石田川上流部の水文地形学的特性(その3)

Abstract

Structure of topsoil horizons on a mountain slope in the Kamagatani Experimental Basinwas investigated to estimate the predominant geomorphic processes on it. The soil profile onthe gentle slope near the ridge represents a standard weathering sequence. In an old land-slide scar, standard weathering sequence is overlain by colluvial material brought aboutthrough the secondary erosion of the landslide cliff. In newer landslide scar, exposed bed-rock has been rapidly weathered. Sometimes it has been covered by the colluvial deposit.If the bedrock is covered as soon as it is exposed, the weathering becomes retarded. Mea-surement of particle size distribution revealed that the matrix part of deposit or weatheredslate is composed of silt particle with a mode at 11 to 16 micron and very little fractions ofclay and fine sand throughout the depths of trench-cut cross sections. It suggests that all soilhorizons have been brought about by the weathering of the bed rock. Some of topsoil hori-zons contain dispersed rock pieces and the content of rock pieces becomes greater downslopeuntil the topsoil consists of pure pebble near the valley bottom. It suggests that the colluvialmaterial loses fine particles due to surface wash as it migrates downslope by the repeatedtopsoil failure. Horizons of minimum cone penetration resistance were found within the bedrock. Though it suggests the possibility of rockslide or rockglide, current denudation throughtopsoil failures and subsequent rain wash seems to prevail over the speed of the growth ofweak horizons within the bed rock.