“Early Wisconsin” is a time term introduced by T. C. Chamberlin more than a century ago. Right or wrong, it has been applied to almost 100 units of drift in North America. These embrace any unit too young to bear a true weathering gumbotil or deep pebble rotting (pre-Wisconsin), and yet too weathered and covered with moderately thick loess to be fresh “classical” Wisconsin (now Wisconsinan). Several such drifts, like the long-argued Iowan, have been denied a separate place in time by showing that they are younger soil on older eroded drift like “Kansan.” The revolution in dating generated by radiocarbon measurements of organic matter, from 1950 onward, eliminated a few other “early Wisconsin” cases, including those of Chamberlin. However, from 1950 to 1970 many >35,000 or >40,000 yr B.P. radiocarbon ages reinforced an early Wisconsin possibility. In Ohio we found six kinds of evidence for early Wisconsin glaciation, but only three proved to be good. The greatest confirmation during these two decades came from sea-floor coring; sea-surface temperatures and oxygen isotopes showed one cool long Wisconsinan stage 75 to 18 ka. By 1970 more precise radiocarbon dating confirmed some actual glacial sections on land, as well, and the records of the majority agreed there were climaxes in extent of ice at 70 ka and 20 ka.

Oxygen-isotope analyses of foraminifera record the timing and magnitude of ice-growth events during ice ages. The timing of the major isotopic shifts during the late Pleistocene is relatively well determined, although small improvements continue to be made. In the SPECMAP marine oxygen-isotope time scale, the major isotopic transitions due to ice-growth events of the last glacial inception are centered at ca. 118–112 ka (substage 5e/5d), ca. 98–92 ka (substage 5c/5b), and ca. 75–68 ka (substage 5a/4). The ice volume at each isotopic event remains only partly known. This is due to difficulties in isolating local temperature effects on δ5 18 O, changes in the isotopic composition of ice, and variations in ocean circulation and atmospheric vapor transport. Considering likely temperature, water-mass effects, and analytical errors, we can not discern any size difference in the part of the δ5 18 O shifts due to ice growth at the substage 5e/5d and the stage 5/4 transitions. Both were roughly the same size, about 0.6‰. Assuming that the mean isotopic composition of glacier ice was −30‰ to −35‰, ice-volume changes are about 25 × 10 6 km 3 across these transitions. The substage 5c/5d isotopic transition, the smallest of the three major ice-growth events of stage 5, was about a 0.3‰ shift, or 13 × 10 6 km 3 of ice, using similar assumptions. In spite of the uncertainties in translating isotope measurements quantitatively into ice volumes, it is clear that the isotopic shifts between 120 and 65 ka require relatively large and rapid changes in global ice volume.

Considerable uncertainty exists as to whether the last interglacial was relatively “short” (~10 ka) or “long” (∼20–60 ka), although most investigators generally agree that the last interglacial correlates with all or part of deep-sea oxygen-isotope stage 5. A compilation of reliable U-series ages of marine terrace corals from deposits that have been correlated with isotope stage 5 indicates that there were three relatively high sea-level stands at ca 125–120 ka, ca. 105 ka, and ca. 85–80 ka, and these ages agree with the times of high sea level predicted by the Milankovitch orbital-forcing theory. At a number of localities, however, there are apparently reliable coral ages of ca. 145–135 ka and ca. 70 ka, and the Milankovitch theory would not predict high sea levels at these times. These ages are at present unexplained and require further study. The issue of whether the last interglacial was “short” or “long” can be addressed by examining the evidence for how high sea level was during the stands at ca. 125 ka, ca. 105 ka, and ca. 80 ka, because sea level is inversely proportional to global ice volume. In technically stable areas such as Bermuda, the Bahamas, the Yucatan peninsula, and Florida, there is clear evidence that sea level at ca. 125 ka was +3 to +10 m higher than present. During the ca. 105 ka and ca. 80 ka high sea-level stands, there is conflicting evidence for how high sea levels were. Studies of uplifted terraces on Barbados and Haiti and most studies of terraces on New Guinea indicate sea levels considerably lower than present. Studies of the terraces and deposits on the east and west coasts of North America, Bermuda, and the Bahamas, however, indicate sea levels close to, or only slightly below, the present at these times. Thus, data from Barbados, Haiti, and New Guinea indicate a “short” last interglacial centering ca. 125 ka, but data from the other localities indicate that sea level was high during much of the period from 125 to 80 ka, and that there were two minor ice advances in that period. If it is accepted that the last interglacial period was relatively “long” and ended sometime after ca. 80 ka, then coastal deposits on the California Channel Islands record a shift in the nature of sedimentation at the interglacial/glacial transition. Marine terraces that are ca. 80 ka are overlain by two eolianite units separated by paleosols. U-series ages of the terrace corals and carbonate rhizoliths indicate that eolian sedimentation occurred between ca. 80 and 49 ka, and again between ca. 27 and 14 ka. Eolian sands were apparently derived from carbonate-rich shelf sediments during glacially-lowered sea levels, because there are not sufficient beach sources for calcareous sediment at present. The times of eolian sedimentation agree well with times of glaciation predicted by the Milankovitch model of climatic change.

