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Although the glaciers would not totally disappear for another 3,500 years, the year 10,000 BP ended the Pleistocene Epoch, or Ice Age. It is believed the Pleistocene Epoch lasted approximately 2 million years with four major glacial periods. The last glacial period, the Wisconsin, lasted approximately 100,000 years. The new geologic period, the Holocene Epoch, is an interglacial period and continues today. We are over halfway through our interglacial period that began about 10,000 years ago (Pielou 1991).
An accelerated warming trend began around 9,500 years BP and reached maximum temperatures between 7,500 to 5,000 years BP. Temperatures during this period were warmer than at any time since (Pielou 1991). Higher temperatures melted the remnants of the glaciers in Canada, and the massive ice sheet east of Hudson Bay disappeared around 6,500 years BP (Hughes 1987) (fig. 24.11).
Due to the west-to-east retreat of the glacier, maximum temperatures were reached in Western North America earlier than in the East. Warm air penetrated northwestern regions, allowing trees to grow farther north than they do today. By 10,000 years BP, trees were growing in today's tundra. After 6,500 years BP, the glacier disappeared in eastern Canada, and tree species began moving north into the Hudson Bay area around 3,500 years BP. The northern forest limit then was 175 miles north of the present forest edge. This broad expanse of woodland reverted to tundra as temperatures declined since then. This warming period (7,500 to 5,000 years BP) is referred to by a variety of names, including hypsithermal, Altithermal, xerothermic, or the Climatic Optimum (Pielou 1991).
The melting glaciers during the hypsithermal caused sea levels to rise, reaching present levels around 5,000 years BP. Across North America the increased warmth also dramatically affected tree species composition and distribution, reshuffling wildlife habitats and species (Edwards and Merrill 1977, Pielou 1991).
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