Illinoian Glaciation Research Articles

The easternmost occurrence of Mammut pacificus (Proboscidea: Mammutidae), based on a partial skull from eastern Montana, USA.

Mammut pacificus is a recently described species of mastodon from the Pleistocene of California and Idaho. We report the easternmost occurrence of this taxon based upon the palate with right and left M3 of an adult male from the Irvingtonian of eastern Montana. The undamaged right M3 exhibits the extreme narrowness that characterizes M. pacificus rather than M. americanum. The Montana specimen dates to an interglacial interval between pre-Illinoian and Illinoian glaciation, perhaps indicating that M. pacificus was extirpated in the region due to habitat shifts associated with glacial encroachment.

The pre-Late Wisconsin stratigraphy of southern Simcoe County, Ontario: implications for ice sheet buildup, decay, and Great Lakes drainage evolution

Recent three-dimensional mapping investigations in southern Simcoe County, Ontario, allow refinement of the existing regional stratigraphic framework. Analysis of 25 continuously cored boreholes has revealed a complex but consistent sediment succession that provides a record of the last two glacial cycles (Marine Isotope Stages 1–6). Five stratigraphic units (SU1–SU5) comprise the pre-Late Wisconsin record. The stratigraphy is floored by a presumed Illinoian glacial complex consisting of a lower, coarse-grained till (SU1), locally overlain by stratified glaciolacustrine and glaciofluvial sediments (SU2), but more commonly capped by a stone-poor, fine-grained till (SU3) of the Georgian Bay lobe. A widespread subaerial unconformity developed on the upper surface of SU3 contains organic-bearing, nonglacial deposits (SU4) ranging between 54 800 ± 3000 years BP (considered beyond the limits of radiocarbon dating) and 37 450 ± 590 14C years BP. SU4 is abruptly overlain by a thick succession of rhythmically laminated lacustrine muds graded upwards into glaciolacustrine silts and clays interrupted by regionally continuous sand bodies (SU5). The succession is capped (and locally truncated) by Late Wisconsin Newmarket Till. The sedimentary record of southern Simcoe County is correlated with other well-studied reference sections in southern Ontario and contains information that informs reconstructions of former ice extents in the lower Great Lakes region following the Illinoian glaciation. Several sediment units host aquifers, but limited thickness and spatial extent, as well as issues with naturally occurring dissolved gases and solids, restrict their use for groundwater supply.

Sphagnum compactumin Southern Illinois

Sphagnum compactum DC is rare in southern Illinois with five known locations some 400 km south of sites in northern Illinois. We examined habitat preferences of this species in southern Illinois, where it occurs exclusively on shaded, north-facing sandstone cliff faces and canyons, all just south of the maximum extent of Illinoian glaciation. We consider these populations to be long-lived and perhaps represent isolated relict occurrences of the species. We also report Sphagnum henryense Warnst. new to the area.

Late Quaternary chronostratigraphic framework of terraces and alluvium along the lower Ohio River, southwestern Indiana and western Kentucky, USA

