Abstract
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.
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.
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.
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.
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.