Questions concerning the initiation and growth of the continental ice sheets at the onset of the last glaciation are explored by comparing Milankovitch forcing, paleoclimatic records, and the results of global circulation model experiments. Decreases in the solar radiation at lat 50° and 60°N over the past 140 ka provide an apparent reason for the cooling intervals, which, in accord with paleoclimatic evidence, started with a dramatic and rapid temperature decrease ca. 115 ka. Subsequent insolation decreases, although in phase with paleoclimatic evidence of cooling, appear to be unrelated to the magnitude of the response, particularly during marine oxygen-isotope stages 4 and 2. The gradual and relatively small insolation changes during stage 3 could not have alone caused the rapid climatic oscillations evident in ice cores, pollen records, and deep-sea sediments. The magnitude of the Holocene wanning appears to be greater than that generated by an equivalent insolation regime 80 ka. A series of experiments performed using the Goddard Institute for Space Studies (GISS) general circulation model driven by modified insolation input corresponding to the time of the initiation of the last glaciation failed to maintain snow cover at high latitudes during the summer, despite reduced summer and fall insolation at 116 ka. Even when the differences between the insolation fields at 116 and 114 ka were magnified by a factor of five, snow did not accumulate. When 10-m-thick ice was added in the model to locations where continental ice sheets existed during the last glacial maximum, the model failed to maintain this ice; melting rates over North America were such that the ice would have disappeared within five years. Only when the sea-surface temperatures and carbon-dioxide concentrations were reduced to the full glacial values was the model capable of maintaining the ice, but only in a restricted region of northern Baffin Island. Modeling results and evaluation of the paleoclimatic records imply that many key questions concerning the response of the atmosphere-ocean-biosphere-cryosphere to orbital forcing remain unresolved. Mechanisms and feedbacks involved in the general pattern and details of global climate change can be defined only as chronological resolution of the paleoclimatic record is improved.

Sediments, soils, and fossils are used to interpret paleoenvironmental conditions during the last interglacial-glacial transition in Illinois. The sediments include the classic Sangamonian-Wisconsinan and Wisconsinan/Farmdalian-Woodfordian successions; both consist of loess overlying pedogenically modified or generally colluviated sediment. Although the contacts between the loess and soil are defined as isochronous, they are diachronous. The age of basal Wisconsinan deposits spans from at least 50 to 22 ka, and the age of basal Woodfordian sediments spans from 25 to 20 ka. Recognition of the last interglacial episode, the Sangamonian Age, is based on Mollisols, Alfisols, and Ultisols that formed in Illinoian glacigenic deposits. Sangamonian flora and fauna, although sparsely preserved, include pollen well represented by deciduous trees, grasses, and Ambrosia. Sangamonian vertebrates included giant tortoise, mastodon, giant beaver, snapping turtle, and short-nosed gar. The Wisconsinan Age (Altonian Subage) began as Roxana Silt (loess) was deposited under periglacial conditions. The vegetation was characterized by coniferous trees that grew in weakly developed, often organic-rich, cryoturbated soils. Periglacial conditions persisted during the Altonian and Farmdalian Subages, from before 50 ka to about 25 ka, after which loess, outwash, and till were deposited under glacial conditions during the Woodfordian Subage and much of the remainder of the Wisconsinan Age. The vegetation that grew during the last glacial episode includes plants that are now found at and north of the treeline in Canada.

