The Unique Geology of Iwahana
Two Cliffs Facing Each Other Across the River
The most distinctive feature of Iwahana is the presence of two enormous cliffs on opposite sides of the Chikuma River. Their steep walls rise about 120 meters above the surrounding landscape, creating the impression that the mountains were suddenly torn apart to make way for the river.
Hangai Iwahana on the left bank is primarily composed of quartz-hornblende porphyrite with well-developed columnar joints. Shimoshiojiri Iwahana on the right bank consists mainly of green tuff, often known as Green Tuff. The striking difference between these rock types provides important clues about the geological history of the area.
The Chikuma River Fault
The contrast in geology is thought to be related to a fault known as the Chikuma River Fault, which lies between the two cliffs. This makes Iwahana an important site for understanding the geological structure and formation of the surrounding region.
Iwahana was officially designated a Natural Monument of Nagano Prefecture in the geological and mineral category on January 14, 1974. The designated area, known as the Iwahana Cliffs of Koizumi, Shimoshiojiri, and Nanjo, covers approximately 13 hectares.
A Rich Natural Environment
Rare Plants Among the Cliffs
The unusual environment around Iwahana supports a diverse range of plant life. Despite being located in central Honshu, the area is home to plants normally associated with colder regions. One example is Moiwa nazuna, a plant also found in Hokkaido and Sakhalin.
The combination of exposed rock, river valleys, and distinctive local climate creates an environment well suited to nature observation and geological exploration.
A Historic Firefly Habitat
Iwahana was also famous for its fireflies. An Edo-period geographical record called Shinano Kisho-roku describes the fireflies of Iwahana, and the spectacular sight of countless fireflies flying together around the summer solstice was once known as a “firefly battle.”
The area was regarded as one of the finest firefly-viewing locations in Shinshu. Today, conservation activities continue around the irrigation channels of Shimoshiojiri in Ueda, helping to protect this traditional natural environment.
A Breeding Ground for Kestrels
The cliffs are also known as a communal breeding site for kestrels. Their presence adds another attraction for birdwatchers and nature enthusiasts visiting the area.
Legends of an Ancient Lake
Stories of a Lake That Once Filled the Valley
Local tradition tells of a large lake that once existed upstream from Iwahana. According to the story, the Chikuma River gradually eroded the surrounding land and eventually created the dramatic cliffs and river valley seen today.
Several place names in the region are said to preserve memories of this legendary lake. The name Shiojiri, for example, is traditionally associated with the expression “Shio-jiri,” meaning the northern end of the lake. Place names such as Uminokuchi and Umijiri in the southern Saku region are also sometimes connected with this ancient-lake tradition.
The Giant Rat and the Chinese Cat
One of the most entertaining local legends tells the story of a giant rat that terrorized a village. The villagers released an enormous cat, known as a Karane-ko or “Chinese cat,” to chase it away.
The giant cat cornered the rat at Iwahana, where the rat is said to have gnawed through the rock. The rock then broke apart, causing the lake upstream to burst and releasing a tremendous flood. According to the legend, both the giant rat and the cat were swept away by the rushing water.
The story is reflected in local place names and shrines. A place called Nezumi, meaning “mouse” or “rat,” remains in Nanjo, Sakaki Town. In Shinonoi, Nagano City, the Karaneko Shrine, whose formal name is Karaneko Shrine (軻良根古神社), is traditionally associated with the cat in the legend.
Daidarabotchi and Koizumi Kotaro
Other traditional stories attribute the formation of Iwahana to legendary figures. One says that the giant Daidarabotchi, a gigantic supernatural being appearing in folklore throughout Japan, created the dramatic landscape.
Another legend tells of Koizumi Kotaro, a heroic figure from the Ueda region who is said to have broken through the rocks. Koizumi Kotaro is sometimes identified with Izumi Kotaro, a legendary pioneer credited in local folklore with opening up the Matsumoto Basin. These stories demonstrate how the landscape of Iwahana has inspired imagination across generations.
