Ice Age Impacts

Maximum ice cover of North America
during Pleistocene.

In addition to crustal uplift, one other dramatic geologic event has had a profound effect on the Appalachian landscape during the Cenozoic: the coming of the ice ages. In the late Tertiary Period, the Earths climate began to cool, and ice sheets formed over Antarctica and Greenland. In the Quaternary Period (the latest 1.8 million years) the cooling increased, and ice sheets began periodically to form in the northern parts of North America. In the United States, glaciers repeatedly have swept southward from northern Canada, at least once reaching as far as northern Kentucky, and then retreated and disappeared. Such episodes of glacial climate are called glaciations, and the relatively warm intervals between glaciations, such as the present (which is still cool enough to maintain the Antarctic and Greenland ice sheets), are called interglaciations.

The latest glaciation, called the Wisconsinan glaciation, ended about 10,000 years ago. At least three, and probably more, glaciations took place before the Wisconsinan. Because later glaciers tend to erode away evidence of previous glaciers, the best evidenceis provided by sediments on the sea floor. Deep sea cores in these sediments show that glaciations and interglaciations have occurred fairly regularly during the Quaternary, with one climatic cycle (one glaciation and one interglaciation) lasting an average of 100,000 years. The climatic cycles appear to be caused, at least in part, by slight periodic variations in the direction and amount of tilt of the Earths axis and the shape of the Earths orbit about the sun.These variationsaffect the distribution of the suns heat on the Earths surface.

Ice sheets never covered the Central or Southern Appalachians, nor were there mountain glaciers. Nevertheless, the effects of glaciations on this region were strong. Based on pollen and other paleovegetation data, some areas had mean annual temperatures as much as 10^o-12^o C cooler than now. As a result, vegetation was greatly different from todays. It wassimilar to that found in present-day Canada. Higher areas were actually above tree line and hadonly tundra-like vegetation (Delcourt and Delcourt, 1981). The intense cold associated with the glacial climates greatly increased the rate of physical weathering processes such as frost wedging. In addition to landforms and deposits, such as sorted patterned ground, that are clearly periglacial in origin, much of the hillslope colluvium and talus in the Appalachians is probably relict from glacial times.