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Schists in Nagatoro

Map
Fig. 1: Location of Nagatoro
 (Google map)

Nagatoro is a well-known scenic spot with wonderful rocky landscapes along the River Ara-kawa in western Saitama Prefecture northwest to Tokyo. In terms of geology, Nagatoro is a famous site that has excellent exposures of schists (crystalline schist). The rocks in this area were designated as national natural treasure to protect those in 1924 (damaging and collecting any rocks are prohibited). Because study of rocks in this area began at the time when modern geology was introduced in Japan about 130 years ago, Nagatoro is the birthplace of Japanese geology.

Schist is a regional metamorphic rock, which is a major component of the basement of the Japanese Islands. Nagatoro is in the Sambagawa metamorphic rock belt. Various kinds of schist are found in Nagatoro and some of them are hardly seen in other areas. I introduce a few rocks in Nagatoro.

Piemontite quartz schist

Piemontite quartz schist is characterized by glossy pink or violet red. This color is that of piemontite, a mineral containing manganese, in the schist. The rock in Photo 1 was reported as the first discovery of piemontite quartz schist in the world in 1888.

Piemontite schist

Photo 1: Piemontite quartz schist

Piemontite schist (closeup)

Photo 2: Color of the piemontite quartz schist

Color of schist varies depending on contained minerals. For example, greenschist is literally green because of containing an abundance of green minerals such as chlorite and epidote. Greenschist, common in the Sambagawa belt, is produced by which volcanic products mainly comprising basaltic tuff are metamorphosed. Major kinds of schist in Nagatoro are shown along with their colors and parent rocks in Table 1.

Table of schists in Nagatoro

Table 1: Schists found in Nagatoro

Schistosity

Schist has a thin parallel-banded or foliated texture. Elongated minerals randomly present in non-metamorphic rocks are arranged in a parallel fashion while pressure is applied to two sides (Figure 2). As a result, the foliated structure develops in metamorphic rocks including schist and gneiss. All of schists contain platy and elongated minerals, the sizes of which are large enough to be clearly visible. Foliation with visible coarse grained minerals is called schistosity. Schist splits along a parallel plane to make a flat surface.

Mineral arrangement

Figure 2: Arrangement of elongated minerals
A: Before pressure is applied; B: After metamorphosed; P: Pressure

The piemontite quartz schist and other schists in Nagatoro have these characteristics (Photos 1 and 3). Green rock in Photo 3 is greenschist (epidote schist). The surfaces of the rocks tilt in the same direction at the same angle because of the inclined rock bed in this area. Almost horizontal planes appear at a place several hundred meters down the stream from the Photo 3 point, which formed a rock terrace called “Iwadatami” [named because the appearance is like thick straw mats that cover the floor (Japanese traditional floor mat, tatami)]. The landscape of Iwadatami is one of the features of Nagatoro. The terrace is about 60 m wide and 500 m long, the rock of which is graphite schist.

Green schist

Photo 3: Greenschist

Iwadatami

Photo 4: Iwadatami (rock terrace)

Toraiwa

Toraiwa is schist with beautiful alternated dark brown and white wavy layers. Toraiwa (“tora” means tiger and “iwa” means rock) was named because the texture is like a tiger coat pattern. Dark brown layers contain stilpnomelane, a silicate mineral with blackish brown or brown color, and white layers contain quartz, albite, and calcite (calcareous schist). Recumbent folds accompanied by flow folds with cleavage well developed in the stilpnomelane schist and the calcareous schist. White short veins crossing brown layers (Photo 7) are calcite (fibrous crystals). When the schist was strained, the stilpnomelane layers fractured because of their brittleness and water got into the fissures in which calcite precipitated. The ductile white layers elongated without fracture. The calcite veins divide the brown layers to be rectangular blocks. Such structure is called boudinage. Long white veins crossing the brown and white layers are quartz veins produced after the calcite veins formed. Toraiwa contacts with greenschist broadly exposed upstream.

Toraiwa

Photo 5: Toraiwa

Folds in Toraiwa

Photo 6: Folds in Toraiwa

Boudinage

Photo 7: Boudinage
White short veins crossing brown layers are calcite (fibrous crystals), producing boudinage. A black bar on the left bottom indicates about 5 cm.

  En echelon veins

Metamorphic rocks are deformed under various stresses. White curved lines in pelitic schist in Photo 8 are en echelon quartz veins, the array of which is called right-handed en echelon. The veins are fissures of the schist filled with quartz, which are caused by shear. The formation of the en echelon indicates that the schist was subjected to left-lateral stress (Photo 9).

En echelon veins

Photo 8: En echelon quartz veins (right-handed en echelon)

En echelon quartz veins 2

Photo 9: En echelon veins subjected to left-lateral stress

Another type of en echelon array is found near the veins. En echelon veins in Photo 10 are mainly calcite veins in greenschist and the direction of array is revers to the veins above, called left-handed en echelon. The direction of the en echelon is northwest (N34°W) and that of the right-handed en echelon as mentioned above is north-northeast, suggesting the relationship between the right-handed and left-handed en echelons is conjugated.

En echelon veins in green schist

Photo 10: En echelon veins in greenschist (left-handed en echelon)

Access

Nagatoro Station on the Chichibu Railway line (the observation area is from Nogami to Oyahana). See the Chichibu Railway website (in English).

Nagatoro is in Chichibu Geopark. There is a museum, Saitama Museum of Natural History near Kami-Nagatoro Station.

Giving any damage to and collecting any rocks are prohibited because Nagatoro is a designated area as national natural treasure.

 

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