谷密度からみた蛇紋岩山地の特性 -2種類の谷の定義による評価- Geomorphological Characteristics of Serpentinite Mountains by Two Definitions of Drainage Network

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谷密度を計測する場合,どの地点までを谷の最上流部と認定するかにより谷密度の値が変化する.そこで,25,000分の1地形図を用いて,等高線の屈曲部をすべてっないだ水系を全水系,等高線の屈曲部において等高線長2mmかっ屈曲角90°以内の基準を満たす水系を深水系として2種の水系図を作成し,地質ごとに1・2次流域の谷密度を測定,比較した.計測対象地域は敏音知,鵡川・沙流川,大島,大江山の各蛇紋岩山地とその周辺である.大江山地域を例に挙げると,全水系の谷密度の値は大きい方から,蛇紋岩<花商岩<古生層堆積岩,深水系の谷密度は,花商岩<古生層堆積岩<蛇紋岩となり,各地質における谷密度の相対的な大きさが水系認定法により異なることが明らかとなった.また,他の3地域においても相対順位が大きく変化した.これらの計測結果は,従来の谷密度の計測方法だけでは,山地の地形特性を正確に把握できないことを示唆する.一方,すべての対象地域において,蛇紋岩流域の全水系の谷密度は他の地質に比較して最大,深水系の谷密度は最小であった.さらに,蛇紋岩流域の全水系と深水系の谷密度比は最小であった.このことから蛇紋岩山地の地形的特徴として浅い水系が多く,深い谷が少ないことが示された.また,そのような地形の原因として地すべり地形の存在が挙げられる.

In Japan, the landforms underlain by serpentinite rocks are characterized by long convex slopes and are known to be distinct from most other lithologies. The geomorphological characteristics of ser-pentinite mountains, however, cannot necessarily be represented by one kind of drainage network. A reexamination of valley identification is necessary because the definition of valley head significantly af-fects studies of landform processes in mountainous terrain. In this paper, drainage densities were meas-ured based on two different definitions of the valley head to quantify the differences in valley land-forms on various lithologies.<br> The two types of drainage networks were identified on a 1: 25, 000 topographical map as follows. All-type valleys were defined as those where topographic convergence at the valley heads could be con-firmed on the map, following Takayama's (1972) method. Deep-type valleys were defined as valleys in which the angle of notched v-shaped contours is less than 90° and the map contour length is long-er than 2mm (actual length 50m), taking the degree of topographic convergence into account. All val-ues of drainage density were measured in second-order drainage basins, and then their average and standard deviation were calculated.<br> The four study areas were in and around the Pin'neshiri, Mukawa-Sarukawa, Oshima, and Oe-yama Serpentinite mountains in Japan. In the Oe-yama area, the drainage density of all-type valleys in moun-tainous terrain underlying three lithologies was in the order of: serpentinite>granite) Paleozoic argillite. In contrast, the drainage density of deep-type valleys was in the order of: granite>Paleozoic argil-lite>serpentinite.<br> These tendencies varied greatly among geologies. In serpentinite mountains, the drainage density of all-type valleys was the highest, while that of deep-type valleys was the lowest. In addition, the ratio of drainage density in deep-type valleys to that in all-type valleys in serpentinite areas was the lowest of all measured areas. Similar results were obtained for the other three areas studied Pin'neshiri, Mukawa-Sarukawa, and Oshima.<br> Using definitions of both deep-type and all-type drainage networks, these drainage densities could be effective in demonstrating geomorphological characteristics. In serpentinite mountains, the correla-tion between stream length and drainage area is lower than for granite and Paleozoic argillite be-cause many large-scale landslide landforms are distributed on the slopes. Based on the results of this study, the author suggests that different landform processes could operate on mountains with differ-ent geomorphometric characteristics because different landform processes produce differences in val-ley landform.

収録刊行物

  • 地理学評論. Ser. A  

    地理学評論. Ser. A 72(1), 30-42, 1999-01-01 

    The Association of Japanese Geographers

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各種コード

  • NII論文ID(NAID)
    10002222925
  • NII書誌ID(NCID)
    AN1016484X
  • 本文言語コード
    JPN
  • 資料種別
    SHO
  • ISSN
    00167444
  • NDL 記事登録ID
    4630582
  • NDL 雑誌分類
    ZG1(歴史・地理)
  • NDL 請求記号
    Z8-571
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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