San’in Kaigan Geopark maintains many natural and historical formations that span from when the Japanese archipelago was a part of the larger Asian continent in ancient times, to the formation of the Sea of Japan, to the present. The formation of various geographical phenomena at San’in Kaigan Geopark spans three distinct periods: when Japan was still physically attached to the Asian landmass; when Japan detached from Asia and the Sea of Japan was formed; and the period from that time to the present.

In ancient days, when dinosaurs roamed the earth, the Japanese archipelago was but a part of the Asian continent.
Volcanoes erupted on the surface of the earth, and magma slowly oozed deep into the ground to form granite.

Tectonic shifts and volcanic activity caused cracks to appear at the edge of the Asian continent. Hollows produced from this rupture gave rise to rivers and lakes, with elephants, deer, and other animals living in these areas. These depressions gradually expanded and produced the Sea of Japan.
Volcanic activity also produced numerous igneous rocks.

Once the Sea of Japan stopped expanding, the Japanese archipelago reached largely the shape it is today. Ongoing activity produced various volcanoes, and rias coastlines and terraces were formed by the ocean, with deep ravines and waterfalls in the mountains. Flatlands, basins, and sand dunes developed, and people came to live on these lands.