The Bizen and Tomihara Coalfields which were situated in northern Okayama City, had produced lignite during 1940s. Distribution of coal measures indicates that the coal measures are intercalated in the middle of the so-called "Mountain Gravels". The "Mountain Gravels" is subdivided into two formations ; the older, about 34～35Ma, and the younger, about 27Ma, of fission track ages. The older formation inculudes the Bizen Coal Measures. On the other hand, the Tomihara Coal Measures are intercalated in the younger formation.