Rikuhira Shell Mound

Okadaira Shell Mound: The Birthplace of Japanese Archaeology

Nestled on a quiet plateau overlooking the southern shore of Lake Kasumigaura in Miho Village, Ibaraki Prefecture, the Okadaira Shell Mound (陸平貝塚, Okadaira Kaizuka) stands as one of Japan's most historically significant archaeological sites. Designated as a National Historic Site in 1998, this remarkable Jomon-period shell midden spans approximately 3,500 years of continuous human habitation — from roughly 7,000 to 3,500 years ago — making it one of the longest-occupied shell mound sites in the entire Kanto region.

What sets Okadaira apart from hundreds of other shell mounds around Lake Kasumigaura is not just its exceptional scale and state of preservation, but its pivotal role in the history of Japanese archaeology itself. In 1879, two young Tokyo Imperial University students — Chujiro Sasaki and Isao Iijima — conducted the very first archaeological excavation carried out entirely by Japanese researchers at this site, earning Okadaira the distinguished title of "the birthplace of Japanese archaeology."

A Jomon Settlement on an Ancient Island

The Okadaira Shell Mound occupies a tongue-shaped plateau measuring approximately 250 meters east-to-west and 150 meters north-to-south, at an elevation of 20 to 30 meters above sea level. During the Jomon period, when sea levels were higher than today, this plateau is believed to have been an island or a narrow peninsula jutting into Lake Kasumigaura, providing its ancient inhabitants with abundant access to marine and freshwater resources.

Eight distinct shell mound deposits are scattered across the slopes of the plateau, forming a ring-shaped pattern known as a "ring shell midden" (環状貝塚). This configuration was confirmed through systematic surveys conducted by the Okadaira Survey Team beginning in 1987. Among these, the A, B, and D shell deposits are particularly large, extending approximately 100 meters in length. The B deposit is especially remarkable, with shell layers reaching a depth of 4.7 meters below the current ground surface — a testament to thousands of years of accumulated daily life.

Why Okadaira Was Designated a National Historic Site

The Okadaira Shell Mound received its designation as a National Historic Site on September 11, 1998, for several compelling reasons that underscore its exceptional value to Japanese cultural heritage and archaeological science.

First, the temporal breadth of the site is extraordinary. Spanning from the Early Jomon period (approximately 7,000 years ago) through the Late Jomon period (approximately 3,500 years ago), Okadaira provides an unbroken record of human habitation over roughly 3,500 years. No other shell mound in Ibaraki Prefecture covers such an extensive time range, making it an invaluable resource for understanding long-term changes in Jomon-period lifeways.

Second, the sheer scale of the site is unmatched among the approximately 180 shell mounds found around Lake Kasumigaura. The eight shell deposits encircling the plateau, combined with the exceptionally deep stratigraphic layers, make Okadaira a representative site for the entire region.

Third, and perhaps most importantly, Okadaira holds a unique place in the history of Japanese archaeology. The 1879 excavation by Sasaki and Iijima — students of the renowned American naturalist Edward S. Morse, who had excavated the famous Omori Shell Mounds near Tokyo just two years earlier — marked the moment when Japanese scholars first took the lead in archaeological fieldwork in their own country.

Finally, the natural environment surrounding the shell mound, including the valleys and slopes that served as the living space for Jomon people, has been preserved in a remarkably complete state, offering visitors an authentic glimpse into the prehistoric landscape.

What the Shell Layers Tell Us

The shells and other organic remains found at Okadaira provide a fascinating window into the changing environment and diet of its ancient inhabitants. In the earliest layers, dating to the Early Jomon period, the predominant species are ark shells (Haigai) and oysters (Magaki) — species that thrive in muddy tidal flats. This suggests that the area around the site was originally characterized by calm, muddy waters.

As time progressed into the Early-to-Middle Jomon periods and beyond, the dominant species shifted to hard clams (Hamaguri), along with surf clams (Shiofuki) and Asian clams (Okishijimi), which prefer sandy-muddy environments. This gradual change in shellfish composition reflects the evolving coastal ecology around Lake Kasumigaura over millennia.

Beyond shellfish, the site has yielded a rich array of animal remains including bones of black sea bream (Kurodai), sea bass (Suzuki), and pufferfish (Fugu), as well as deer, wild boar, and rabbit. Artifacts recovered from the site include Jomon pottery, stone arrowheads (sekizoku), grinding stones (ishizara), polished stone axes, bone and antler tools, stone sinkers, and shell ornaments — evidence of a diverse and resourceful community that drew sustenance from both sea and mountain.

