遠野盆地の緩斜面と段丘の成因について

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Geomorphic surfaces in Tôno Basin are composed of Tôno gentle slope-I (Tg-I), Tôno gentle-slope-II (Tg-II), Tôno terrace (Tt) and alluvial plain.The highest surface, Tg-I, is the oldest and erosional one shaved the deep-weathered granitic rocks, which is covered with thin fragmentary deposits red-weathered in their lower part. Denudation of base rocks have brought talus-like or fan-like deposits consist of gravel and sand at the foot of Tg-I. Perhaps it occurred in a stage of slope-instability before the Würm maximum, and then chemical weathering of gravel bed, or red-weathering of slope deposits on Tg-I, followed under the warmer climatic condition.The terrace (Tt), the lowest surface, was continually formed under periglacial condition through the Würm maximum to the next warmer period (the Alleröd Stage?). The terrace deposits consist of solifluction products along the Sarugaishi River in the west, of fluvial products like fan deposits on the right of the Hayase River in the south, and of intermingled deposits of solifluction and fluvial products along the Ogarase River in the north, as well as on the left of the Hayase and the Rainai in the south-west.Thereafter, Tg-II was formed under the cold climate probably of the last substage in the Würm, as depositional gentle slope with solifluction products, overlying the Tt surface.Alluvial plain has been developed since the Holocene, resulted from the swinging and shifting of river channels.There are some regional patterns in distribution and materials of these geomorphic surfaces, especially of Tg-II and Tt. The area along the Sarugaishi River is colluvial in character, the area on the right of the Hayase River is fluvial, and the area along the Ogarase River, on the left of the Hayase and the Rainai River, is colluvio-fluvial.
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遠野盆地の緩斜面と段丘の成因について

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