Until about 20 million years ago, Oga Peninsula was part of the mainland. At Nishikurosawa Beach, you can see the stratum from the formation of the Sea of Japan 15 million years ago. In this stratum you can find fossils of scallops, sand dollars (a type of sea urchin), and operculina (large forams), fossilized plants such as pinecones, and even fossils of paleoparadoxia (a small hippo-like mammal). It was a warmer climate with a higher sea temperature. The flat surface along the beach is called a wave-cut terrace and was formed by wave erosion.