacial in the western Transylvanian Basin, Romania and their paleoenvironmental significance

A Weichselian Pleniglacial cyclic sedimentation sequence at Floresti, near Cluj-Napoca in western Transylvania, Romania represents an alternation of slope deposits (coarse fan structures and reworked loess-like sediments) and paleosols. Detailed grain-size analysis together with pedological, pollen and malacofaunal investigation allows the recognition of four main morphoclimatic cycles similar to those of stadial–interstadial nature. Boreal forest-steppe to periglacial steppe oscillations developed during the Weichselian Pleniglacial. According to the U/Th and radiocarbon ages, the period between 70 and 26 ka BP was cool and variably dry after which the climate became colder, probably in response to the Last Glacial Maximum. The Floresti section shows strong similarities to other Central and South-Eastern European pedocomplexes of Pleniglacial age, notably the Moravian section of Dolní Ve˘stonice and loess sections from Vojvodina, Serbia.

The vegetation of the Russian Plain during the second part of the Late Pleistocene (33–18 ka)

The paleofloral records from 108 sections of the Russian Plain for the Briansk interstadial (33–24 ka) and the maximum of the Late Valdai glaciation (24–18 ka) have been combined in a palynological database. A series of electronic maps of indicator plant species and different plant communities, determining certain vegetation paleophytocoenoses and the landscape as a whole, were reconstructed. Forest communities occurred fragmentarily. They were mainly associated with dissected areas. Subarctic and steppe plants were represented almost everywhere. The predominance of open landscapes (periglacial steppe and periglacial forest steppe formations) during the Middle and Late Valdai on the Russian Plain indicates a moderately cold climate during the Briansk interstadial and a cold continental climate during the Late Valdai glaciation.

Landscape, climate and mammoth food resources in the East European Plain during the Late Paleolithic epoch

Typical mammoth inhabited the East European Plain during the second half of the Late Pleistocene glaciation. Under conditions of extremely arid climate, periglacial, mostly open landscapes formed a vast hyperzone (cryohyperzone) that occupied the place of the modern tundra, forest and steppe zones. To assess the available foodstuffs for mammoth provision, data concerning productivity and nutritive value of modern herb and grass vegetation that may be considered as more or less close analogues of periglacial communities can be used. The central part of the Late Pleistocene periglacial hyperzone was most favorable for mammoths. Those regions were well endowed with water (in large rivers, as well as snow and ice) and presented the richest fodder base, because trees and bushes persisted in valleys, while higher watersheds were occupied by periglacial steppe. Climate warming and consequent degradation of permafrost resulted in instability of the land surface, thermokarst, and expansion of wetlands. The snow thickness increased due to more abundant snowfall in winter and made grazing difficult for mammoths. The first interstadial warming affected the less hardy early mammoths, while the progressive warming towards the Holocene appeared fatal to the typical mammoth.

Glacial anticyclone recorded in Palouse loess of northwestern United States

Climate simulations for the Last Glacial Maximum using general circulation models typically show a glacial anticyclone that produced an easterly wind anomaly south of the North American ice sheets. Evidence of this phenomenon has not been found in eolian sedimentary deposits that record surface wind patterns. Luminescence ages of loess and accompanying paleoecologic records from opal phytoliths and paleosol units across the Columbia Plateau, Washington State, United States, document a decrease, up to fivefold, in regional dust production and accumulation from 35 ka to 15 ka. This interval corresponds to simulations indicating that a glacial anticyclone weakened prevailing south-southwesterly winds that have shaped the eolian landscape of the Columbia Plateau since at least 75 ka. At the same time that the glacial anticyclone suppressed dust production and accumulation, enhanced cold and dry conditions resulted in soil formation dominated by intense bioturbation in periglacial steppe accompanied by shallow calcium carbonate precipitation. This is the first evidence from eolian deposits in North America verifying that the glacial anticyclone altered surface wind patterns and affected eolian systems during the Last Glacial Maximum.

Pleistocene climate change, natural environments and palaeolithic occupation of the upper Yenisei area, south-central Siberia

The upper Yenisei River area, near the geographical centre of Asia, is of major importance for mapping the Pleistocene climate evolution over Siberia. An extensive loess cover with a suite of buried palaeosols from the Northern Minusinsk Basin has provided one of the most complete, high resolution Late Quaternary palaeoclimatic archives currently described in north-central Asia. Coupled with the pollen evidence, the loess–palaeosol records indicate establishment of periglacial steppe–tundra during cold stadial intervals, replaced by parkland steppe and boreal forest during warm interstadial intervals. Zonal transformations of the Pleistocene environments due to marked climatic variations are reflected by the diversity of fossil faunal species, including non-analogue communities. Geoarchaeological records incorporated in the loess–palaeosol formations represent important palaeoenvironmental multi-proxy data complementing the fossil biotic and palaeopedological evidence. Abundant “pebble tool” assemblages provide explicit evidence of human occupation of the upper Yenisei area prior to the last interglacial (>130 ka BP). The morphological and technological variability of the palaeolithic industries displaying a differential degree of patination and aeolian abrasion attest to several stages of the initial peopling of the southern Siberian territory. The cultural finds associated with the Middle Pleistocene cold-adapted fossil fauna indicate that early humans were able to survive in local periglacial environments. Favourable climatic conditions during the last interglacial promoted expansion of the more advanced Mousterian (Levallois) tradition that persisted until the early last glacial stage (OIS 4). Warming during the mid-last glacial stage (OIS 3) accelerated formation of the Late Palaeolithic cultures dominated by developed blade reduction techniques. Intervals of significant climatic deterioration and onset of full glacial conditions are manifested by the absence of any archaeological record. The main focus of the current Quaternary investigations is to refine the established chronological framework and detail contextual settings of the cultural records within the larger study area.

