A flood debris assemblage of fossil Coleoptera is described from a 10 600 year old terrace deposit in southern Ontario. It is a small sample of the fauna that had colonized the region after deglaciation and although the species are extant, the assemblage is a surprising mixture of faunistic elements for which there is no known modern analog. Species with wide-spread boreal, arctic-alpine, northwestern boreal, southern boreal, and eastern distributions are recorded. The present ecologic and geographic distributions of the species indicate a valley environment with similar habitat diversity and climate to those which characterize the tundra–forest transition zone of northern Canada. This interpretation is supported by macroscopic plant evidence but conflicts with pollen evidence which implies a climate with warmer summers. To resolve the problem it is proposed that the valley fauna and flora survived in an isolated cold microenvironment surrounded by regionally warmer conditions and for which analogies presently exist on the north and east shores of Lake Superior.