Formation of river terraces along the upper Mabechi River in northeast Japan is investigated tephrochronologically and its relation to piedmont gentle slope is discussed.
Four terraces, that is, the Higher Terrace, Terrace I, Terrace II and Terrace III, are recognized in the study area. Lower terraces, II and III, are distributed in the uppermost reach dominated by undulating or gently rolling landscape of the northern part of the Kitakami Mountains. Apart from the Higher terrace, Terrace I appears to continue downstream to the marine terrace of the last interglacial stage. Tephrochronologically, the uppermost part of Terrace II deposits intercalate the Oide Black Ash of c. a. 33, 000-34, 000yr. B. P. and Terrace III is partially overlain by the Hachinohe Pumice of c. a. 13, 000yr. B. P. These evidences indicate that both Terrace II and III were formed in the last glacial stage. The angular gravel-deposits composing the piedmont gentle slope covers the Terrace II, while deposits of its lower part and the Terrace II deposits interfinger each other.