兵庫県内陸部における最終氷期以降の地形形成

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  • Geomorphological processes since the last glacial age in inland Hyogo prefecture.
  • ヒョウゴケン ナイリクブ ニ オケル サイシュウ ヒョウキ イコウ ノ チケイ

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The gentle slopes along the feet of the low mountains in the inland part of Hyogo prefecture are colluvial slopes, which were developed where rhyolite and chert occur. The area is further characterized by the existence of geomorphological features such as stream capture and formation of wet lowlands. Geomorphological processes which contributed to forming these topographic features were studied on the basis of tephro- and radio-carbon chronologies as well as geomorphological investigations.<br>Volcanic ashes which are recognized in the area are useful in determining the period in which the geomorphological features were formed. They are: DNP (Daisen Namatake pumice; 65, 000-70, 000 y.B.P.); MsP (Misen pumice; 16, 000-18, 000 y.B.P.), derived from Daisen Volcano; and AT (Aira-Tn ash; 24, 700 y.B.P.) and K-Ah (Kikai-Akahoya ash; 6, 300 y.B.P.), derived from volcanoes in southern Kyushu.<br>The colluvial slopes are divided into four surfaces: I, II (further divided into II and II 2), III and IV. The II 2 can be further divided into three parts: c(talus cone), d (debris flow) and f (alluvial fan). The ages of formation of these surfaces are as follows: surface I was formed before the last glacial age; II 1 and II 2, during the first and second halves of the last glacial age, respectively; III, between the late glacial age and approximately 6, 000 years ago; IV, after the formation of the former.<br>The distribution of the colluvial slopes is influenced by the lithology: they are best developed in the area of rhyolite, and next best in that of chert. Rhyolite and chert are easily subjected to freeze-thaw action and discontinuous rock disintegration. The surface II 2 is judged to have been formed in the cold period of the last glacial age, when freeze-thaw action was dominant and broke these rocks into fragments. The facies of I and II 1 indicate that they were formed by the process as II 2. Surface III was formed as an alluvial fan during the period when the climate was getting warmer and wetter. The debris of II 1 is thicker than that of II 2, and was transported by debris flow. This suggests that the formed period was wetter than the latter.<br>Discharge of the basin and lowland lie inland area gradually diminished during the last glacial age, because of the development of colluvial slopes. The colluvium was deposited onto the outlet of Sasayama basin, leading to stream capture. After this event, the drainage of this basin improved. In Hikami lowland, however, wet conditions remained.

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  • 第四紀研究

    第四紀研究 27 (4), 219-228, 1989

    日本第四紀学会

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