Oboke-Koboke and Iya Valley Area

Oboke Gorge and Iya Valley contain a small amount of “conglomerate schist”—a type of crystalline schist that includes conglomerate. About 70 to 90 million year ago, layers of conglomerates were deposited on the ocean floor.
Due to tectonic plate movements, they were later pushed down to depths of around 15 kilometers underground.
There, under intense pressure, the conglomerates were stretched. Today, the area of Sanmyo and Iya clearly show evidence of this underground geological process. These unique rocks, known as conglomerate schist, have also been designated as a Prefectural Natural Monument.

In the Oboke-Koboke Gorge, you can observe a dome-shaped large bending strata made of crystalline schist that stretches for several kilometers. Because this is a major fold in the rock layers, the geological features vary depending on where you look. For example, 1) at the downstream area near the River Station West West (a tourist rest stop), the layers tilt to the north, 2) around Dotoko the layers are nearly horizontal, and 3) near the jetty for the Pleasure Cruise, the layers tilt to the south.

Along the Oboke Gorge Pleasure Cruise route, you will spot a unique rock formation called “Lion Rock.” It looks just like a lion’s face turning to the side. If you look closely at the chin area, you will see rocks sticking out like a beard. This natural sculpture was shaped over time by the flowing river, gradually forming the face of a lion.

Iya Valley lies beyond Mt. Kunimi, to the east of the Oboke-Koboke Gorge. One of its most striking features is the “Hi-no-ji Valley,” a sheer cliff named for its resemblance to the Japanese letter “ひ” (hi). The winding river below curves in a way that looks like it’s writing the letter “ひ” across the landscape. This splendid scenery is carved into hard rocks such as psammitic schist, which are common in this area.

The Shiozuka Plateau is covered with Japanese pampas grass (Miscanthus sinensis), which was once used as material for thatched roofs and natural fertilizer. To maintain these grasslands, controlled burning has been practiced for many years and continues today. Thanks to this traditional method, new plants continue to grow, creating a rich habitat for insects and birds. This area is especially important for species that can only survive in grassland environments.

In the rocky areas of Oboke that are often flooded, you can find unique plants that have adopted to survive in these harsh conditions. To cope with the strong river currents and frequent flooding, these plants have changed their shape—some have become smaller, while others have developed longer, narrower leaves. It is fascinating to see how these tiny plants continue to thrive, perfectly adapted to their challenging environment.

Around Mt. Nokanoike, linear depressions, formed by landslides have filled with water, creating unique marshland. The marshland is home to rare species such as bog moss (Sphagnum palustre) and Honshakunage (Rhododendron japonoheptamerum var. hondoense), the latter being a species endemic to Japan. The mountain is mainly covered with deciduous broad-leaved trees, and as you walk from the marshland toward the summit of Mt. Nokanoike, you can also observe Japanese beech forest (Fagus crenata) along the mountain pass.