The lower Ohio River valley is a terraced fluvial landscape that has been profoundly influenced by Quaternary climate change and glaciation. A modern Quaternary chronostratigraphic framework was developed for the lower Ohio River valley using optically stimulated luminescence (OSL) dating and allostratigraphic mapping to gain insights into the nature of fluvial responses to glacial–interglacial/stadial–interstadial transitions and Holocene climate change. River deposits, T0 (youngest) to T7 (oldest), were mapped along a 75 km reach of the lower Ohio River and were dated using 46 OSL and 5 radiocarbon samples. The examination of cores combined with OSL and radiocarbon dating shows that fluvial sediments older than marine oxygen isotope stage (MIS) 2 are present only in the subsurface. Aggradation during MIS 6 (Illinoian glaciation) filled the valley to within ∼7 m of the modern floodplain, and by ∼114 ka (MIS 5e/Sangamon interglacial) the Ohio River had scoured the MIS 6 sediments to ∼22 m below the modern floodplain surface. There were no fluvial sediments in the valley with ages between MIS 5e and the middle of MIS 3. The MIS 3 ages (∼39 ka) and stratigraphic position of T5 deposits suggest the Ohio River aggraded 8–14 m during MIS 4 or MIS 3. Near the end of MIS 3, the Ohio River incised the mid Last Glacial (mid-Wisconsinan) deposits ∼10 m and began aggrading again by ∼30 ka. Aggradation continued into MIS 2, with maximum MIS 2 aggradation occurring before ∼21 ka, which is coincident with the global Last Glacial Maximum (LGM). As the Ohio River adjusted to changing fluxes in sediment load and discharge following the LGM, it formed a sequence of fill-cut terraces in the MIS 2 outwash that get progressively younger with decreasing elevation, ranging in age from ∼21 ka to ∼13 ka. From ∼14 ka to ∼13 ka the Ohio River rapidly incised ∼3 m to form a new terrace, and by ∼12 ka at the onset of the Holocene, the Ohio River established a meandering channel pattern. The river formed a broad floodplain surface from ∼12 ka to ∼6 ka, and then incised ∼1 m and formed a fill-cut terrace from ∼6 ka to ∼5 ka. After ∼5 ka, likely in response to mid-Holocene drought in North America, the Ohio River incised ∼5 m, and by ∼4 ka the river began aggrading again. The Ohio River has aggraded ∼4 m since aggradation began at ∼4 ka. The chronostratigraphic framework and reconstructed history developed here suggest that the lower Ohio River is highly sensitive to glacial–interglacial transitions and abrupt Holocene climate change and responds rapidly to these allogenic forcings.

Phylogeography of striped skunks (Mephitis mephitis) in North America: Pleistocene dispersal and contemporary population structure

Pleistocene climate fluctuations rearranged ecosystems, and influenced the contemporary distribution of modern species. Although specialist species were often restricted to isolated refugia by Pleistocene climate change, generalist species may have been less constrained in their distribution and movements. We used a combination of genetic data and previously published fossil data to investigate the phylogeography and contemporary population structure of a generalist species, the striped skunk (Mephitis mephitis). We sequenced a portion of the mitochondrial cytochrome-b gene (601 base pairs) and amplified 8 microsatellite loci from 314 striped skunk specimens. Phylogenetic analysis of the cytochrome-b gene revealed the presence of 4 distinct phylogroups, and examination of microsatellite data indicated a pattern of secondary contact among these clades. We infer from these data that during the Rancholabrean stage prior to the Illinoian glaciation, striped skunks emerged from a southern refugium in the Texas–Mexico region and colonized the southeastern United States, forming a 2nd, later refugium in the east. This colonization was followed by a 2nd dispersal event from the southern source population to west of the Rocky Mountains during the Illinoian glacial period. During the Sangamonian interglacial stage, 2 distinct subclades formed on either side of the Sierra Nevada. During the Holocene, the subclade that colonized the Great Basin then expanded east across the northern Rockies and recolonized the Great Plains to create an area of secondary contact with the southern phylogroup. Secondary contact occurred to a lesser extent with individuals from the eastern phylogroup east of the Mississippi River. It appears that periodic Pleistocene glacial expansions and retreats caused a series of range expansions and secondary contact events in this native North American species to create a complex pattern of population structure today.

