Abstract
In and around the Shinmurabashi Bridge in Kamikochi Valley of northern Japanese Alps, debris hummocks and a minor ridge features both 10 m high and with angular gravels are developed on the alluvial fan along Benten-sawa River and Okumatashiro-dani River, respectively. These landforms are not moraines but rock avalanche origin supplied from the east face of Maehotakadake North Ridge about 2500-3000 m ASL. Terrestrial cosmogenic nuclide chronology reveals that these features were formed in the period during 6.0-7.9 ka (hummocks) and 0.8-1.1 ka (minor ridge).