The Pleistocene Legacy: Glaciation
It has been less than 10,000 years, and in some places less than 6000 years, since the two major ice sheets that once covered North America finally disappeared leaving behind a legacy of sediments and landforms that cover slightly more than 60% of the continental land surface (fig. 2.1). The consequence of this last glaciation, and the several previous glaciations, that affected North America is immense in terms of the impact on soils, groundwater, river systems, topography, vegetation, fauna, and ultimately human society.
Debate as to when the Pleistocene Epoch began is fraught with considerable controversy because the criteria for identifying the start of the Pleistocene are debatable. In general, it is thought to be marked by (1) the onset of colder conditions than prevailed during the Pliocene, as evidenced by the increased presence of cold-water species in the waters of the Atlantic and Pacific Oceans or by reductions in ring growth in trees in the southwestern United States, and (2) significant changes in oxygen isotope ratios of waters that form stalactites and stalagmites in caves or in deep sea sediments. It is now generally agreed that the boundary between the Pliocene and the Pleistocene can be fixed at about 1.8 Ma at a time when cold-water mollusks are detected in sediments at Vrica, Calabria, on the shores of the Mediterranean Sea (Pasini and Colalongo, 1997; Van Couvering, 1997). In North America, this date may be accepted with the proviso that conditions prevalent in Europe were somewhat out of phase with those in continental North America. Lindsay (1997) notes that in North America several changes in the mammalian fauna around this time reveal slight climatic shifts symptomatic of changing climatic and oceanographic conditions with the onset of the Pleistocene. It is fundamental, however, to realize that these changes occurred nonpervasively throughout the continent. In some places, no changes are noted, whereas elsewhere dramatic changes occurred (Van Couvering, 1997).
The impact of continental glaciation is broadly twofold. First, those areas directly overrun by glacier ice have suffered a consequent devastating set of effects that is the substance of this chapter; second, those areas that remained unglaciated were affected indirectly through major environmental alterations whose imprint can also be detected today (see chapter 3). The effect of glaciation on any landscape is devastating as is the aftermath of glacial retreat when soils develop, plants and animals begin colonization, river systems are established, and groundwater systems evolve. Glaciation results in large volumes of sediment being deposited, much of it from distant sources, in topographic changes that disrupt older river systems, in lakes being formed, and in new landscapes that either replace or are imposed on earlier landscapes. Within the areas covered by glacier ice, zones of terrain can be differentiated where distinctive processes occurred and dominated