Abstract
Low-relief erosion surfaces on high ridges in the Akaishi Mountains have been considered to be “remnants of peneplains”. The purpose of this study is to reconsider the origin of those surfaces by using a slope process-response model based on the continuity equation and on the rates of slow mass-movement processes investigated on the periglacial smooth slopes.
In the Akaishi Mountains periglacial smooth slopes mainly formed during the last glacial age are preserved in a high altitudinal zone (Figs. 1, 2). Low-relief erosion surfaces on high ridges are the gentlest parts (slope angle is less than 20°) of those periglacial smooth slopes (Fig. 3).
There are several quantitative studies about the slow mass-movement due to ground frost in the alpine zone of the Akaishi Mountains (Koaze, 1964; Okazawa et al., 1975; Higuchi, 1987). According to these studies the annual rate of surface gravel movement is in proportion to the slope gradient (Fig. 5; equation (1)). In Daishyoji-daira (Fig. 4), the vertical profiles of the annual rate of downslope soil movement (Fig. 6) were also measured (Okazawa et al., 1975) on periglacial smooth slopes mantled with surface rubble layers. These data indicate that the potential rates of weathering is greater than the rate of transport; in other words, the transport limited condition on those slopes.
In this case, based on the above empirical process laws of debris transport, the following continuity equation is deduced:
dy/dt=-5.8×10-2 {(d2y/dx2)+(dy/dx)/x}
where, x [m] is the distance from the divide, y [m] is the altitude and t is time elapsed [year].
The development of an initially straight slope under slow mass-movement (Fig. 7) obtained from this equation shows that, without flat interfluve as an initial landform, the erosion surfaces can be formed over a period of tens thousands of years under periglacial climatic conditions. Therefore the low-relief erosion surfaces observed on high ridges do not necessarily have to have been derived from flat interfluves such as “peneplains”. But they probably have been formed at about their present height by periglacial processes in the glacial ages during the late Quaternary.
