Oigawa river originates from the Aidanotake peak located at the boundaries of the Shizuoka, Nagano and Yamanashi prefectures. Its length is 168 km² and coverage is 3,189 km². It runs in the middle of the Shizuoka prefecture to the south into the Suruga bay with merging many tributaries such as Sumatagawa and Sasamagawa rivers.
The valley of the Oigawa river is situated in a wedge bounded by the Itoigawa-Shizuoka Tectonic Line and the Median Tectonic Line, both of which are major tectonic line in Japan. The valley of the Oigawa river is mostly covered by the Shimanto belt, a Cretaceous to Tertiary accretionary complex. Although folding in the Shimanto belt makes repeated occurrence of the beds, depositional age generally becomes older towards north.
Shimanto belt makes steep topography, and has many fractures inside to enhance weathering. These features cause many landslides especially in rainy and thaw seasons.

Oi River rocks

Valleys of the Oigawa river is basically covered by the Cretaceous to Tertiary Shimanto accretionary complex, composed mainly of sandstone and mudstone associated with blocks or lenses of chert, greenstone and limestone. The majority of the accreted materials is terrigenous clastic such as sand and mud however, a small amount of oceanic crust materials (chert, greenstone and limestone) and uppermost mantle beneath are also off-scraped and accreted. Therefore, a small amount of these oceanic origin rocks are also found in accretionary complex incorporated in sandstone and mudstone.

Sandstone
Mainly composed of quartz and feldspars (K-feldspar and plagioclase) with a small amount of lithic fragments. Feldspars give dusty looking by alteration, whereas quartz gives clear looking because it is not affected by alteration. Other minerals are very minor except for a few micas, and matrix is also very small. Clastics are moderately rounded and sorted. This type of sandstone is called as arenite (this sample particularly called as felspathic arenite because of the large volume of feldspars).
(top: plane-polarised light, bottom: crossed polars; Qtz: quartz, Kfs: K-feldspar, Pl: plagioclase).

Serpentinite
Serpentine is formed by alteration of peridotite. Olivine, a main constituent mineral of peridotite is not resistant to alteration and weathering. As a result of alteration of most olivine to serpentine, peridotite became serpentinite. However, careful observation by microscope enables to find remnant olivine grains.
(top: plane-polarised light, bottom: crossed polars; Ol: olivine, Serp: serpentine).

Bulk sand

Quartz occupies the half of the total modal proportion, and the sum of quartz + albite + muscovite occupies 90 % of modal proportion. It is noted that Ca-bearing plagioclase is very rare which is a characteristic mineral of young volcanics. This feature is similar to that of the Abakawa River.

Heavy Minerals

Because heavy minerals are relatively minor, many kinds of minerals seem to share a small modal proportion and none of them occupies a large modal proportion. Chlorite, pumpellyite, epidote and actinolite are considered to be derived from greenstones contained in the Shimanto accretionary complex. Hornblende is relatively abundant however, there is no candidate for its source except for a small diorite body in the upstream of the Aigadanigawa river, a small tributary that merges to the main stream of Oigawa river at Shimada. Although the body is small, it is close to the sampling point so it might produce relatively large amount of hornblende. A small amount of chromite and olivine are contained, which are characteristic to peridotite. These minerals are considered to be derived from peridotite (serpntinitised) body that associates the above-mentioned diorite / gabbro complex.

Although the number of analysis is not large, Mg value of the analysed orthopyroxenes are concentrated to 66 - 70 except for one analysis. The observed orthopyroxenes did not show euhedral grain shape, so they are more likely to be gabbro origin than tephra origin.