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Pollen samples from the Eastern United States indicate vegetation changes in response to a glacial retreat around 16,500 years BP (Delcourt and Delcourt 1984). The retreat was not constant but was interrupted by at least six or seven advances, each lasting 700 years. However, no advance reached the glacial maximum of 18,000 years BP (fig. 24.8). A number of physical changes in the glaciers were associated with the glacial retreat. By 12,500 years BP, the extent of the Laurentide glacier had slightly diminished, but the height of the dome had decreased by more than half to 5,900 feet (fig. 24.9) (Hughes 1987). The glaciers now contained 25 percent of the earth's water available for precipitation (Kutzbach 1987).
Although somewhat less massive, the glacier continued to dominate the climate of the United States, and especially land east of the Mississippi River. A significant ice cap still covered most of Canada (Hughes 1987) (fig. 24.9). After 16,500 years BP, the glacial retreat was occasionally interrupted by readvances brought on by temperature fluctuations. Readvances were within 100 to 200 miles north of the maximum extent of 18,000 years BP. This occurred at least three times during the late glacial phase—at 12,900, 11,700, and 9,900 years BP (Edwards and Merrill 1977, Mickelson and others 1983) (fig. 24.8). Logically, the increased presence of ice during readvances came at the expense of precipitation over the continent.
Precipitation models indicate that current precipitation averages were not attained in the Southeast until after 9,500 years BP (Kutzbach 1987). Webb and others (1987) also reported that annual precipitation in the Southeast remained below present levels. Barry (1987) states that temperature and precipitation rose in the Southeast between 12,800 to 10,000 years BP but remained below modern averages. Kutzbach (1987) reported lower sea-surface temperatures in the western Atlantic, which contributed to reduced summer temperatures in the Southeast. Summers were droughty, which controlled vegetation composition and distribution. This information supports a drier climate as opposed to a climate with abundant precipitation as indicated by Delcourt and Delcourt (1984).
The retreat of the glacier was accompanied by a rise in ocean levels. By 12,000 years BP, the ocean rose to within 30 to 60 miles of the present shoreline. The rising ocean, accompanied by an increase in mean annual precipitation, lifted coastal river levels, increased streamflow, and raised water tables. However, both sea level and precipitation remained lower than at present (Edwards and Merrill 1977).
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