松本盆地周辺のいわゆる"山砂利"について

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Matsumoto Basin, situated between the Hida Ranges and the mountain ranges occupying the western margin of the Fossa Magna, is bordered by fault lines along its western margin, and is probably a structural basin., It is partially filled by rather thick fanglomeratic deposits of Pleistocene Epoch., Fluviatile boulders on the mountain have been called "Yamajari" or "Mountain gravel" in Japan, and have been attributed by NAKAMUMA (1927) to represent a past pluvial or torrential deposit., HOMMA (1927) recognized mountain gravels on the eastern side of the Matsumoto Basin., The Mountain gravels are named "Omine gravels" or "boulders" in strict sense, and among them are found some that attain three meters in diameter; these are residual., The Omine boulders are composed mainly of granitic rocks and have been considered to be derived from the Hida Ranges by remote transportation., The possible derivation of these boulders from the Hida- Ranges may be an evidence that consequent streams from the Hida Ranges traversed the Matsumoto Basin., The mechanisms of transportation are suggested by the following evidences: (1) The boulders observed are almost subrounded granitic rocks measuring 3〜0.,5 meters in diameter., (2) The surface previous to the deposition are represented by small relief of the Omine erosion surface, and therefore, the stream gradient on this surface must have been considerably reduced., Therefore, without crustal movement or heavy precipitation, previous to or at the time of deposition of the Omine gravel, large boulders could not have been transported by the streams flowing over the Omine erosion surface., The deposition of boulders indicates the commencement of a new phase of stream regimen, in other words, an increase of stream gradient took place., Rapid uplift of the Hida Ranges occurred in the middle of the Pleistocene, resulting in the rejuvenation of the upper drainage of the river and an increase of the stream gradient., The scarcity of boulders exceeding one meter in diameter indicates that they have not been transported by normally flowing streams, but by some occasional and accidental floods., Although the Omine boulders may have been transported by occasional heavy rainfall, it can not be proved that their deposition was during the pluvial age., The reference of the Omine gravel to early upper Pleistocene age is from that its deposition was during the second interglacial or in the third glacial ages., From the studies on the particle size distribution of the present fan of the Takase, the writers conclude that the large boulders among the Omine gravel were transported by rapid streams, influenced by the increased gradient of the upper drainage of the river and by occasional heavy rainfalls during the rainy season (summer time) of the glacial or interglacial age.,
日本地質学会の論文
1955-01-25

松本盆地周辺のいわゆる"山砂利"について

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