Diatomaceous Earth, Shionohama Beach

Here you can see diatomaceous earth, which serves as evidence that the land of Oki was at the bottom of the sea around 16–10 million years ago.
Diatomaceous earth is a rock formed by the accumulation of the fossilized shells of diatoms (algae) that lived in the sea. Due to its fire resistant and insulating properties, this rock is used as a material in walls and shichirin charcoal grills.