Phylogeography of the North American red fox: vicariance in Pleistocene forest refugia

Fossil, archaeological, and morphometric data suggest that indigenous red foxes in North America were derived from vicariance in two disjunct refugia during the last glaciation: one in Beringia and one in the contiguous USA. To test this hypothesis, we conducted a phylogeographical analysis of the North American red fox within its presettlement range. We sequenced portions of the mitochondrial cytochrome b (354 bp) gene and D-loop (342 bp) from 220 historical red fox specimens. Phylogenetic analysis of the cytochrome b gene produced two clades that diverged c. 400,000 years before present (bp): a Holarctic and a Nearctic clade. D-loop analyses of the Nearctic clade indicated three distinct subclades (> or = 99% Bayesian posterior probability); two that were more recently derived (rho estimate c. 20,000 bp) and were restricted to the southwestern mountains and the eastern portion of North America, and one that was older (rho estimate c. 45,000 bp) and more widespread in North America. Populations that migrated north from the southern refugium following deglaciation were derived from the colonization of North America during or prior to the Illinoian glaciation (300,000-130,000 bp), whereas populations that migrated south from the northern refugium represent a more recent colonization event during the Wisconsin glaciation (100,000-10,000 bp). Our findings indicate that Nearctic clade red foxes are phylogenetically distinct from their Holarctic counterparts, and reflect long-term isolation in two disjunct forest refugia during the Pleistocene. The montane lineage, which includes endangered populations, may be ecologically and evolutionarily distinct.

Middle to Late Pleistocene loess record in eastern Nebraska, USA, and implications for the unique nature of Oxygen Isotope Stage 2

New subsurface data reveal a nearly continuous stratigraphic record of Middle to Late Pleistocene loess sedimentation preserved beneath upland summits in eastern Nebraska, USA. Thickness and grain size trends, as well as pedologic evidence, indicate significant changes in loess sources, accumulation rates, and depositional environments. The newly defined Kennard Formation accumulated in the Middle Pleistocene, and may represent multiple thin increments of distal loess from nonglacial sources on the Great Plains. The overlying Loveland Loess, up to 18 m thick and deposited during Oxygen Isotope Stage 6 (OIS 6) (Illinoian glaciation), probably records the emergence of the Missouri River valley as a major glaciogenic loess source. The prominent Sangamon Geosol formed through long-term pedogenic alteration of the upper Loveland Loess during OIS 5 and 4. Thin loess of the Gilman Canyon Formation records slow loess accumulation and pedogenic alteration in OIS 3. The Peoria Loess (OIS 2) is similar in thickness to Loveland Loess, but may have accumulated more rapidly in an environment less favorable to bioturbation. More importantly, comparison of Peoria and Loveland loess thickness trends indicates much greater influx of nonglaciogenic loess from the Great Plains during OIS 2 than in OIS 6, suggesting colder and/or drier conditions in the Midcontinent during OIS 2 than in earlier glacial stages.

Geologic characteristics of the central stretch of the Ticona Channel, north-central Illinois

The Ticona Channel is located in north-central Illinois and occurs in Grundy, LaSalle, and Putnam counties. It is a buried bedrock valley that served as the principal paleodrainage system in north-central Illinois during the Illinoian and pre-Illinoian. This study focused on the part of the Ticona Channel within the Leonore 7.5 Quadrangle. The geometry and stratigraphy of sediments that fill the Ticona Channel were investigated using high-resolution, shallow seismic reflection profiling, traditional field geologic mapping techniques, borehole data, and water-well-log data. The valley is about 2 km (1 mi) wide and approximately 60 m (200 ft) deep. The U-shape channel is straight, trends east-west, and has only one mappable tributary. The valley is carved into the Pennsylvanian Carbondale Formation in the eastern part of the study area; it has incised into the Ordovician Prairie du Chien Group in the west. At its base, the Ticona Channel is filled with the Pearl Formation, which is coarse-grained sand and gravel that was deposited during the Illinoian glaciation. The Pearl Formation is overlain by Illinoian till of the Glasford Formation and is capped by Wedron Group sediments from the Wisconsinan stage.

