The area was surveyed by the Geological Survey of Hokkaido over 40 years ago. The underlying rock was found to be pre-Cretaceous diabase (dolorite), diabasic tuff and chert (indicated in flecked green on the map below). This was described as part of the 'Yamabe formation, Sorachi group of the Hidaka supergroup' of which the "diabasic complex, represented by the Tomamu diabasic rock" was "characteristic".

This map below has been adapted from the 1;50,000 Ochiai (Kushiro 28) map published by the Hokkaido Development Agency in 1967.

geology1.jpg9 July 2010