This rock formation is called “Jusangjeolli” in Korean, which literally means “pillar-shaped joints.” The following information is a rough translation from the Naver Encyclopedia (Korean original):
(These formations) are formed from volcanic rock dikes, lava, or welded tuff. “Joint” refers to the gaps or split faces between the rocks that have no or almost no distortion... There are two types of joints, sheeting joints and pillar-shaped joints, depending on the direction in which the joints split, and pillar-shaped joints refer to those joints which have a hexagonal or triangular cross-section and are shaped like long pillars.
It was early in the morning when we saw the formations, so it was difficult to get any good photos because of the angle of the sun. I liked the way the water ran down through the joints in this picture, though.