Foundations from the Past: Clues to Understanding Late Quaternary Stratigraphy Beneath Cleveland, Ohio

Laboratory data from a rare set of samples collected during the 1920s from foundation borings for the Cleveland Union Terminal and Eagle Avenue Bridge provide new information about Late Quaternary stratigraphy beneath Cleveland, Ohio, on the south shore of Lake Erie. Samples represent ten unconsolidated units ranging in age from the Illinoian glaciation through modern anthropogenic fill. The oldest deposits may include the Illinoian Keefus, Mogadore, and Northampton tills and their associated lacustrine and outwash deposits. There is no evidence of Sangamonian deposits or the Late Wisconsinan Kent advance. Hiram Till deposited in close association with proglacial lakes is the only Late Wisconsinan diamict found in the area. Radiocarbon dates on twigs in lacustrine deposits suggest the landscape was deglaciated and vegetated by 14,500 yr BP. Lakes Maumee, Arkona, and Whittlesey immersed the area, and as the water level dropped to the Warren levels, deltaic deposits may have been reworked into beach ridges. The post-glacial Cuyahoga River downcut possibly 40 m into Illinoian deposits in response to the drop in water level to that of Early Lake Erie. Subsequently, this post-glacial valley filled with Holocene alluvium as Lake Erie rose to modern levels.

A >130,000-Year-Long Pollen Record from Pittsburg Basin, Illinois

Pittsburg Basin, in south-central Illinois, contains a sediment record extending from the present back to the end of the late Illinoian glaciation, when central Illinois was covered with Picea/Pinus forest. During the last interglaciation, a temperate deciduous forest more diverse than Holocene Quercus/Carya forest replaced the Illinoian late-glacial boreal forest. Prairie pollen types and the charcoal/pollen ratio, indicating fire frequency, temporarily increased. Then forest, with high Juniperus percentages, became dominant once more, as the charcoal/pollen ratio dropped. After the last interglaciation, the charcoal/pollen ratio increased again and prairie and wetland surrounded Pittsburg Basin through the entire Wisconsinan glacial age. The area was still prairie in late Wisconsinan time, but with some Picea and Pinus. During the Holocene, the region has been a mixture of prairie and Quercus/Carya forest. During the last interglaciation, Pittsburg Basin was surrounded by vegetation different from that surrounding it during the present interglaciation. Rather than indicating substantial differences in climate between analogous phases of different glacial/interglacial cycles, this variation may be due to changes in fire frequency, which could be caused by small changes in climate, human activity, or differences in soil.

Multiple pre-Wisconsinan glaciations along the northwestern edge of the Allegheny Plateau in Ohio and Pennsylvania

Stratigraphic studies of the glacial geology of the northwestern margin of the Allegheny Plateau of northeastern Ohio and northwestern Pennsylvania include evidence for multiple pre-Wisconsinan glaciations. Near the former eastern margin of the Scioto lobe at Mount Gilead, Ohio, Millbrook, Gahanna, and Chesterville tills have an inferred Illinoian age based on thermoluminescence ages of 146 000 ± 25 000 and 124 000 ± 16 000 BP from an overlying silt interpreted as loess. Northampton Till overlies Millbrook Till of the Killbuck lobe and its correlative Mogadore Till of the Cuyahoga lobe, but does not overlie the correlative Titusville Till of the Grand River lobe. There are signficant differences in carbonate contents among these tills. Millbrook Till and its equivalents contain more kaolinite than other tills, and the Northampton Till contains more clay. Millbrook Till and its correlatives represent the most extensive Illinoian glaciation of the plateau, whereas the other Illinoian tills were deposited by ice that flowed a shorter distance onto the Allegheny Plateau. Butler, Mapledale, and Slippery Rock tills, formerly assigned to an Illinoian glaciation, are pre-Illinoian in age. Scarcity of materials for the dating and the lack of recognizable paleosols separating the tills from Wisconsinan deposits inhibits the reliability of the chronological classification of these tills.

