Jungmun-Daepo Coast represents a very unusual geological occurrence - gigantic vertical columnar-joints formed after undergoing rapid cooling and contraction of lava. Volcanic rocks formed by cooling of hot lavas commonly have cooling joints, which are typically expressed as vertical columnar jointing. The best exposures of columnar-jointed lavas in Jeju Island are found along the coast of Jungmun and Daepo where dark gray trachybasalts crop out for about 3 km. The Jusangjeolli, vertical stone pillars, are as high as 25 m. The upper units are poorly developed and ash flow tuffs are observed in the upper levels. Columnar Joints were formed by the flow of lava from the Nokhajiak Oreum about 140,000 to 250,000 years ago. The columnar joints of Jungmun-Daepo Coast are also referred to as 'Jisatgae Rocks' from their old name, 'Jisatgae'. More columnar joints can be observed at Jungmun Yerae Dong Coast, Andeok Valley, Cheonjaeyeon Waterfall, and Sanbangsan Mountain in Jeju Island. The joint systems in Jungmun-Daepo Coast are mostly six-sided, but there are also some four-sided to seven-sided joints. In addition to the polygonal jointing in plan, the columnar-jointed lava of Daepo-dong shows well-developed colonnade (the lower zone of columnar jointing that has thicker and better-formed columns) and entablature (the upper zone of columnar jointing that has thinner and less regular columns) in vertical section. The columns of the lava are locally curved and inclined, indicating that the lava had a lobate geometry.