You can observe unconformity which is an overlapping joint between deposit in the shallow sea called Bikai Formation in Middle Miocene ca.16-12 million years before present and Ugetsuzawa Pyroclastic Flow Deposit shaped in 2.8 million years before present.

Bikai Formation is stratum of mud, sands, and pebbles in rare cases in a shallow sea. It includes fossil seashells, which are mainly bivalves. The fact that it contains seashells which live in warm seas indicates that warm current came to flow into this area at that time.

Ugetsuzawa Pyroclastic Flow Deposit was found in between Biei town and Asahikawa city, as well as in mountain area of Shintoku town.
Investigation shows the deposit was formed by eruptions of the volcano located at the centre of Hokkaido like Tokachi Pyroclastic Flow. The volcano used to exist around Tokachidake and Daisetsuzan Mountains.