The Mt. Fox basalt field encompasses late Tertiary plugs and flow remnants and the Pleistocene Mt. Fox volcano. The flow remnants suggest an earlier largely westerly flowing plateau drainage, captured by southerly flowing drainages which cut through the old valley fills.
The basalts are mostly undersaturated types, basanitic and alkali basaltic rocks, but include transitional olivine tholeiite. Geochemically, the younger Mt. Fox lavas are the most undersaturated and show relative enrichment in alkali and alkaline earth elements.