ATLANTIC PERIOD: in Europe, a climatic optimum following the last Ice Age. This period was represented as a maximum of temperature, and evidence from beetles suggests it being warmer than average for the interglacial. It seems to have begun about 6000 bc, when the average temperature rose. Melting ice sheets ultimately submerged nearly half of western Europe, creating the bays and inlets along the Atlantic coast that provided a new, rich ecosystem for human subsistence. The Atlantic period was followed by the Sub-Boreal period. The Atlantic period, which succeeded the Boreal, was probably wetter and certainly somewhat warmer, and mixed forests of oak, elm, common lime (linden), and elder spread northward. Only in the late Atlantic period did beech and hornbeam spread into western and central Europe from the southeast.