The Oshoro Circle has puzzled academics and laymen alike for years.
via The Japan Times
In 1861 at Oshoro, southwestern Hokkaido, a party of herring fishermen, migrants from Honshu, were laying the foundation for a fishing port when they saw taking shape beneath their shovels a mysterious spectacle — a broad circular arrangement of large rocks, strikingly symmetrical, evidently man-made. What could it be? An Ainu fortress?
They would have been astonished to learn, as in fact they never did, that the Oshoro Stone Circle is a relic from a time before even war — let alone fortresses — likely existed in Japan.
Oshoro today is part of the city of Otaru, on its western fringe, 20 km from the city center and 60 km west of Sapporo.
The Late Jomon period (circa 2400-1000 B.C.) was an age of northward migration. The north was warming, and severe rainfall was ravaging the established Jomon sites, primarily in the vicinity of today’s Tokyo and Nagoya.