Hwangtogul is a sea cave , formed after parts of the tuff were eroded by waves. The ceiling of the cave is formed of trachyte. It is 6 m high, 32 m wide, and 44 m long. The cave has flat surfaces and a dome-shaped ceiling. Red clay is found in the cave. According to the records, when government officials returned to Hanyang after their term at Ullueng-do, they were required to take a sample of red clay to the royal court as a proof of their service. Once, a magistrate of Samcheok and his men landed on the island after the ship was caught in a storm. With nothing to eat, they survived on red-clay of Hwangtogul. Since then, the red clay of Hwangtogul has also been known as ‘Hwangtoghumi’ for its diverse tastes. Weathering of rocks containing iron oxides in the tuff is responsible for the red color of the clay. Inside the clay wall, the hanging wall has dropped relative to the footwall. The surface of the red beds is covered with salt, which accelerates oxidation of iron.