Sanbasekikyō Gorge: A 1.5-Kilometer Riverbed of Green Schist on the Gunma–Saitama Border
The Kanna River cuts a narrow trench through forested ridges in northwestern Saitama and southwestern Gunma, and for roughly 1.5 kilometers below the Shimokubo Dam the channel is lined with great blocks of greenish stone. These are the boulders that give the gorge its name. Locals have called them Sanbaseki for at least four centuries, and each of the forty-eight largest blocks carries an individual title — "Kabuto-ishi" (Helmet Stone), "Tora-ge-ishi" (Tiger Fur Stone), "Suzuri-ishi" (Inkstone), "Hora-gai-ishi" (Conch Stone), and so on — counted in order from "Ichiban-ishi" downstream to "Amida-ishi" near the dam.
The site sits on the prefectural boundary: the right bank is the Yuzurihara district of Fujioka City, Gunma; the left bank is the Yanō district of Kamikawa Town, Saitama. The river itself, a major tributary of the Tone, traces the line.
What Makes the Stones Green
Geologically, Sanbaseki is a green schist — a low-temperature, high-pressure metamorphic rock laced with white veins of quartz. The bedrock here belongs to the Sanbagawa metamorphic belt, a band of similar formations that runs roughly 800 kilometers from the Kantō region through Shikoku and into Kyūshū. Geologists consider it one of the most coherent metamorphic belts in the world.
The name Sanbagawa entered the scientific literature in 1888, when Kotō Bunjirō, a geologist at Tokyo Imperial University, surveyed the nearby Sanba River and labeled the distinctive crystalline schist he found there. The rock began life far away. Basalts and volcanic ash erupted on the Pacific floor were carried under the Japanese arc by plate movement, compressed and altered deep below the surface, and finally pushed back up. The blue-green tint comes from minerals such as chlorite, epidote, and actinolite formed under those conditions.
Designated in 1957 — and Why a Single Site Holds Two Categories
The gorge was designated on 3 July 1957 (Shōwa 32) under both classifications administered by the Agency for Cultural Affairs: a Place of Scenic Beauty (meishō) and a Natural Monument (tennen kinenbutsu). The dual listing is unusual. It was given on three grounds: as a representative example of the geological occurrence of rocks and minerals; as a notable instance of rock and cave formation; and as a gorge with valleys, pools, and rapids of outstanding character. Management responsibility was assigned to Fujioka City and Kamikawa Town in 1959.
The boulders themselves were quarried as garden stones from the early Edo period onward — records from the Kan'ei era (1624–1645) already mention Sanbaseki extraction — and Japanese gardens across the country still use the stone. The 1.5-kilometer stretch in the gorge is, by official protection, the only place where Sanbaseki remains in its natural state. Removing even a small fragment is illegal.
Walking the Gorge
The standard approach starts from a parking area immediately below the Shimokubo Dam and follows a short stretch of road that hugs the river. Six numbered descents lead from the road down to the bank, and most visitors walk between them rather than attempting the full length in one go. The shoreline is uneven — the route involves stepping among the boulders themselves, not a paved promenade — so sturdy shoes are useful.
From the road you catch glimpses of the riverbed framed between cedars and hardwoods. The view changes sharply with the season: new green in May, deep summer foliage reflected in the pools, and a hard contrast of red and orange against blue stone in late November. The stones reveal their truest color when wet, the quartz veins flashing white against the chlorite green. The Sōseki Bridge, a short footbridge across the gorge, is the most photographed vantage point. Downstream, the red truss of the Tōsenkyō Bridge marks the lower end of the protected reach.
Be aware that some elevated walkways have been closed since storm damage in recent years, and bear sightings have been reported on the surrounding hill trails. The Fujioka City tourism office at Onishi maintains current information.
The Dam Above — and the Effort to Bring the Gorge Back
The Shimokubo Dam, completed in 1968, towers directly above the gorge. It is a working dam, supplying drinking water to metropolitan Tokyo, and its construction profoundly altered the river below. With normal floods held back, sediment that once tumbled down the channel stopped reaching the gorge. Algae and silt darkened the green stones, and the riverbed grew duller year by year.
Beginning in 2003, the Japan Water Agency, in cooperation with local communities, introduced a periodic "sediment-flushing flow" — controlled releases that send gravel and sand through the gorge to scour the boulders clean. The program continues today, and the riverbed has gradually regained much of the brightness that earlier travelers described.
