Columnar Jointing at Kinujima
The columnar jointing at Kinujima is a rare dike formation and a designated National Natural Monument.
Columnar jointing formations often have hexagonal basalt columns arranged vertically — but the ones on Kinujima are even more unusual, consisting of four-to-six-sided columns that run horizontally.
As the rock formations are not too far from the coast, you can also see them up close via standup paddleboarding, etc.
The area around Higashikagawa has many other sites with fascinating geological features as well.