Permafrost, tectonics, and past and future regional climate change, Yukon and adjacent Northwest Territories

Late Tertiary changes in the general circulation of the atmosphere, regionally enhanced by uplift of the Wrangell – Saint: Elias and Coast mountains, were sufficient to promote permafrost development in the western Arctic. Permafrost developed in Yukon Territory and adjacent Northwest Territories during early Pleistocene glacial periods, after continued tectonic activity led to further modification of regional climate, but degraded in the interglacials. Permafrost has been present in northern parts of the region since the Illinoian glaciation, but most ground ice in central Yukon formed in the Late Wisconsinan. The present interglacial is the only one with widespread evidence of permafrost, which is maintained in the valleys of central and southern Yukon by the Saint Elias Mountains blocking continental penetration of maritime air from the Gulf of Alaska. This reduces snow depth in winter, while cold-air drainage in the dissected terrain of the Yukon Plateaus enhances the near-surface inversion, leading to continental minimum temperatures. General circulation models used to simulate climate represent the physiography of northwest Canada crudely. As a result, the simulations are unable to reproduce conditions responsible for the development and preservation of permafrost in the region.

Thermoluminescence chronology of Toronto-area Quaternary sediments and implications for the extent of the midcontinent ice sheet(s)

We report direct dating, by thermoluminescence (TL) methods, of key Quaternary stratigraphic horizons in the Toronto area. TL ages indicate that (1) most of the exposed sediments of the Don Formation at the sampled site were probably deposited during the closing phase of the Sangamon Interglacial (∼80 ka; oxygen isotope stage 5a), not the climatic optimum substage 5e (∼120∼130 ka),as generally supposed; (2) overlying deltaic strata of the Scarborough Formation, which accumulated in an ice-dammed ancestral Lake Ontario, were deposited between 60 and 50 ka (early Wisconsin, oxygen isotope stage 4); (3) the Sunnybrook diamict, overlying the Scarborough Formation, was deposited in the middle Wisconsin at ∼40-45 ka (oxygen isotope stage 3), likely by glaciolacustrine processes, rather than before 65 ka, as previously supposed; (4) sediments of the Upper Thorncliffe Formation lying stratigraphically just beneath the last-glacial-maximum Hallton Till were deposited at ∼23 ka, consistent with reliable 14 C dating. Thus, the earliest Wisconsin incursion of the (proto) Laurentide Ice Sheet into the Lake Ontario basin, as recorded by the Scarborough Formation, probably did not take place until at least 60 ka, and did not extend south and southwest of the Toronto area until after ∼25 ka. Ice-free conditions probably persisted over large areas of midcontinent North America from the end of the Illinoian glaciation at ∼130 ka to the late Wisconsin glacial maximum at ∼20 ka.

The phylogenetic relationship of the muskox and takin based on high resolution, G-banded, chromosome analysis

Muskoxen, Ovibos moschatus, currently inhabit parts of the Canadian mainland tundra, numerous Arctic islands, regions of Alaska, Norway, Sweden, northern and eastern Greenland, and the Taymyr Peninsula (Lent, 1988). The takin, Budorcas taxicolor, is an Asian species presently found in mountainous areas of west central China, Burma, Bhutan and India Qia-Yan, 1989; Neas and Hoffman, 1987). Both species appear to have originated from a common ancestor in Asia and exhibit morphological similarities. Muskoxen apparently dispersed from north central Asia to North America during the Illinoian glaciation Qia-Yan, 1989). Both the muskox and the takin are members of the Bovidae. The species in this family often have markedly different diploid numbers of chromosomes, but the fundamental number (number of chromosome arms) differs only from 58 to 62 (Hsu and Benirschke, 1967-1977; Wurster and Benirschke, 1968). The consistency in the. number of chromosome arms suggests Robertsonian fusion, or the joining of chromosomes at their centromeres, to be a dominant phenomenon in the evolution of the Bovidae. The relationship of the muskox to other members in the Bovidae is somewhat unclear. Many related genera of bovids adapted to colder climates are now extinct, leaving only the muskox and takin (Jia-Yan, 1989; Lent, 1988; Neas and Hoffman, 1987). Despite the apparent close relatedness of the two species, banded karyotypes of the muskox and takin have yet to be compared. In the Bovidae, individual autosome pairs cannot be reliably distinguished using conventional staining (Lin et aL, 1977), but they have been identified using G-banding (Wang and Federoff, 1974) in a variety of species (Evans et ai, 1973; Lin et aL, 1977). High resolution or elongated banding techniques have further improved resolution and facilitated chromosome comparisons (Mensher et aL, 1989). The object of our research was to compare the individual chromosomes of the muskox and takin, and to determine the cytotaxonomic relationship of the two species using high resolution G-banding. Karyotypes were prepared from blood samples from three male muskoxen and two female takin: the muskoxen are part of