What to See Nearby
The Shimokubo Dam itself is worth a stop. It is an L-shaped concrete dam — an unusual configuration designed to fit the topography — and at 605 meters its crest is the longest of any concrete dam in Japan. Guided interior tours descend into the inspection galleries when scheduled. Just downstream, the Michi-no-Eki Jōshū Onishi roadside station has a small exhibit corner on Sanbaseki, including polished samples that show what the stones look like in a finished garden.
A few kilometers away, Sakurayama Park is famous for fuyu-zakura, a variety of winter-blooming cherry that flowers from late October through December — a curiosity celebrated in the regional Jōmō Karuta playing cards together with the stones of this gorge. For walkers, the Kantō Fureai-no-Michi long-distance trail passes directly through, linking the gorge with the Jōmine highlands across the river.
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Q&A
- How do I get to Sanbasekikyō without a car?
- Public transit is limited. The usual route is a bus from JR Honjō Station (about 40–50 minutes) to the Onishi Post Office stop, then a connecting local bus toward Shimokubo. Allow plenty of time for return connections, or use a taxi from Honjō or Honjō-Waseda Station (around 35 minutes by car). Most visitors come by private car via the Kan'etsu Expressway, exiting at Honjō-Kodama and driving about 40 minutes.
- Can I collect a Sanbaseki stone as a souvenir?
- No. The entire 1.5-kilometer reach is designated as a Natural Monument, and removing stones — even small pebbles — is prohibited by law. Polished Sanbaseki samples are sometimes available for purchase at the nearby Michi-no-Eki Jōshū Onishi roadside station and similar shops in the Onishi district.
- When is the best season to visit?
- Late October through mid-November brings autumn colors against the green stone, which is the most photographed combination. Fresh green in May is equally striking, and the rocks show their deepest color after rain. Winter access is possible but the riverside path can be icy.
- Is the riverside walkway safe for casual visitors?
- Parts of the elevated walkway have been closed following storm damage in recent years, and shoreline footing among the boulders is uneven. Sturdy shoes are essential. Several short descents from the road allow you to reach the water without committing to a long traverse.
- Why is the water sometimes cloudy?
- The flow through the gorge is regulated by the Shimokubo Dam upstream, so water clarity varies with releases. During scheduled sediment-flushing flows, deliberately introduced sand and gravel cleans the boulders but temporarily clouds the water. After such releases, the riverbed regains its characteristic green brilliance.
Basic Information
| Name | Sanbasekikyō (三波石峡) |
|---|---|
| Designation | Place of Scenic Beauty and Natural Monument (national) |
| Designated | 3 July 1957 (Shōwa 32) |
| Designation criteria | Geological occurrence of rocks and minerals; rocks and caves; gorges, waterfalls, rapids, and deep pools |
| Location | Yuzurihara, Fujioka City, Gunma Prefecture / Yanō, Kamikawa Town, Kodama District, Saitama Prefecture |
| Managing bodies | Fujioka City and Kamikawa Town (designated 2 February 1959) |
| Extent | Approximately 1.5 km of the Kanna River below the Shimokubo Dam |
| Geology | Green schist of the Sanbagawa metamorphic belt |
| Access | Approximately 40 minutes by car from the Honjō-Kodama Interchange on the Kan'etsu Expressway; bus from JR Honjō Station to Onishi Post Office, then local connection |
| Admission | Free; outdoor site open at all hours, though night access is not advised |
| Contact | Onishi General Branch Office, Fujioka City Tourism Section (Tel: 0274-52-3111) |
References
- National Database of Designated Cultural Properties — Sanbasekikyō (Agency for Cultural Affairs)
- https://kunishitei.bunka.go.jp/bsys/maindetails/401/3090
- Sanbasekikyō — Cultural Heritage Online
- https://bunka.nii.ac.jp/heritages/detail/203256
- Sanbasekikyō — Fujioka City Board of Education (Cultural Properties)
- https://www.city.fujioka.gunma.jp/soshiki/kyoikuiinkai/bunkazaihogo/2/3/meisyou_tennenkinenbutu/1043.html
- Sanbasekikyō (National Designated Place of Scenic Beauty and Natural Monument) — Kamikawa Town, Saitama
- https://www.town.kamikawa.saitama.jp/soshiki/shogaigakushu/bunka_rekishi/kunishitei/564.html
- Course 1: Sanbasekikyō-no-michi (Kantō Fureai-no-Michi) — Gunma Prefecture
- https://www.pref.gunma.jp/page/1175.html
- Transformation of Sanbasekikyō — Japan Water Agency, Shimokubo Dam Office
- https://www.water.go.jp/kanto/simokubo/sanba-hensen/index.html
Last verified: 2026.05.18