The next layers of rock seen on the peninsula formed during the Miocene. These layers include andesite, tuff breccia, and a system of sills and dikes left over from an extinct volcano that was centered at Mount Sasamori, a foothill of Mount Mizugasawa. The volcano's dikes can be seen along the west coast of the peninsula. Layers from the Quaternary are mainly made up of sedimentary deposits and tephra from Mount Iwaki.