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.