Alloisoleucine/isoleucine (aIle/Ile) ratios obtained from fossil mollusc shells collected at localities in southwestern Ohio and southeastern Indiana, where they occur in silt beds associated with the Whitewater and Fairhaven tills, indicate a pre-Wisconsinan age for these tills, which had previously been thought to be early or middle Wisconsinan. The aIle/Ile ratios in shells from beneath the buried soil (Sidney soil) and till exposed near Sidney, Ohio, are most similar to values in shells obtained from Illinoian sediments at Clough Creek in Hamilton County, Ohio; Mechanicsburg southwest, Illinois; and Trousdale Mine in Vermillion Co., Indiana. The first well-developed weathering profile in the sequence above the implied Illinoian age silt at the Sidney cut, therefore, probably represents Sangamonian, early and middle Wisconsinan weathering. Molluscs from an organic silt, exposed near the base of the Bantas Fork cutbank section, also have aIle/Ile ratios that are similar to those measured in shell recovered from the silt at the Sidney cut and from the silt inclusion in inferred Illinoian till at Clough Creek. These data indicate that the organic silt is pre-Wisconsinan. Therefore, the Fairhaven Till, which overlies the silt at the Bantas Fork locality, could be pre-Wisconsinan and the weathering profile developed in the Fairhaven Till may be correlative with the Sangamon Soil of Illinois. The New Paris Interstade silt overlies Whitewater Till at the American Aggregates quarry at Richmond, Indiana. Shells from the silt have aIle/Ile ratios that are intermediate between those obtained from inferred Illinoian age sediments at Bantas Fork, Sidney cut, and Clough Creek, and magnetically reversed sediments at Handley Farm, near Connersville, Fayette County, Indiana. These data suggest a pre-Illinoian age for the silt unit and the underlying Whitewater Till.

Three sections critical to the interpretation of upper Pleistocene deposits in northern Ohio are the Titusville, Pennsylvania, site and the Garfield Heights and Mt. Gilead sites in Ohio. At Titusville, Pennsylvania, the relation of peat dated at about 40,000 yr B.P. to the Titusville till is unclear. The interpretation of early Wisconsinan glaciation at Garfield Heights, Ohio, has been based on the Garfield Heights till and a subsequently derived accretion gley, both of which overlie a truncated paleosol (Sangamonian soil?) and two formerly exposed weathered tills. Current interpretation assigns the Garfield Heights till to the Illinoian. Previous interpretations for a middle or early Wisconsinan age of the Millbrook till of north-central Ohio have been placed in question by thermoluminescence ages of 146 ± 25 ka and 124 ± 16 ka on overlying loess at Mt. Gilead, Ohio. Much of what has been called Millbrook till has been traced to areas previously mapped as Illinoian. Thus, there is no evidence to support early to middle Wisconsinan glaciation in northern Ohio and adjacent northwestern Pennsylvania. During the early to middle Wisconsinan, landscapes in Ohio may have been geomorphically unstable, and deposits from this time may be in buried valleys or may have been eroded by late Wisconsinan glaciers.

