The various lobes and segments of the southern periphery of the Laurentide ice sheet reached their maximum extension at different times between 21,000 and 14,000 yr ago, but the CLIMAP date of 18,000 yr ago is taken as a reference level to review the distribution of major vegetational formations in central and eastern United States. Tundra was apparently confined to a narrow belt peripheral to the ice margin only in the Minnesota area and from northern Pennsylvania to New England, with extensions down the crest of the Appalachian Highlands at least as far as Maryland. Some areas south of the Great Lakes may later have been marked by treeless vegetation briefly as the ice retreated. The boreal forest to the south in the central United States was dominated by spruce; the jack pine that had prevailed during previous times was apparently eliminated by the time the ice reached its maximum. In the Appalachian Highlands and the Atlantic Coastal Plain, however, jack pine occurred along with spruce, which decreased in importance southward. The southern limit of the boreal forest in the Southeast was perhaps somewhere in southern Georgia and Alabama. Oak and other temperate deciduous trees were minor components of the boreal coniferous forests especially in the southern Appalacchians, but there is no evidence yet in the southeastern states for a relic mixed mesophytic forest 18,000 yr ago similar to the rich modern deciduous forests of the region, except possibly in the Lower Mississippi Valley. The climate in much of the Southeast was apparently dry as well as cool at that time; in Florida oak/pine scrub and prairie-like openings prevailed, and all but the deepest lakes dried up.

A model for predicting the growth and decay of ice sheets based on the astronomical theory of climate change is presented. The purpose of the study in part is to isolate the role of the ice-sheet physics and earth response under varying ice load by simplifying to the extreme the role of the hydrosphere-atmosphere. Ice sheet physics and the response of the lithosphere-asthenosphere under the ice load are modeled explicitly. Insolation anomalies (taken at a fixed latitude) directly force latitudinal displacement of the snow line. Accumulation rate a, and ablation rate a′ evaluated at mean sea level are specificed as external constants; a,a′ decrease linearly with ice sheet elevation. Rough tuning of the model to the general shape of the ice-volume record of the last two major glacials determines the external constants. Model predictions of the ages of several events in the last major glaciation compare well with the radiological ages. The six glacial terminatios (I–VI) over the last 600,000 yr are identified and the predicted ages compare reasonably well with the δ18O record for two deep-sea cores. A direct comparison of model power spectra of ice volume as a function of period with spectra of the δ18O record shows apparent underprediction of power near 100,000 yr. When a quantitative but heuristic method for taking into account the “red noise” spectrum evident in the geological records is used, a much more favorable comparison is possible. The model prediction lends support to the hypothesis that the nonlinearity of the ice-sheet physics is responsible for the 100,000-yr periodicity in the geological record of the late Pleistocene.

Temperatures for the past 2700 yr are estimated using well-dated pollen data from northwestern lower Michigan. The pollen data were sampled from sediment cores of four lakes along a 75-km transect, with fine-grained morainic soils around the two western lakes and sandy outwash soils around the lakes to the east. Climatic reconstructions based on the pollen data from the sandy sites show less temperature change than the reconstructions from the other sites, because variations in the composition of the vegetation at the sandy sites are edaphically restricted. One of the cores studied was dated by counting visible annual laminations (varves). The cores from the other lakes were dated based on three radiocarbon dates per core as well as the historically determined age of the settlement horizons. All the time scales were cross-checked using pollen-stratigraphic correlation between the four sites. A calibration function was developed using a network of modern pollen and climate data covering all of lower Michigan. Based on this calibration function, the 2700-yr reconstruction for Marion Lake indicates an estimated growing-season temperature range of 1.3°C between extreme 30-yr means. Mild conditions persisted prior to ca. A.D. 400, but a cold interval occurred between ca. A.D. 500 and 800. The well-marked warm period evident from ca. A.D. 1000 to 1200 was the last time when temperatures were about equal to the 1931–1960 mean. A prolonged longed cooling occurred after A.D. 1200 and reached 1°C below the 1931–1960 mean by the 1700s. A warming of 0.5°C is indicated from ca. A.D. 1750 to 1850. The estimated temperatures for the 1830s at Marion Lake agree with the instrumental data for that period and this provides some validation of the calibration-function results.

The sequence of vegetation phases in the late Glacial was studied in a sediment section from the bog Tourbière de Chirens by means of pollen analysis. 18O/16O ratios of samples of lacustrine marl, obtained from the same profile, reflect variations of 18O/16O in precipitation and thus provide an additional, independent paleoclimatic record. The observed 18O/16O variations agree well with the climatic history as deduced from pollen analysis. The climatic transition from the Oldest Dryas to the Bølling period sensu lato, as well as the beginning and end of the Younger Dryas cold phase, is marked by abrupt changes in the 18O/16O ratio which were observed also in other regions of the Alps. These drastic climatic changes probably took place simultaneously over large areas of Central Europe and occurred within short time spans.

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.

K-Ar ages (A. Abdel-Monem, P. D. Watkins, and P. W. Gast, 1971, American Journal of Science 271, 490–521; this paper) and revised paleontological determinations (J. Meco, 1977, “Los Strombus neogenos y cuatenarios del Atlantico euroafricano”, Las Palmas, Ediciones del Excmo. Cabildo Insular de Gran Canaria) show that “Quaternary” (R. Crofts, 1967, Quaternaria 9, 247–260; G. Lecointre, K. J. Tinkler, and G. Richards, 1967, Academy of Natural Science of Philadelphia Proceedings 119, 325–344) littoral deposits on Lanzarote and Fuerteventura are early Pliocene and late Pleistocene. Early and middle Pleistocene strand lines are not represented. Early Pliocene littoral and marine deposits contain a characteristic fossil assemblage: Strombus coronatus, Nerità emiliana, Gryphaea virleti, Patella cf. intermedia, and Rothpletzia rudista. Differences in elevation record differential post-Pliocene uplift of the coastal platforms on which they lie. Late Pleistocene beach deposits at low elevations belong to two groups, an older with Strombus bubonius and a younger without. Differences in elevation of early Pliocene littoral deposits are reflected by differences in elevation of late Pleistocene beach deposits nearby.