LANDFORMS AND BLOCK STREAMS IN THE MINEYAMA HIGHLAND AT THE CENTRAL PART OF HYOGO PREFECTURE, WEST JAPAN

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  • 兵庫県中央部,峰山高原の地形と岩塊流
  • ヒョウゴケン チュウオウブ ミネヤマ コウゲン ノ チケイ ト ガンカイリュウ

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Abstract

The mountainous district at the eastern end of the Chugoku mountains in the central part of Hyogo Prefecture is called “Bantan Sanchi, ” and is composed mountains rising 1, 000-1, 300m above sea level. The Mineyama Highland and other; small plateaus, at the height of 800-1, 000m, are located south of the watershed in these mountains. We investigated the geomorphology and block stream in these highlands. The results obtained can be summarized as follows.<br> 1) The surface of these highlands consists of smooth debris-mantled slopes, block streamI and IT, and upland fans.<br> 2) A debris-mantled slope has a smooth surface with an inclination less than 25°, and exists mainly in relatively higher areas of the highland. It is composed mainly of andesitic rocks and is covered by Kuroboku (black soil) and thin debris deposits on the surface. Blocks 3-4m in size sometimes lie on the surface. A block stream measures less than 500m in length and less than. 40m in breadth with an inclination below 10°, and is seen at lower areas contiguous to the smooth debris-mantled slope. The block measures less than 1-5m in size and the thickness of the block stream deposit is less than approximately Sm. An upland fan usually lies on the downstream side, of a smooth debris-mantled slope or block stream. It incises the slope and block stream on the upstream side and overlies it on the downstream side. It is composed of small and large blocks and coarse sand as the matrix.<br> 3) Geomorphic chronological characterization of these highlands may be achieved using Kikai- Tozurahara volcanic ash (K-Tz, fall of 70, 000-80, 000 years ago), Aira Tn volcanic ash (AT, fall of 24, 000-25, 000 years ago), Misen pumice (MsP, fall of 16, 000-18, 000 years ago), and Akahoya volcanic ash (Ah, fall of 6, 300 years ago) components of the widespread tephra.<br> 4) The thick weathered crust and its rock core, which is formed on these highlands composed of andesitic rocks, were produced by deep chemical weathering followed by periglacial mass movement in the cold period of the Last Glacial Age.<br> 5) The smooth debris-mantled slope was formed as a result of periglacial mass movements-that is, frost shattering, frost creep, slope wash, and. solifluction-on the abovementioned highland surfaces.<br> 6) The core stones were first rafted down slopes by solifluction from their initial positions on summits, smooth slopes and interfluves, then moved downvalley as “block streams” at the bottom, finally, flowed as block streams.<br> 7) As block stream I overlying K-Tz ash was covered by AT and MsP ash, it was formed before 24, 000 y. B. P., perhaps during the first half of the Last Glacial Age. Block stream II was formed between the fall of MsP and that of Ah-that is, in the Late Glacial Age.<br> 8) As can be presumed from the landform arrangement and the Ah ash that covers it, the formation of upland fans seems to have occurred as a result of erosion of the smooth debris-mantled slope and block stream and the resultant deposition of debris on the downstream side which were caused by the action of water flow produced by hydrological change with the warming of the climate.

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