geomorphometry.org

Characteristic or threshold slope angles with higher frequencies than other angles have been related to slope stability determined by soil mechanics, bedrock strength, tectonics and climate. High relief mountains in Japan and Taiwan tend to have a characteristic slope angle of ca. 35°, which corresponds to the typical form of V-shaped valleys. Regolith is usually thin in these mountains, and differences in bedrock, climate and uplift rates exert little influence on slope angles. Slope histograms are negatively skewed in most parts of mountains where the characteristic slope angle is observed. These results differ from previous observations elsewhere. The angle of 35° corresponds to the angle of repose of dry regolith. Therefore, the angle permits repeated shallow slope failure due to rainfall in almost all parts of the slope even near ridgelines, permitting parallel retreat of the whole slope. Temporal occurrence of steeper or gentler slope segments may be adjusted by localized change in erosion rates. This mechanism combined with river down-cutting can maintain the constant shape of v-shaped valleys.