Great Britain and nearby parts of continental Europe, showing wide tracts that lie less than 600 feet below sea level. During the Ice Age there was free communication between the islands and the continent, and plants and animals migrated in both directions. A large river, of which the Rhine and Thames were tributaries, drained into the North Sea. The last separation of Great Britain from the mainland took place about 7000 years ago. Present lands are shown in gray, chief rivers and waters over 600 feet deep in back and shallowly submerged land in white. (Stokes, 1960, p. 402)