Ueno Village Kikkōseki Stone Locality is a geological heritage site in Narahara, Ueno Village, Tano District, Gunma Prefecture, designated as a National Natural Monument of Japan. The site is known for kikkōseki, unusual stones featuring distinctive patterns resembling the shell of a turtle.
Importantly, the designation applies not simply to the stones themselves, but to the geologically valuable locality where kikkōseki occurs.
The kikkōseki found in Ueno Village range from several centimeters to several tens of centimeters in diameter. Some of the largest specimens are approximately 73 centimeters across. They are generally flattened spheres or oval-shaped stones, with striking crack patterns resembling a turtle shell across their surfaces.
Kikkōseki is not the name of a particular type of rock, but a term describing a distinctive shape and surface pattern. Geologically, these stones are a type of septarian concretion, a kind of nodule with a structure resembling natural concrete.
The kikkōseki of Ueno Village formed when calcareous material accumulated within muddy sandstone and gradually hardened through diagenesis, the physical and chemical changes that take place in sediments after deposition.
As the concretions formed, cracks developed inside them. Carbonate minerals and other materials later precipitated within these cracks, leaving behind the characteristic turtle-shell-like patterns visible today.
Like kikkaseki and shinobuseki, kikkōseki is not a biological fossil. Rather, it is considered a type of pseudofossil created by inorganic natural processes. Its unusual appearance gives it both geological and aesthetic value.
A hillside on the left bank of the Kanna River in Narahara, Ueno Village was once a particularly valuable place for observing kikkōseki in its natural geological setting. Stones that would normally remain underground were exposed at the surface, allowing their occurrence within the surrounding strata to be studied.
Recognizing its scientific importance, the site was designated a National Natural Monument on August 8, 1938.
Today, however, direct observation of the stones in their original geological setting has become difficult due to changes to the slope, including the application of protective concrete and other construction work.
Because the designation applies to the locality itself, collecting stones or carrying out activities that alter the land is strictly prohibited under Japan’s cultural property protection laws.
Kikkōseki collected before the area received its protected status is preserved and displayed at facilities including Ueno Village Fureai-kan at Michi-no-Eki Ueno and the Gunma Museum of Natural History in Tomioka, where visitors can view these unusual geological specimens.
Ueno Village lies in the southwesternmost part of Gunma Prefecture and is surrounded by the mountains of the Chichibu Mountain Range. The area’s geology consists mainly of strata belonging to the Jurassic Chichibu Belt and the Cretaceous Sanchu Graben.
The kikkōseki is thought to have formed in the Ishido Formation, considered to be among the oldest geological formations in the area. Within its dark-gray, fine-grained muddy sandstone, calcareous concretions formed, some of which developed the distinctive cracks that characterize kikkōseki.
Millions of years ago, this region was covered by a shallow sea. Mud and sand accumulated on the seafloor, while calcium-rich material became concentrated within the sediment and gradually formed hard concretions.
Later, dehydration and shrinkage caused cracks to develop across the concretions. Carbonate minerals filled these cracks, eventually producing the distinctive turtle-shell-like pattern seen today.
However, the precise reason why these cracks tend to form such characteristic polygonal patterns has not yet been completely explained.
Kikkōseki-like formations have been known in Europe and the United States since the 19th century and were described in geological textbooks. In Japan, however, they were not widely recognized for many years.
The beginning of their academic study in Japan is generally associated with geologist Wakisui Tetsugorō, who reported kikkōseki from the Oku-Chichibu area to an academic society in 1907.
During the late Edo period, these stones were also appreciated by enthusiasts of unusual natural stones because of their distinctive shapes. Some were displayed as bonsai-style viewing stones or ornamental stones.
Today, it is believed that numerous similar examples and fossils occur within geological formations extending from the Oku-Chichibu region through Ueno Village and onward toward the Saku area of Nagano Prefecture.
There are few signs remaining around the National Natural Monument site, making its exact location difficult for visitors to identify. Although topographic maps published by the Geospatial Information Authority of Japan indicate the cultural property with a designated symbol, clear signage at the site itself is limited.
The following location is outside the officially designated area, but near Benten Bridge in the Narahara district, visitors looking upstream along the Kanna River can see sections where white, turtle-shell-like patterns appear within dark mudstone along the riverbank. This provides an opportunity to observe how kikkōseki occurs within sedimentary rock.
At Ueno Village Fureai-kan at Michi-no-Eki Ueno, visitors can see kikkōseki specimens from Ueno Village. The specimens display a variety of rock characteristics rather than a single uniform type, offering an accessible introduction to the geological diversity of the area.
Narahara, Dōdokomyoigasawa, Ueno Village, Tano District, Gunma Prefecture, Japan
From Shimonita Station on the Joshin Electric Railway Joshin Line, take the Ueno Village shared taxi service and get off at Ueno Village Fureai-kan. The journey takes approximately 65 minutes.
From the Shimonita Interchange on the Joshinetsu Expressway, take Prefectural Route 45, Shimonita-Ueno Line, via the Yunosawa Tunnel. The site is approximately 30 kilometers away and takes about 35 minutes by car.
Ueno Village Kikkōseki Stone Locality offers a fascinating glimpse into geological processes that unfolded over tens of millions of years. Although it is now difficult to observe the stones directly in their original geological setting, visiting local exhibition facilities and exploring the surrounding landscape can help visitors understand how these remarkable formations developed and why they are scientifically valuable.
Combining a visit with the natural scenery around Jikkoku Pass and the Kanna River basin provides an opportunity to discover another side of Ueno Village’s rich geological and natural heritage.
As you encounter these unusual patterns created by natural processes, take a moment to imagine the immense span of geological time recorded in the rocks beneath your feet.