The Dawn of Japanese Archaeology

The story of Okadaira's place in archaeological history begins with one of the most colorful figures in Meiji-era Japan: Edward Sylvester Morse, an American zoologist who came to Japan in 1877 to study brachiopods. During a train ride from Yokohama to Tokyo, Morse spotted layers of shells exposed in a cliff near Omori Station, leading to the excavation of the famous Omori Shell Mounds — the first scientific archaeological dig in Japan.

Among Morse's students at Tokyo Imperial University were Chujiro Sasaki and Isao Iijima. Inspired by their mentor's work at Omori, the two young scholars traveled to the shores of Lake Kasumigaura in 1879, where Sasaki had initially gone to survey freshwater shellfish. There, they discovered and excavated the Okadaira Shell Mound, producing a report that compared Okadaira's pottery with the "Omori-type" pottery documented by Morse. Their English-language report, "Okadaira Shell Mound at Hitachi," published in 1883, became one of the earliest archaeological publications by Japanese researchers.

Sasaki would later go on to become a distinguished entomologist — the national butterfly of Japan, the Great Purple Emperor (Omurasaki), bears the scientific genus name "Sasakia" in his honor — but his contribution to launching Japanese-led archaeology remains one of his most enduring legacies.

Preservation Against the Odds

The survival of Okadaira as an intact archaeological site is itself a remarkable story. In the 1970s, a residential housing development was planned for the plateau, which would have destroyed the shell mounds entirely. Fortunately, the project was canceled before any damage was done. In the 1980s, a resort development plan emerged, but this time, the principle of "development and preservation in parallel" guided discussions from the outset. When the economic bubble burst, this project too was abandoned.

In the 1990s, local residents formed volunteer groups dedicated to raising awareness about the shell mound's importance. Their grassroots efforts, combined with support from researchers and the cooperation of former development companies, culminated in the 1998 designation as a National Historic Site, ensuring the site's protection for future generations.

Highlights for Visitors

Today, the Okadaira Shell Mound is part of the Okadaira Shell Mound Park, a beautifully maintained public space that invites visitors to walk through the same landscape where Jomon people lived thousands of years ago. The park features reconstructed pit dwellings (tateana-shiki jukyo) based on actual excavation findings, allowing visitors to step inside and imagine daily life in the Jomon period.

The Miho Village Cultural Properties Center (also known by its affectionate nickname "Okadaira Research Institute") is located within the park and houses a permanent exhibition of artifacts excavated from the site, including pottery, stone tools, and a cross-section of the actual shell layers. The center also shows a guidance video about the shell mound excavations. Admission is free.

Hands-on activities are available by reservation. Through the "Hands-on Okadaira" program and the volunteer group "Okadaira wo Yoisho suru Kai" (the Okadaira Cheering Society), visitors can try making Jomon-style pottery, clay whistles, or even Jomon cookies. These workshops cost approximately 300 yen per person and require advance booking for groups of 5 to 20 people.

Every October, the Okadaira Shell Mound Festival brings the site to life with Jomon-themed activities including fire-starting demonstrations, archery, straw crafts, and a scarecrow contest. The surrounding satoyama (rural woodland) area features walking paths where volunteers cultivate ancient rice varieties and practice charcoal-making, recreating the traditional landscape of rural Japan.

Nearby Attractions

While visiting the Okadaira Shell Mound, take the opportunity to explore the surrounding area of Miho Village and its neighbors. Kihara Castle Ruins Park (木原城址公園) is a hilltop park built on the site of a medieval castle, offering panoramic views of Lake Kasumigaura and beautiful tulip displays each spring. The Tatenui Shrine (楯縫神社), historically known as the first-ranked shrine of ancient Shida District, provides a peaceful atmosphere for cultural exploration.

Lake Kasumigaura, Japan's second-largest lake, offers recreational activities including windsurfing, fishing, and lakeside cycling along its scenic shores. The JRA Miho Training Center, one of Japan's premier horse racing training facilities, opens its public gallery "Turf Plaza" on weekends for visitors interested in equestrian sports.

Miho Village is also renowned as a production center for mushrooms and paprika. Local specialty products including original brand rice "Hikari Itten" and gourmet items using local mushrooms and paprika are available at the "Miho Fureai Plaza" direct sales shop.