Major Northern Hemisphere deglaciation caused by a moisture deficit 140 ka

A variety of marine evidence indicates that much of the deglacial activity that ended the Illinoian glaciation occurred under cold climate conditions prior to the glacial termination 2 warming at 127 ka in the {delta}{sup 18}O deep-sea core record. This anomalous deglaciation is explained by a reduction of water-vapor input to the ice sheets caused by weaker summer atmospheric circulation due to a known smaller latitudinal insolation gradient associated with Earth's orbital variations. A major factor in the eventual marine and atmospheric warming of termination 2 was the formation of unusually warm deep water in the eastern North Atlantic and its circulation to Antarctica, with the probable consequence that the marine-based West Antarctic ice sheet melted, resulting in a eustatic sea level 6 m higher than present.

Long continental pollen sequence from Washington State (U.S.A.): correlation of upper levels with marine pollen-oxygen isotope stratigraphy through substage 5e

Fossil pollen from a 7.70-m core and a pit excavated 2.20 m deep in a mire at Humptulips, located on glacial drift of the Olympic Peninsula in western Washington State, presents a continuous record of vegetation since the Middle Pleistocene. Chronostratigraphic relations, established from sedimentation rates based on finite and infinite radiocarbon dates in the upper 2 m of the deposit, cover the last glacial cycle, Illinoian glaciation, and apparently older millennia. The pollen stratigraphy, contained in 16 pollen assemblage zones, shows a sequence of vegetation types including tundra, park tundra, woodland, and closed confierous forest. Comparison with pollen and oxygen isotope stratigraphy of marine core Y72 11 1 from the Pacific Ocean off Oregon establishes direct continental-marine correlation of upper levels at Humptulips through substage 5e.

Stratigraphy, paleoecology, and glacial history of the Gillam area, Manitoba

Sections along the Nelson River in northern Manitoba, outcropping upstream and downstream from Limestone Dam, record a long succession of late Quaternary events. The oldest sediment exposed consists of sandy, nonfossiliferous Sundance till of northwestern provenance and related to a Kansan or Illinoian glaciation. The paleosol developed in the Sundance till is assigned to the Yarmouthian or Sangamon interglacial on its stratigraphic position and depth of weathering. Fossiliferous, clayey Amery till of eastern provenance overlies the Sundance till and underlies the nonglacial Nelson River sediments. Aspartic acid D/L ratios of wood fragments from the Nelson River sediments correlate with an aspartic acid D/L ratio of similar wood from the Missinaibi Formation in Ontario. Beetle analysis indicates the Nelson River sediments were deposited north of the tree line under conditions more severe than those found in the area today. The deposits are believed to be of latest Sangamon or possibly Mid-Wisconsinan age. The Wisconsinan Stage is represented by the Long Spruce and Sky Pilot tills deposited by ice from the east. These tills are texturally and compositionally similar but are different colours. The overlying Henday sediments record glaciofiuvial deposition and mass wasting along the eastward retreating ice margin. Varves indicate the area was covered by glacial Lake Agassiz for less than 100 years after the ice retreated. The breakup of the Laurentide Ice Sheet in Hudson Bay and the final drainage of Lake Agassiz took place 7800–8000 years ago when the Hudson Bay Lowland was inundated by the marine water of the Tyrrell Sea. The area emerged from the Tyrrell Sea about 6500 years BP.

