 The clays, which contain layers of good coal and abundant fossilized vegetation, show that during the Miocene period, Sakhalin formed part of a continent which comprised north Asia, Alaska, and Japan, and enjoyed a comparatively warm climate.The Pliocene deposits contain a mollusc fauna more Arctic than that which exists at the present time, indicating that the connection between the Pacific and Arctic Oceans was probably broader than it is now.Main rivers