多摩川の段丘地形とその形成過程
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概要
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The Tama River flows from the Kanto Mountains, 1, 000 to 2, 000 meters high, and across the Kanto Plain into Tokyo Bay. Fluvial terraces from the Late Pleistocene are well developed along the river. They are classified into four groups according to the differences in the relative height above the river floor and in the tephra-cover. These are, from older to younger, the Musashino (80ka-60ka), Tachikawa (30ka), Aoyagi-Haijima (20ka-15ka), and Lower terraces (15ka-).Concerning the longitudinal profile of the terraces along the river, the Aoyagi-Haijima terrace, whose profile is steeper than those of the Musashino terrace and the present river-floor, crosses both of them and converges into the Tachikawa terrace in the upstream direction. Moreover, the Aoyagi-Haijima terrace is assigened downstream to the buried valley floor beneath the recent formations in and around Tokyo Bay. Further, in the gorge of the Kanto Mountains in the upper reaches of the river, a prior buried valley bottom is exposed under terrace deposits, which tends to be assigned downstream to the Musashino terrace in the lower reaches of the river. On the basis of the above results, the author interprets fluvial processes and terrace formation along the Tama River. The results are as follows.1) In the upper reaches of the Tama River, the profiles of the Aoyagi-Haijima terrace diverge from the prior buried valley bottom in the upstream direction. This is attributed to the fact that fill deposits in the prior valley accumulated more thickly in the upper reaches, and that the increase in gradient is larger in the upper reaches. This is because the amounts of debris which were yielded in this drainage area were in excess of stream capacity at that time. This accumulation in the upper reaches took place from 80ka to 50ka.2) In the upper reaches, the valley remained filled for a lengthy period of time, from 50ka to 20ka in terms of tephrochronology. Meanwhile the valley wall was worked backwards by lateral erosion. Afterwards major terracing by vertical erosion occrred, resulting in an undersupply of debris during Post Glacial time.3) Although the drainage basin of the Tama River was in a non-periglacial region, fluviatile accumulation took place during the transition from interglacial to glacial because of an increase of sediment yield in the upper reaches. Therefore the terraces are climatic terraces. The phenomena of crossing terraces are related to the difference in terrace type between climatic terraces in the upper reaches and thalassostatic terraces in the lower reaches.