How the Iwahana Landscape Was Formed
Rocks, Faults, and River Erosion
The geological contrast between the two cliffs is one of the key features of Iwahana. Hangai Iwahana consists mainly of quartz-hornblende porphyrite with prominent columnar joints, while Shimoshiojiri Iwahana is dominated by green tuff.
Geological evidence suggests that the two sides were once part of a continuous landform extending from Mt. Kammuriki to Mt. Azumaya. Over a long period, erosion by the Chikuma River gradually cut through the land and produced the spectacular cliffs that now stand on opposite sides of the river.
Gravel from an ancient riverbed can still be found near the upper parts of the cliffs. These deposits provide clues to the changing course and elevation of the Chikuma River over geological time.
Iwahana in History and Culture
A Scenic Landmark Known Since Medieval Times
Iwahana has been recognized as a notable landscape since ancient times. It appears in the medieval historical tale Genpei Seisuiki, where the area is referred to as Shiojiri-se.
The name Iwahana has also been written using different characters, including Iwabata, Iwahana, and Gan-ka. These variations reflect the long history of the place in regional records and culture.
An Important Route Through Northern Shinano
The area around Iwahana has long been an important transportation corridor. During the Edo period, the Hokkoku Kaido, one of the major highways connecting the regions around central and northern Japan, passed through the area.
Iwahana was also located near the point where the former districts of Hanishina, Sarashina, and Chiisagata met. Because the terrain was narrow and difficult to cross, it was an important geographical point for travelers and travelers’ routes.
Local tradition even tells of the Maeda family of Kaga Domain sending messengers to Kanazawa during their journeys for alternate attendance to report safely after passing through this difficult terrain.
Chikuma Park: A Viewpoint Above the Cliffs
For visitors who want to appreciate the scale of Iwahana from above, Chikuma Park is located on top of Hangai Iwahana. The park was developed in connection with a photograph featuring Iwahana that was associated with Japan’s “100 Scenic Views” tradition during the Taisho period.
From the park, visitors can look across the Chikuma River toward the surrounding mountains and enjoy a broad panorama of Ueda City. In spring, the park becomes especially atmospheric when cherry trees are illuminated by traditional lanterns at night, combining cherry blossoms, the river valley, and the lights of Ueda in a memorable evening view.
Visiting the Two Sides of Iwahana
Hangai Iwahana
From Ueda Station, take a local bus for approximately 18 minutes and get off at the Ueda Michi to Kawa no Eki bus stop.
By car, Hangai Iwahana is approximately 9 km, or about 15 minutes from the Ueda-Sugadaira Interchange on the Joshin-etsu Expressway. Parking for approximately 50 regular vehicles is available.
Chikuma Park
Chikuma Park can be reached from Ueda Station by bus in approximately 16 minutes, followed by a walk of around 30 minutes. By car, it is approximately 10 km, or about 30 minutes from the Ueda-Sugadaira Interchange.
Shimoshiojiri Iwahana
Shimoshiojiri Iwahana can be reached by taxi in approximately 5 minutes from Tekuno-Sakaki Station on the Shinano Railway. By car, it is approximately 4 km, or about 5 minutes from the Sakaki Interchange on the Joshin-etsu Expressway. There is no parking area at this site.
Experience Nature, Geology, and Folklore at Iwahana
Iwahana is much more than a dramatic pair of cliffs. Its distinctive geology reveals the long history of the Chikuma River, while rare plants, fireflies, and breeding birds demonstrate the richness of its natural environment.
At the same time, legends of an ancient lake, a giant rat and cat, Daidarabotchi, and Koizumi Kotaro reveal how generations of local people have interpreted this extraordinary landscape. From the viewpoints above the cliffs to the river flowing between them, Iwahana offers overseas visitors a fascinating opportunity to encounter the natural history, folklore, and cultural heritage of Nagano in one remarkable setting.