The Rostock site is situated in a peat-covered wetland where fragments of a mammoth (Mammuthus primigenius) were unearthed in 1982. Excavation showed a descending sequence of peat, silty marl, gravel, clayey silt, and till. Approximately 300 kg of sediment was taken from the section for pollen, invertebrate, and geochemical analyses. The peat–marl transition is believed to be approximately contemporaneous with the spruce–pine transition in southern Ontario (ca. 10 600 years BP). A date of 10 790 ± 150 years BP on collagen from mammoth tusk fragments recovered from the same level substantiates this inference.Insects, especially Coleoptera, were common throughout the marl and represent at least 2000 individuals from 16 different families. On the basis of beetle ecologies and modern distributions, the marl was subdivided into lower and upper horizons. The lower horizon contains many species that are today restricted to western and northern North America. They are typical residents of the northern Boreal Forest Region and are frequently found on barren, sparsely vegetated ground. The upper horizon contains species found in and to the south of the boreal forest. Phytophagous species are more numerous and indicate a change from sparse to abundant vegetation. The insect assemblages suggest a shift from cold, tree-line conditions to a cool, temperate environment between approximately 13 000 and 11 000 years BP.