Arashima Kofun Group

Arashima Kofun Group

In 1911, farmers clearing land on a low hill south of what is now Arashima Station turned up a stone burial chamber holding a bronze mirror cast with images of deities and mythical beasts, along with an iron sword tipped by a plain ring-shaped pommel. The tomb they had stumbled on, Ōnari Kofun, would later be recognised as one of the largest square burial mounds of its age anywhere in Japan. It is one piece of a much larger story written into these hills above the Nakaumi lagoon in Yasugi, at the eastern edge of Shimane Prefecture.

The Arashima Kofun Group gathers the tombs scattered across this ridge into a single nationally designated Historic Site. Together they record how the rulers of eastern Izumo buried their dead across roughly six centuries, from the late Yayoi period into the Kofun period, and how the shape of those tombs changed as the region was drawn into a wider political order.

What the site contains

The group occupies the Arashima hills in the Arashima and Kushira districts of Yasugi, looking out over the Nakaumi lagoon. The designated area brings together three clusters: the freestanding Ōnari Kofun, four tombs of the Tsukuriyama subgroup, and eight of the Shiotsuyama subgroup.

Ōnari Kofun is the oldest of the lot. It is a square mound measuring about 60 metres east to west and 44 metres north to south, with a long pit-style stone chamber built up from stacked stones around a wooden coffin. The grave goods recovered from it, including the triangular-rimmed mirror and the ring-pommel sword found in 1911, mark it as a leading example of an early Izumo tomb.

A short way west lies the Tsukuriyama subgroup. Its No. 1 mound is a square tomb roughly 60 metres on each side. Pit-style chambers were uncovered there in 1936 and again in 1938, yielding bronze mirrors of two types, a spindle-whorl-shaped stone object, glass tube beads, and iron swords, daggers, and knives. No. 3 is a comparable large square tomb. No. 2 breaks the pattern: built in the sixth century, it is a square-fronted, square-backed mound about 50 metres long that produced a large quantity of haniwa, the unglazed clay cylinders and figures set on Kofun-period tombs.

The Shiotsuyama subgroup, on the eastern side, preserves the deepest layer of local history. Its No. 6 and No. 10 mounds are four-cornered projecting tombs, a Yayoi form in which the corners of a roughly square mound flare outward into ramp-like spurs. These two rank among the largest of their kind, with sides of more than 30 metres that reach about 40 metres once the projecting corners are included. No. 1 is an early Kofun square tomb that still carries traces of those projecting corners, a hinge between two eras of tomb building.

From Yayoi mounds to Kofun tombs

The four-cornered projecting tomb is a distinctly regional form. It is found above all in present-day Shimane and Tottori, the heart of ancient Izumo, and rarely elsewhere. Building one of these mounds meant raising earth into a square platform and then drawing each corner out into a spur faced with stone, a labour-intensive design that signalled the standing of the person buried beneath.

What makes Arashima unusual is that the change of form can be read on a single ridge. The late Yayoi projecting mounds of Shiotsuyama give way to the early Kofun square tombs of Ōnari and Tsukuriyama, and Shiotsuyama No. 1 sits between them with its softened corners. Visitors and archaeologists alike can follow, mound by mound, how one tradition flowed into the next rather than being swept aside.

That continuity carries meaning. As the Kofun period opened and the keyhole-shaped tomb spread across much of Japan as the mark of the highest rank, the rulers of Izumo kept building square mounds and square-fronted variants instead. They were holding to a local custom inherited from their Yayoi predecessors even as a new order took hold around them.

Why it is designated

The site was first protected in 1936, when Tsukuriyama No. 1 was named a national Historic Site under the name Zōzan Kofun. In 1999 the surrounding tombs of the Tsukuriyama and Shiotsuyama subgroups and the Ōnari mound were added, and the whole was renamed the Arashima Kofun Group.

The Agency for Cultural Affairs sets out the value of the group on several grounds. The Shiotsuyama No. 6 and No. 10 mounds are among the largest four-cornered projecting tombs known. Ōnari and Tsukuriyama No. 1 rank among the largest square tombs of the early Kofun period in the country. The site lets researchers trace the shift from the Yayoi projecting mound to the early Kofun square tomb, and it shows that the Yasugi area held a stable and dominant position within Izumo from the late Yayoi into the early Kofun period. Taken together, the tombs define the regional character of Izumo within the wider history of Japanese tomb building.

Visiting the hill

The tombs form the core of an open-air historical park known as the Hill of the Ancient Izumo Royal Mausolea (Kodai Izumo Ōryō no Oka). Several of the mounds have been laid out for walking, and the Tsukuriyama park holds the square and square-fronted tombs while the Shiotsuyama mounds preserve the four-cornered Yayoi forms. Shiotsuyama No. 1 has been restored to something close to its original shape, and from its summit the broad cone of Mount Daisen rises to the east.

The burial mounds sit at an elevation of about 50 metres, and the view from the top takes in the Nakaumi lagoon and, beyond it, the Shimane Peninsula. The outlook is counted among the so-called Eight Views of Nakaumi. Standing where the chieftains of eastern Izumo chose to be buried, looking out over the water they once controlled, it is easy to understand why this ridge was claimed by the powerful for so long.

The park is freely open and has a car park. To put the tombs in context, the Yasugi City History Museum displays objects excavated from the mounds and other local sites, tracing the area's history from antiquity onward.

Around Arashima

The hill stands close to the San'in coast, wedged between the San'in Expressway and the JR San'in Main Line. The nearest station, Arashima, is about a twelve-minute walk away, and the Yasugi interchange on the expressway is roughly eleven minutes by car. Trains on the San'in Main Line connect Arashima with Matsue and Yonago.

A few minutes' drive from the tombs is the Adachi Museum of Art, famous for a Japanese garden that is regularly ranked among the finest in the country and for its collection of modern Japanese painting. Pairing the ancient mounds with the museum makes for a full day in the Yasugi area, joining the deep past of Izumo to one of the region's best-known cultural draws.