Iwajuku Ruins

Iwajuku Site: Where Japan's Earliest History Was Recovered from the Red Soil

One afternoon in 1946, a young man pushing a cart of natto along a country lane in eastern Gunma stopped to look at the reddish soil exposed in a road cutting between two small hills. He pulled from it a piece of chipped obsidian. The find, made by amateur archaeologist Aizawa Tadahiro, would three years later overturn a foundational assumption of Japanese prehistory: that no human had lived on the archipelago before the pottery-using Jōmon period.

The site he stumbled upon is now the Iwajuku Site (岩宿遺跡), located in the Asami district of Kasakake-chō, Midori City. It was designated a National Historic Site of Japan in August 1979, with an additional area added in 2017. Together the protected zone covers roughly 189,000 square meters across the gentle ridges of Inariyama, Yamaderayama, and Konpirayama — small isolated hills southeast of Mt. Akagi on the west bank of the Watarase River.

A Discovery That Rewrote Japanese Prehistory

Before Iwajuku, Japanese archaeologists treated the thick layer of reddish volcanic loam known as Kantō rōmu as bedrock — the floor below which one stopped digging. The prevailing view held that human occupation in Japan began with the Jōmon culture some 15,000 years ago. Anything older, scholars assumed, lay outside the archipelago's habitable history.

Aizawa worked without a university affiliation. He supported himself by selling natto door to door across the Kantō region, and used his travels to investigate sites on his own. After the 1946 find he returned to the cutting again and again, recovering more chipped stone — but never pottery alongside it. By the summer of 1949 he had a clear obsidian spear point in hand, an artifact too unambiguous to dismiss. He brought his finds to Tokyo, and Meiji University quickly organized an excavation under Professor Sugihara Sōsuke. On September 11, 1949, the team broke ground on the hilltop.

What they found settled the question. The site yielded two stratigraphically distinct assemblages of stone tools, with no pottery in either layer. The earlier — now called the Iwajuku I culture and dated to roughly 35,000 years ago — sat about 1.5 meters below the surface within the dark band at the top of the middle loam. It contained two oval stone axes with partially polished cutting edges, side scrapers, blade-like flakes, and cores, worked mainly from shale, with scattered carbonized chestnut wood in the same level. The upper, lighter layer held the Iwajuku II culture, dated to about 25,000 years ago, defined by chisel-shaped knife tools made from agate, shale, obsidian, and andesite. Above the dark band the team identified the Aira-Tn (AT) volcanic ash — a regional marker layer from a massive eruption in southern Kyūshū — giving the older assemblage a firm chronological anchor.

Why the Site Was Designated

The Agency for Cultural Affairs designated Iwajuku as a National Historic Site on August 17, 1979, recognizing it as the first place in Japan to yield scientifically demonstrated Paleolithic artifacts and to confirm that the country's cultural history extends into the Old Stone Age. The designation singled out three points of significance: the stone tools rank among the oldest reliably attributed human artifacts found anywhere on the islands; the layered assemblages established at the outset that the Japanese Paleolithic itself had internal phases; and the discovery opened the entire framework of pre-Jōmon archaeology in Japan.

Within a decade of the 1949 excavation, similar finds were reported from every region of the country. Today around ten thousand Paleolithic sites are known across Japan, but Iwajuku remains the point from which all of them count. In October 2017, an additional area south of B point — designated F point — was added to the protected zone after stone tools were confirmed there during a 2015–2016 survey, bringing the total designated area to 189,225.97 square meters.

What to See at the Site

The hilltop today is a quiet park threaded with footpaths. Information panels mark the original A and B excavation points, and the road cutting where Aizawa first pulled stone tools from the loam still runs between the two small hills. It takes only minutes to walk the full length of the cutting, but the spot itself is worth standing at: this is the actual ground where the Japanese Paleolithic stopped being theoretical.

The principal feature on site is the Iwajuku Dome (岩宿ドーム), a small protective shelter built over the stratigraphy at B point. Inside, visitors stand beside an exposed section of the Kantō loam and can trace the bands where the Iwajuku I and II tool layers sat. A short video explains the geological context. Admission is free and the building is open year-round.

Adjacent to the historic site stands the Iwajuku Museum (岩宿博物館), opened in October 1992 and operated by Midori City. It is the only permanent museum in Japan dedicated to what it calls the "Iwajuku period" — the term it uses for the Japanese Paleolithic, since traits such as partially polished stone axes don't quite fit the European Old Stone Age model. A life-sized mammoth skeleton replica anchors the entrance gallery, and the collection ranges across Paleolithic sites from around the country. Note that the museum's permanent exhibits are closed for renovation from September 2025 through approximately September 2026; check ahead before visiting.

Nearby

The walk from JR Iwajuku Station on the Ryōmō Line passes through residential lanes and farmland — about twenty minutes on foot to the site. The wooded slopes immediately west, known locally as Katakuri-no-Sato, are carpeted in late March with the small purple dogtooth violets the area is named for; a Katakuri Cherry Festival runs at that time. For visitors continuing further afield, the Watarase Valley Railway departs from nearby Kiryū and follows the river upstream into the Ashio mountains, one of the more atmospheric local-line journeys in the Kantō region.