An outcrop of metagabbro (metamorphosed gabbro) is found above this tunnel. The original gabbro was at the bottom of the sea between the two continental plates that later combined to form Hokkaido. When these plates collided and pushed up the Hidaka Mountains, the gabbro on the oceanic plate between them was sandwiched and thrust up. The intense pressure applied caused a transformation to the different rock type of metagabbro.

*Prior approval is needed to access this geosite. For more information, contact the Commerce, Industry and Tourism Department, Samani Town Government (tel. 0146-36-2120).