The Bradtville drift, the Canning till, and their correlatives in southwestern Ontario have been previously thought to be early Wisconsinan in age. Here another alternative is offered, whereby the Bradtville drift is assigned to the Illinoian stage, the lowermost Member A of the overlying Tyrconnell Formation to the late Sangamonian; Eowisconsinan, or the earliest part of early Wisconsinan, and its Member B to the early Wisconsinan substage. The age of the Canning till is still unknown. Member A of the Tyrconnell Formation is an accretion gley that formed about 20 m below the present level of Lake Erie, thus requiring a low outlet for the Erie basin. At that time, the Erie basin was drained probably by the buried Erigan channel, which extends about 50 m below the present level of Lake Erie. Member B of the Tyrconnell Formation is varved glaciolacustrine silt and clay, the deposition of which required a rise of lake level above the present one. This rise could have been caused by the Ontario lobe overriding the Niagara peninsula, possibly as far as Gowanda, New York; however, the ice margin remained in the eastern part of Lake Erie. The above hypothesis is supported by available lithologic and paleoecologic data from the region adjoining the north-central and eastern part of Lake Erie, but supporting numerical age determinations beyond the range of radiocarbon dating are still lacking.

A substantial Sangamon interglacial (isotope stage 5) and subsequent Wisconsin glacial sedimentary record is preserved at Toronto, Canada. The age of individual stratigraphic units is poorly constrained, however. An interglacial sequence records climatic deterioration from warm temperate to subarctic. The interglacial Don Beds, resting on a presumed Illinoian till, were deposited on a storm-influenced shoreface of an ancestral Lake Ontario in water depths that increased over the recorded time interval from about 2 to 20 m. Pollen and faunal analyses identify a climatic deterioration in the upper Don from warm-temperate conditions, with mean annual temperatures some 2 °C warmer than at present, to cool temperate, with temperatures lowered by about 3 °C. Continued cooling is recorded in overlying deeper-water deltaic sediments of the Scarborough Formation, but later climatic amelioration and the return of mixed forest are suggested by the pollen record and by caddisfly fauna. The youngest deltaic sediments, lying immediately below a Wisconsin glacial complex, were deposited in a subarctic setting in an ice-dammed lake, with local mean annual temperatures depressed by at least 7 °C. This chapter identifies the likely continuity of the Don Beds and Scarborough Formation and places them in an expanded “Sangamon” interglacial possibly equivalent to the whole of stage 5 (i.e., 130–75 ka). The biological record at Toronto for this stage indicates one or more phases of cooling when continental-glacier ice may have developed in North America; this record can be favorably compared with the marine oxygen-isotope record with its evidence of increased global ice volumes during stage 5. The subsequent Wisconsin record at Toronto is not well constrained by radiometric age dating but indicates that maximum regional expansion of the Laurentide Ice Sheet in the eastern Great Lakes area occurred after 25 ka.

Our reinvestigation indicates that Sunnybrook drift represents a glaciolacustrine-subglacial-glaciolacustrine cycle, the Ontario lobe being the main glacial agent. Glacier ice overrode the present Toronto area during Sunnybrook deposition, deforming the substrate and depositing subglacial till. We propose that Sunnybrook drift comprises three or four members: a diamictic Sunny Point member (mainly till) sandwiched by two glaciolacustrine members (Sylvan Park and Bloor), and possibly a subglacial fluvial member (Pottery Road) associated with till deposition. The Sylvan Park member represents glaciolacustrine fine sediment that was overridden, deformed, and incorporated by ice that deposited the Sunny Point member. Parallel striated stone pavements, parallel and transverse stone and magnetic fabrics, stone facet and stoss-lee relations, and shear planes within the Sunny Point member indicate that it was formed by a combination of lodgement and subglacial deformation. Deposition under active grounded ice is further supported by the sharply erosive contact of the Sunny Point member with the top of the buried Scarborough delta, as well as glaciotectonic fractures and folds in substrata. Sunnybrook drift deposition concluded with the glaciolacustrine Bloor Member. Regional correlation suggests that the glaciolacustrine members were not formed far inland from the Scarborough Bluffs.

