Along a 420 km transect in northwestern Ontario, Canada, sediments from four lakes were analyzed with respect to lithology, pollen, and macrofossils. Radiocarbon dates show that the region was deglaciated between ca. 11 500 and 8000 years BP, and periods of both rapid ice retreat and readvance influenced the history of Glacial Lake Agassiz. In the south the ice sheet was succeeded by a lengthy interval of park–tundra with stands of spruce, ash, and elm. The ash and elm seem to have disappeared during a suggested cool period (11 100–10 200 years BP). Farther north the park–tundra phase lasted not more than 50–100 years after ca. 10 200 years BP before boreal trees dominated. The climatic change around 10 200 years BP permitted the very rapid migration of spruce, larch, birch, and jack or red pine into northwestern Ontario from northern Minnesota. The migration routes for Pinus strobus (white pine), Alnus rugosa, and A. crispa were divided, however: one from the south (south of Lake Superior) and one from the east-southeast (north of Lake Superior). White pine reached its maximum distribution 6500–6000 years BP, when the limit was probably 150–200 km north of today's. The composition of the boreal forest during the altithermal was only slightly changed, but the influx of presumed prairie pollen reached a peak ca. 8000–7000 years BP. Since then Picea mariana (black spruce) gradually became the dominating tree species.