Okuruwa Shell Mound

A Shell Mound Found Beneath a Running Track

In the spring of 1939, a crew leveling a riverside field in southeast Nagoya for a new athletics ground began turning up broken pottery and quantities of shells. The find, on the left bank of the Yamazaki River, marked the start of one of the most closely studied prehistoric sites in the Tōkai region: the Ōguruwa Shell Mound (Ōguruwa kaizuka).

The mound dates to the Early Jōmon period, the long age of foragers, hunters and fishers who lived across the Japanese archipelago thousands of years before rice farming arrived. It sits at the tip of the Yagoto hills, on ground that once met a far higher sea. During the warm climatic peak archaeologists call the Jōmon marine transgression, the coast reached deep inland, and the people here gathered blood cockles, oysters and clams from tidal flats close at hand.

Discarded over generations, those shells built up into thick layers. The lime they released preserved bone and other organic material that would otherwise have rotted away in Japan's acidic soils, which is why the site has so much to say about how its inhabitants actually lived.

From Trial Dig to National Historic Site

The site takes its name from Ōguruwa, the old place name of the district where it came to light. After the first discovery, Kitamura Ayao, a teacher at the nearby Mizuho Elementary School, carried out a trial dig. Ōguri Tetsujirō of the Aichi prefectural survey commission for historic, scenic and natural monuments then led four seasons of excavation, tracing a shell deposit of roughly 360 square metres. As a leading example of Early Jōmon settlement in the Tōkai region, about 350 square metres of that shell layer was designated a National Historic Site in January 1941.

What lay underground proved larger than anyone had expected. When the stadium was completely rebuilt in 1980, excavation of the adjoining land, some 2,200 square metres, showed that the shell layer ran on beneath the grandstand itself. The dig also uncovered Late Jōmon graves and pit dwellings cut into the earlier shell deposit, evidence of a village that people returned to over thousands of years. Those features were lost to construction once the survey ended, a loss that pushed the city to clarify and protect what remained. After further investigation from 2015 confirmed the shell layer extending continuously toward the stands, the designated area was formally enlarged in October 2021.

What the Ground Gave Up

For all its scientific weight, the mound is not a monument you stand and admire. The shell layers stay sealed underground, conserved in place. What rewards a visit is what came out of the ground, and how that record has been made visible.

The finds read almost like an inventory of Early Jōmon life: stone arrowheads, axes, drills, spatula-shaped knives, net sinkers and hammerstones, set beside cord-marked pottery, bone and antler tools, and fired clay tablets. Among the figurine fragments is a flat, plate-shaped clay figure pierced with several holes through the head. That type is far more common in eastern Japan, and its appearance here helps archaeologists trace how figurine ritual spread westward into the Tōkai region during the Early Jōmon. The animal remains are just as telling. Bones of deer and wild boar, of rays and sharks, sea bream and sea bass, together sketch a diet drawn from both forest and sea.

The 1980 excavation produced the site's most arresting find: a near-complete adult male skeleton, laid on its back with the limbs drawn up in a flexed burial. Eight of his teeth, four upper and four lower, had been deliberately removed, a practice linked to rites of passage into adulthood. Another burial held a person with the bones of a single dog resting across the chest, which has been read as a sign the animal was laid to rest alongside its owner.

Visiting the Site Today

The site now forms part of Mizuho Park, and a redevelopment tied to improvements at the neighbouring stadium has turned it into something a visitor can actually read. The historic site is an open lawn that anyone may enter, with the underground extent of the shell mound traced out on the surface in rubber chips. A wall along the adjacent slope, named the "10,000-Year Wall," carries drawings and three-dimensional models of the pottery and stone tools. A guidance facility close by displays genuine artefacts recovered from the site and held by the city board of education, where they can be examined at close range.

The wider park rewards a longer wander. The Yamazaki River, which runs past the site, is lined with cherry trees that draw crowds in spring, and the park's sports grounds and green spaces spread out within easy walking distance. For visitors who want to see how the finds fit the wider story of the region, the Nagoya City Museum in the same ward periodically exhibits material from Ōguruwa and the related Shimouchida shell mound, once located across the river.