Faunal burrows as indicators of paleo-vegetation in eastern Washington, USA

The Palouse loess deposits in the Pacific Northwestern United States contain buried paleosols with distinctive biological fabrics. The Washtucna Soil, formed 40,000–15,000yr ago during the last full glacial, contains up to 90% by volume distinctive cylindrical pedotubules that are 1–2cm in diameter formed by burrowing fauna. The overlying post-glacial loess and modern surface soils that support perennial grasslands lack these cylindrical burrows. We investigated the paleoecological significance of the burrows. Observations of several types of burrowing fauna under native vegetation indicated that nymphs of cicadas (Homoptera: Cicadidae) are responsible for cylindrical, back-filled burrows that match those in the Washtucna Soil. We measured the abundance of active cicada burrows in soils in four native vegetation zones (Soil sites), establishing that cicada burrows comprise 19% of the rooting zone volume in sagebrush steppe, but just 1–3% of the rooting zone volume in bunchgrass steppe, meadow steppe and coniferous forest. This suggests strongly that abundant cicada burrows in paleosols are a proxy for the geographic extent of plant communities that contained sagebrush. To assay the activity of cicada nymphs through time, we measured burrowed volume by depth from the soil surface down through the Washtucna Soil (i.e. from the present to ca. 40,000yrbp) at five stratigraphic research sites (Paleosol sites). Cicada-burrowed volume is generally less than 5% in the rooting zone of the surface soils and throughout Holocene loess, but is up to 94% in the rooting zone of the Washtucna Soil at Paleosol sites that currently support bunchgrass steppe outside the present-day sagebrush steppe zone. Cicada host preferences in vegetation zones today suggest that sagebrush was the dominant shrub as the Washtucna Soil formed. From this reconstruction, combined with information from studies of pollen and plant opal, we infer that periglacial steppe plant communities dominated by sagebrush spread, from 40,000 to 15,000yrbp, into areas of the Columbia Plateau that support bunchgrass steppe, meadow steppe, and even some coniferous forest today. Trace fossils of soil fauna in paleosols are indicators of fluctuations in biological activity in response to climatic changes of the Quaternary Period in Palouse loess deposits.

The steppes in the Late Palaeolithic

A set of papers on the human settlement of the steppes, epoch by epoch, is not complete unless it pays attention to the last glaciation, when periglacial steppe adaptations show remarkable cultural development, especially in the famous mammoth-bone houses. The subsistence basis depended - as a coincident paper in this issue (pages 719-32 above) shows – on an unusual and unusually early development of food storage, with all that implied for social evolution.Pressure of present commitments did not allow the writing of a new single paper on this time-period. Instead, we print here a collected set of five short contributions, by N.D. Praslov and colleanues that indicate well the evidence and the range of Soviet approaches to it. We thank Olga Soffer and George Frison for allowing us to draw on the outlines of papers given at the joint USSR-USA meeting on the Upper Palaeolithic-Palaeolndian held in the USSR in August 1989.

Late Quaternary vegetation in the Southwestern Columbia Basin, Washington

A 33,000-yr pollen record from Carp Lake provides information on the vegetation history of the forest/steppe border in the southwestern Columbia Basin. The site is located in the Pinus ponderosa Zone but through much of late Quaternary time the area was probably treeless. Pollen assemblages in sediments dating from 33,000 to 23,500 yr B.P. suggest a period of temperate climate and steppe coinciding with the end of the Olympia Interglaciation. The Fraser Glaciation (ca. 25,000–10,000 yr B.P.) was a period of periglacial steppe or tundra vegetation and conditions too dry and cold to support forests at low altitudes. Aridity is also inferred from the low level of the lake between 21,000 and 8500 yr B.P., and especially after about 13,500 yr B.P. About 10,000 yr B.P. Chenopodiineae and other temperate taxa spread locally, providing palynological evidence for a shift from cold, dry to warm, dry conditions. Pine woodland developed at the site with the onset of humid conditions at 8500 yr B.P.; further cooling is suggested at 4000 yr B.P., when Pseudotsuga and Abies were established locally.

Periglacial wedges and the late Pleistocene environment of Wyoming's intermontane basins

Nonsorted polygons in the uppermost 2 to 3 m beneath Pleistocene surfaces indicate permafrost at 1340 m and higher elevations in the intermontane and piedmont plains of Wyoming during the Wisconsin, and perhaps earlier, glacial maxima. The polygons, as much as 10 m in diameter, are delineated by wedges that vary in depths, range from narrow to moderately flared forms, and deform host materials. The wedges have silty fine-to-medium sand matrices (largely eolian) with pebbles or clasts from hosts of gravel or bedrock. Some wedges may reflect seasonal cracking in a periglacial active zone, but most are either permafrost sand-wedge relics or, less commonly, ice-wedge casts. Alternative explanations are rejected largely because similar features are apparently lacking in the lower and warmer plains from eastern Colorado southward. The wedges suggest an arid, windy, periglacial environment whose mean-annual temperatures are conservatively estimated as some 10° to 13°C colder than those at present. Although late Wisconsin-early Holocene floral and faunal evidence indicates lowered montane biotic zones, the eolian and periglacial features indicate a lack of extensive forest cover on the basin floors. In conjunction with vertebrate-fossil associations of grazing and tundra animals, the wedges may provide a parallel line of evidence for a former periglacial steppe, or “steppe-tundra”, in the Wyoming basins.