Tills that discontinuously underlie the late Wisconsinan till throughout New England represent the penultimate full glaciation of the region. In southern New England, the late Wisconsinan till and the tills that locally underlie it are informally referred to as upper and lower tills, respectively. For the most part, the ages of the lower tills are not firmly established, and regional correlations between occurrences of lower till, including those on Long Island, New York, are tenuous. Where a lower till underlies deposits having limiting middle Wisconsinan radiocarbon ages (e.g., the Montauk till member of the Manhassett Formation on Long Island at Port Washington, New York, and the lower till at New Sharon, Maine), many workers have assigned the till an early Wisconsinan age. However, lower tills throughout much of New England may be Illinoian or older in age and may correlate with a lower till exposed at Sankaty Head, Nantucket Island, Massachusetts, that is pre-Sangamonian in age. The till at Sankaty Head lies below marine beds containing marine faunas indicative of sea-water temperatures both warmer and slightly cooler than those off Nantucket today and that have uranium-thorium and amino-acid racemization (AAR) age estimates suggesting a Sangamonian age (marine oxygen-isotope stage 5). The lower till at Sankaty Head and the Montauk till member on Long Island were deposited during a full glaciation of New England that was at least as extensive as the late Wisconsinan advance of the Laurentide ice. Global ice-volume data from the marine oxygen-isotope record and the late Pleistocene eustatic sea-level record inferred from raised coral terraces support an advance of this magnitude during marine oxygenisotope stage 6, but not during stage 4. An early Wisconsinan age of the southern New England lower tills and, hence, of the penultimate glaciation there is problematic in terms of the pre-Sangamonian age of the lower till on Nantucket, and in terms of the late Pleistocene global ice-volume and sea-level records. An Illinoian age for the tills and for the penultimate full glaciation of New England is compatible with all the available evidence except some equivocal radiocarbon ages and AAR age estimates.

In the St. Lawrence Lowland of southern Quebec, an early Wisconsinan glacial advance deposited the Levrard Till. This glacial event, known as the Nicolet Stade, is tentatively correlated with marine oxygen-isotope stage 4. Radiocarbon and thermoluminescence ages bracket the Nicolet Stade between 90 and 70 ka. This advance was preceded and followed by periods of free drainage during which were deposited the Lotbiniere Sand and St. Pierre Sediments, two nonglacial units dated at or beyond the limit of the radiocarbon method. Available evidence suggests that the Deschaillons Varves were deposited ca. 80 ka in a large glacial lake that was impounded in front of the Laurentide Ice Sheet as it advanced up the St. Lawrence Valley. In the Appalachian Uplands, fluvial and lacustrine sediments of the Massawippi Formation were probably deposited at the end of the Sangamonian Interglacial. These sediments underlie the Chaudiere Till, a unit in which the occurrence of distinctive lithological indicators is taken as evidence that a regional episode of westward to southwestward ice flow prevailed at the onset of the last glaciation. The proposed paleogeographic reconstruction suggests that the development of an independent ice cap in the northeastern Appalachians played a key role during the early part of the Wisconsinan Glaciation in southern Quebec. This independent ice mass flowed southwestward across the Appalachian Uplands of southern Quebec and eventually coalesced with the Laurentide Ice Sheet, which was advancing up the St. Lawrence Valley.

