Located at the end of Dongdo, the rock is called Dongnimmunbawi as it looks like Dongnimun Gate (Liberation Gate). The rocky islet, composed of tuff, has well-developed columnar joints. 30-40 m high steep sea cliffs (steep to vertical or overhanging rock slope at the coast) marine erosion are found on the cape protruding to the east. There is a sea cave on the front side of the cliff, formed as the rock layers fell apart along the columnar joints in the process of weathering and erosion caused by wave, wind, and saline solutions. Dongnimmunbawai, ‘a sea arch’, was formed at the end of the narrow cape after the sea caves developed and connected with each other. Dongnimmunbawi is surrounded by several rocky islets or sea stacks, formed as the weak rock layers eroded by waves.