Piled on top of green tuff are rock layers that contain fossils of marine creatures. This tells us that sea water rushed into the depression that had formed between the spreading Japanese Archipelago and the Eurasian Continent. This event formed the Sea of Japan.
Rocks formed in various environments during this period. However, the most common is rock layer formed by the fossilized remains of diatoms.
Diatoms are a type of phytoplankton that inhabit most bodies of water on the face of the Earth. Their shells are made of a glass-like substance. When diatoms decompose, layers of these shells become diatomaceous earth. Despite its appearance, diatomaceous earth is very light because it is composed of these porous, lass-like shells and is filled with cracks. This makes it is a very good conductor for heat and it is often used to manufacture fire-resistant bricks and earthen charcoal braziers.
Diatomaceous Earth is found on the southwestern side of Dōgo Island and was formerly mined for use as construction material.