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
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.
- 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