Stratigraphy and correlation of the glacial deposits on the Montana Plains

Till units representing at least three pre-Illinoian continental glaciations, at least one Illinoian glaciation, and a late Wisconsin glaciation have been identified on the Montana Plains. Early Wisconsin or middle Wisconsin till units have not been identified. The southernmost limit of late Wisconsin glaciation is in Montana, more than 170 km south of the Lethbridge moraine in Alberta, Canada (Figs 1 and 2). An interpretation that the Lethbridge moraine is the limit of late Wisconsin glaciation (Stalker, 1977, 1980, 1983b; Stalker and Harrison, 1977; Rutter, 1980, 1981; Jackson, 1983) is incompatible with stratigraphic and morphologic evidence from Montana.

Summary of the Quaternary History of Michigan, Ohio and Indiana

Deposits found within the Teays-Mahomet valley system show that system was blocked by ice at least two times prior to −730,000 y. Evidence of several other glaciations and of an extensive erosion interval between that time and the onset of Illinoian glaciation is also found in the subsurface. The Illinoian was a time of multiple glaciation. The Wisconsinan ice sheet, in general, did not reach quite as far south as did that of Illinoian age. Wisconsinan history can be viewed as consisting of an early episode of glaciation (Early Wisconsinan), a long interval in which the three-state area was, for the most part, ice free (Middle Wisconsinan) and a period of time (Late Wisconsinan) marked by four major episodes of ice advance, culminating at about −21,000, −15,500, −13,000 and −11,800 y, respectively. The first three of these were, in turn, marked by oscillations of the ice front. Much of our detailed knowledge of the position of the ice front during the interval from −15,500 to −10,000 y is based on study o...

Multiple glaciations of the Cordon del Plata, Mendoza, Argentina

Evidence exists for four glaciations in the Río Blanco basin west of Mendoza, Argentina. Morphology, superposition of tills, soil-profile development, loess thickness, and boulder weathering have been the techniques most useful in mapping the tills. Glaciers of Vallecitos (= Wisconsinan) age extended to 2600 m and left distinctive moraines. Tills of two pre-Vallecitos glacier advances cover the floor of the valley from 2600 m to below 2100 m, and remnants of one of the tills extend nearly to the junction with Río Mendoza (1400 m) 13 km below the lowest Vallecitos moraines. These deposits were considered to be mudflows rather than tills by Polanski; however, the quartz sand grains examined with SEM have surface textures characteristic of glacial abrasion. The sediments, thus, are more likely glacial than mudflow deposits. One still older till caps ridges as much as 200 m above the present valley floor. Vallecitos glaciers did not smooth the walls of the wide valleys through which they flowed; the valleys had been enlarged by the larger ice tongues during one or more of the earlier glaciations. Frost shattering of the rhyolite and quartzite has altered much of the distinctive glacial valley shape, and talus lies between valley walls and Vallecitos lateral moraines. Holocene glaciations seem to be in phase with those of the Northern Hemisphere, and the Vallecitos glaciation coincided with oxygen isotope stage 2 and the Wisconsin of North America. The next older glaciation may correlate with oxygen isotope stage 6 and the Illinoian glaciation of North America. A till that underlies it is difficult to correlate and is clearly much older, but postdates the last major uplift of the range. It may represent isotope stage 12, and the period that followed it was more moist than other interglacial ages in this area. The oldest, Los Mesones, may correlate with Mercer's “Greatest Glaciation” of Patagonia, 1.0 to 1.2 m.y. ago. Although it is beyond the established oxygen isotope stages, a long cold period is present on the curves at this time, nd it coincides with one of the glaciations of the “Nebraskan—Kansan” complex of North America.