Pillar-shaped joint is a shape generated when lava cools down into a distinct figure; such form can be found at the basalt Canyon of the Daegyo River in Dongsong-eup. The Hantangang River basin is very important to understand the development of the Korean landscape and geology of the 4th period. The Daegyo River is a branch of the Hantangang River yet, from the basalt layer beneath the water, experts consider that Daegyo River was originally the path of the Hantangang River. The bedrock at the entrance of the Daegyo River converging to the Hantangang River is granite from the Jurassic Period.
The Daegyo River basalt valley is 25 ~ 40 meters wide and 30 meters deep where the bottom of the valley or river bottom is covered with granite unconformity; this was created in the 4th period while a sedimentary layer or Choogaryeong basalt unconformity layer could be found at the valley including lava flow or pillar-shaped joints.
The Daegyo River valley provides valuable evidence for understanding the 4th period geology and landscape development as well as offers beautiful natural scenery which is protected by its designation as Natural Monument No. 436.