Senjojiki is a broad, sloping sandstone formation that extends into the Pacific Ocean. Its uneven surface has been formed by erosion caused by many long years of surging waves. Along with the Sandanbeki Cliffs, it is one of Shirahama's most magnificent sights.

Senjojiki Rock Plateau means "a thousand tatami mats." It is made of sandstone and conglomerate rock, formed 15 to 18 million years ago as sediments on a shallow seabed, then uplifted and shaped by wave erosion into a broad platform.