Mt. Yashima used to be an island in the Seto Inland Sea. Its flat-topped mountain formation, called a mesa, can be seen from many places along the shores of Seto Inland Sea and in the city of Takamatsu. The mountain is both a beloved landmark and an island filled with nature and history.
Mt. Yashima is a cliff-shaped lava plateau created when a volcanic eruption 14 million years ago left behind a type of andesite rock known as sanukite, which remained even after subsequent erosion of the mountain.
There are many historical sites on Mt. Yashima, including Yashimanoki Castle, an ancient mountain castle that Prince Naka no Oe (later Emperor Tenji) ordered built in 667 to protect the territory; Yashimaji Temple, said to be the work of Ganjin (Jianzhen), a famous Chinese monk from the Nara Period (710-794); and the remains of the battlefield that served as the stage for the Battle of Yashima during the Genpei War. The castle gate and castle walls made of stacked stone at Yashimanoki Castle have been restored, enabling visitors to have an authentic experience of the ancient castle.
　Based on its high geological and historical value, Mt. Yashima was designated as both a historical site and a natural monument in 1934.