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Yoshino Kumano NP, Wakayama
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Researchers mapped 1,311 boulders around Hashigui-iwa; one reaches 7 m. Models suggest even the 1707 Hōei tsunami couldn’t move some of the largest, pointing to an even bigger past wave—whose date remains unknown.
Local legend says Kukai set out to build Kii Oshima a bridge in one night. Amanojaku, a trickster spirit, mimicked a rooster to make him think dawn had come, stopping the work and leaving the stone “bridge piers” behind.
The rock pillars do more than look like bridge posts: Wakayama records Hashigui-iwa as a breeding colony for Pacific swifts. These birds nest on sea cliffs and spend almost all their nonbreeding time flying.
About 14 million years ago, magma intruded soft mudstone underground and cooled. Waves later wore the mudstone away, exposing the tougher igneous rock as the long, broken line of “bridge posts.”
Japan had plenty of igneous dikes, but the 1924 cultural-property record said it knew no other sea-eroded row of offshore stone pillars like Hashigui-iwa. The protected area was expanded in 1935.
The ancient dike preserves a magma “fingerprint”: a 2021 study interpreted its folds, scour marks and fractures as traces of rising fingers of magma pushing through mudstone and eventually merging.
A study of Kushimoto’s folklore notes that some local versions put villagers named Ta-kichi (太吉) or Yohei (与兵衛) at the story’s center instead of Kukai. The Kukai-and-rooster version is the best known today.
