In terms of geography, Hokkaido and Sakhalin were connected, Tsugaru Straits was frozen in the winter, and Hokkaido was connected to the present Honshu Island.
また地学的に見ても、北海道とサハリンは繋がっていたし、津軽海峡は冬には結氷して北海道と現在の本州が繋がっていた。