The dating of events older than 50 ka, the limit of the radiocarbon method, has been a major drawback in assessing the chronology of the Quaternary. Several new methods have been applied to the dating of pre-late Wisconsinan organic beds in Nova Scotia. These methods include U/Th disequilibrium dating of wood and shells, amino-acid racemization dating of shells, and wood and electron spin resonance dating of shells. These methods are not without problems, and must be assessed together, and in concert with geologic evidence, in establishing a chronology. Evidence of the penultimate interglacial (marine isotopic stage 7) has been found in southern Nova Scotia. A raised marine platform, forest beds beneath till, and glacially-resedimented marine deposits were all formed during the last interglacial or Sangamonian stage (stage 5). Middle Wisconsinan U/Th and radiocarbon dates are questionable, so the chronology of post-Sangamonian events is not well constrained. Post-Sangamonian erosional and depositional stratigraphy indicates that at least four phases of ice flow have affected the Nova Scotia region. The earliest of these flows was a major advance that crossed the Gulf of St. Lawrence and Bay of Fundy (ice-flow phase 1). Later, separate ice caps and divides formed in areas adjacent to the province and the province itself (ice-flow phases 2–4). There is evidence for ice retreat between phases 1 and 2 in offshore areas and locally on land. Nova Scotia was probably covered with ice throughout the Wisconsinan stage (marine isotopic stages 4 to 2).

Information from river sections in the Hudson Bay lowland indicates that two pre-Holocene nonglacial episodes separated by glacial advances postdate the oldest recognized glaciation. Amino-acid data from in situ and transported marine shell fragments provide relative ages for glacial and nonglacial intervals. Absolute ages for non-glacial sediments as recent as mid-stage 3 were obtained from thermoluminescence (TL) data, although no finite radiocarbon ages have been obtained from wood. Déglaciation and deposition of the Bell Sea marine sediments are correlated to substage 5e by extrapolation from TL data. Ensuing stage 5 glaciation was dominated in Ontario by west-northwestward ice flow emanating from Quebec, and in Manitoba by southwestward ice flow. Deglaciation dated by TL at about 75 ka was followed by isostatic recovery and subaerial exposure in a climate which could have been warmer, but was no more than slightly colder than present. Extensive glaciolacustrine sediments deposited at the close of this interstade were TL dated at about 40 ka in Manitoba. If the TL method has systematically underestimated age, glaciolacustrine sedimentation may date to very late stage 5 or stage 4, or the two nonglacial episodes could be reassigned to substage 5e and stage 7. A resurgence of Quebec-derived ice that culminated as late Wisconsinan glaciation first flowed westward across the entire lowland, but was displaced in the north by southward ice flow. Southwestward and, locally, southward ice flow occurred during final ice retreat along a saddle extending across Hudson Bay and linking domes in Keewatin and Quebec.

The records of glaciation and climate change preserved in sediments on the Canadian and northwest Greenland margins of Baffin Bay pertaining to the last interglacial-glacial transition are remarkably similar. In both regions, warmer than present terrestrial and nearshore marine facies of the last interglacial sensu stricto (s.s.) are overlain by glacial sediments that represent the most extensive advance of continental ice during the last glaciation. Chronometric controls ( 14 C, thermoluminescence, amino acids) indicate an isotope stage 5 age for this advance. Evidence for extensive high-latitude glacial erosion during stage 5 is recorded by abundant pre-Quaternary palynomorphs in Baffin Bay sediment cores, in contrast to a much reduced flux during the remainder of the last glaciation. Warm nearshore marine conditions (seasonally ice free) also occurred near the end of stage 5 along both the eastern Baffin Island and northwest Greenland coasts after the maximum glacial advance; surface water in central Baffin Bay apparently was dominated by meltwater at this time. Subsequently (isotope stages 4, 3, and 2), terrestrial conditions were colder and drier, sea-surface temperatures were lower, and ice margins were retracted. Minimum summer insolation at high latitudes, coupled with mild winters and vigorous meridional oceanic (and presumably atmospheric) circulation characterized the inception phase of the last glaciation during isotope stage 5. In contrast, the 20 ka B.P. (isotope stage 2) “last glacial maximum” was characterized by a zonal circulation regime that resulted in cold and dry conditions over Baffin Bay; the margins of the northwest Greenland and northeast Laurentide ice sheets did not extend beyond the fiords at this time.

