If Saitama has a popular natural tourist attraction, it’s the gorge at Nagatoro. Nagatoro Town, situated along the Ara River and enclosed in the Chichibu Basin, has done all it can to benefit from this natural wonder. There are river rafting and “line” boating (these are long wooden boats that carry many passengers) trips down through the gorge; there are shops selling kakigori (shaved ice) using natural ice from the mountains; there are numerous souvenir shops including a Gibli store; there’s Mt. Hodo with its blossoming tree gardens and cable car that goes up to a monkey park at the top, and a natural history museum. Because of Nagatoro’s abundance of exposed metamorphic rocks, it also holds claim to being the birthplace of geology in Japan.
The rocks at Nagatoro form part of a belt of metamorphic rock known as the Sanbagawa Metamorphic Belt. It is named after the Sanba River in Gunma Prefecture where those rocks are also exposed. This particular belt is quite long, extending from Kyushu and roughly following the Chuo (Median) Tectonic Line. The belt is gently curved with the curve of the Japanese Pacific coastline, but near the Izu Peninsular, it takes a sharp turn inland. This is because the Izu Penunsula is acting like a miniature India and pushing its way into Honshu, bending the Sanbagawa Metamorphic Belt inland in the process.
The metamorphic belt was formed long ago during the late Jurassic and early Cretaceous as coastal sedimentary deposits, largely mudstones, which were later subducted by forces of plate tectonics, heated and pressed deep below the surface, and then uplifted by the same tectonic forces. The rocks are exposed anywhere that rivers have cut through or the rocks have been uplifted. Though I’m unfamiliar with this rock belt in other parts of Japan, there is a Sanbagawa Gorge between Gunma and Saitama Prefectures not far from Nagatoro and a smaller Sanbagawa Gorge park in Tokigawa Town, also in Saitama.
The rocks have different appearances depending on the type of mud stones, varying in colour from blues and greens to browns and dark, ash greys. In many places, quartz veins can be seen in the rocks, and indeed much of the belt is comprised of schists.
Though Nagatoro’s “iwadatami” (rock tatami) is the most prominently recognized formation, a trip to the riverside from Kami Nagatoro Station offer views of many more varieties of schist. In fact, it is possible to follow the river from Kami Nagatoro Station to Nagatoro Station, a delightful walk with beautiful riverside scenery and a natural exhibit of fascinating rocks.