The name Oni no Tawarakorogashi Rocks (Ogre’s Rolling Bale Rocks) comes from the way the alternating stripes of pink rock (granite) and brown rock (basalt) look like marks made from an ogre rolling a bale of rice. The pink rock is granite, the oldest rock on Oga Peninsula (about 90 million years old), the blackish rock on top of it is a conglomerate of volcanic rock (about 70 million years old), and the brown rock that runs through the granite is basalt (about 20 million years old), so you can observe rocks from three different time periods at once. The basalt stripes in the granite of Oni no Tawarakorogashi Rocks were formed by magma that welled up through cracks in the rock and then hardened, forming what is called a dike.