Widespread till sheets, glacial lake and glacial-marine sediments on Banks, Victoria, and Melville islands, and on the Beaufort Sea Coastal Plain of the Canadian mainland, may record a late Pleistocene glacial advance which extended to the area as early as the Sangamonian (broad sense) to early Wisconsinan. These sediments overlie beds of interglacial character and underlie in places nonglacial deposits, which have provided both nonfinite and finite ages, and glacial sediments of unquestionable late Wisconsinan age. In other places only a single till sheet is observed between the last interglacial and Holocene sediment suites. Although some workers have argued that the glacial units mentioned above are all late Wisconsinan, stratigraphic, paleoecologic, and chronologic data ( 14 C, Th/U, and amino acid analyses), from several localities, indicate that the glacial sediments are of likely Sangamonian (broad sense) to early Wisconsinan age and that the nonglacial beds underlying or overlying these date respectively from the Sangamonian and middle Wisconsinan. The dispersal centre during the ice advance was situated, as during other advances in northwestern Canada, west of Hudson Bay. The ice generally extended further during the Sangamonian (broad sense)/early Wisconsinan than the late Wisconsinan but not as far as it did during the early and middle Pleistocene. To help resolve apparent incongruities in interpretation of the late Pleistocene deposits and ice limits it is postulated that extensive Keewatin Sector Ice of the Laurentide Ice Sheet may have first advanced in northwestern Canada during the Sangamonian (broad sense)/early Wisconsinan and remained there until it finally disappeared in the late Wisconsinan.

Lithostratigraphic and geochronologic data from Yukon Territory indicate relatively limited glaciation in the northern Canadian Cordillera during the early Wisconsinan. If the Cordilleran Ice Sheet existed in south and central Yukon during the early Wisconsinan, it was less extensive than during either the Illinoian or late Wisconsinan. In contrast, ice cover during the early Wisconsinan in British Columbia and northern Washington may have been comparable to that of the late Wisconsinan, as suggested by the widespread occurrence of glacial deposits between middle Wisconsinan and presumed Sangamonian nonglacial strata. Sediments of probable Sangamonian age have been studied for pollen and plant and animal macrofossils. Climate during deposition of these sediments was warmer and drier than today. Plant communities probably had different distributions than at present, and permafrost may have been absent or more restricted over some areas in which it currently occurs. Little is known about the transition from the Sangamonian to the early Wisconsinan in western Canada and Washington, although limited data suggest that during the early and middle Wisconsinan there were perturbations in climate ranging from full glacial to temperate.

Weathering-rind thicknesses were measured on volcanic clasts in sequences of glacial deposits in seven mountain ranges in the western United States and in the Puget lowland. Because the rate of rind development decreases with time, ratios of rind thicknesses provide limits on corresponding age ratios. In all areas studied, deposits of late Wisconsinan age are obvious; deposits of late Illinoian age (ca. 140 ka) also seem to be present in each area, although independent evidence for their numerical age is circumstantial. The weathering-rind data indicate that deposits that have intermediate ages between these two are common, and ratios of rind thicknesses suggest an early Wisconsinan age (about 60 to 70 ka) for some of the intermediate deposits. Three of the seven studied alpine areas (McCall, Idaho; Yakima Valley, Washington; and Lassen Peak, California) appear to have early Wisconsinan drift beyond the extent of late Wisconsinan ice. In addition, Mount Rainier and the Puget lowland, Washington, have outwash terraces but no moraines of early Wisconsinan age. The sequences near West Yellowstone, Montana; Truckee, California; and in the southern Olympic Mountains have no recognized moraines or outwash of this age. Many of the areas have deposits that may be of middle Wisconsinan age. Differences in the relative extents of early Wisconsinan alpine glaciers are not expected from the marine oxygen-isotope record and are not explained by any simple trend in climatic variables or proximity to oceanic moisture sources. However, alpine glaciers could have responded more quickly and more variably than continental ice sheets to intense, short-lived climatic events, and they may have been influenced by local climatic or hypsometric effects. The relative sizes of early and late Wisconsinan alpine glaciers could also reflect differences between early and late Wisconsinan continental ice sheets and their regional climatic effects.