Glacial and periglacial erosion of the Appalachians

Abstract

The marine 18O record indicates that the first ice advance into the Appalachians equivalent to that of the late Wisconsin occurred in the Pliocene at about 2.4 Ma. Only periodic localized glacial and periglacial activity occurred in the northern Appalachians from 2.33 to 0.85 Ma. After 0.85 Ma (middle to late Pleistocene) eight out of ten glaciations covered the northern Appalachians and brought periglacial conditions to the southern Appalachians. During the last 850,000 years, glaciation extended to the late Wisconsin limit within the Appalachians for 15% of the time and extended to the early Wisconsin limit just north of the Appalachians for 25% of the time. Pleistocene glacial erosion of the Appalachians of 120–200 m is necessary to account for the estimated volume of Pleistocene ocean sediment derived from the Appalachians and the estimated volume of Pliocene-Pleistocene glaciogenic ocean sediment derived from the entire Laurentide ice sheet. This amount of glacial erosion, occurring over the maximum expectable duration of Pliocene-Pleistocene ice cover of 247,000 years, yields glacial erosion rates from 0.5–0.8 m ka −1. Such depths of erosion by thick, warm-based ice streams were localized in the lowlands and estuaries of the northern Appalachians. In the Maritimes the presence of thin, cold-based ice caused minimal erosion of uplands. Periglacial landforms and tundra-boreal forest were distributed throughout the Appalachian Highlands and the northern Coastal Plain during the glacial maxima. Periglacial erosion of 0.15–0.30 m ka −1 near the ice in Pennsylvania decline to 0.1–0.2 m ka −1 in the southern Piedmont. Each cold period almost completely renewed the regolith by eroding slopes and filling valleys. The cumulative effect of the eight to ten major cold events could have produced as much as a few ten's of meters of periglacial ridge-top lowering during the Pleistocene. Both estimated glacial and periglacial erosion rates greatly exceed estimated current fluvial erosion rates. Therefore glacial and periglacial processes should be considered the dominant processes in shaping the present Appalachian landscape. The northern Appalachians may be evolving towards a form where the landscape provides just the shape necessary to permit the least work conveyance of the ice across the region during glacial maxima. The southern Appalachians may be evolving towards a form where the landscape provides just the hillslope form necessary to transport the debris provided by periglacial conditions.