Abstract
This paper describes the glacial fluctuations during the Last Glacial Age in the Uchikomi Valley, in the southwestern part of the Northern Japanese Alps, on the basis of the distribution of glacial landforms and glacial deposits, such as trough edges, terminal moraines, and the differences in the dissection of glacial landforms. The author defines four glacial stages: Ichinosawa, Futamata, Migimata and Kitakenkoku, in order from oldest to youngest. The glacial terminus in each stage was located at 1, 700 m, 2, 000 m, 2, 070 m and 2, 470 m, from older to younger (Fig. 7). The glacier distribution at each stage is shown in Fig. 8.
Aira-Tn volcanic ash (AT; 24, 000 y.B.P.) was discovered on the moraines of the Ichinosawa and Futamata stages. Especially on the medial moraine of the Futamata stage, till is directly covered. with AT (Fig. 6). Moreover, on the ground moraines of Futamata stage, volcanic glasses of AT are contained in the basal part of solifluction deposits which directly cover the till (Fig. 5). This stratigraphy shows that the geological age of the Futamata stage is about 25, 000 y.B.P., just before the fall of AT. Therefore, the Futamata stage corresponds to the Akakurazawa stage (25, 150±210 y.B.P.; Koaze et al., 1974) in the Matsukawa Kitamata Valley, in the northeastern part of the Northern Japanese Alps, and also corresponds to the valley-filling period in many rivers around the Northern Japanese Alps (Machida, 1979). The distribution of AT on the ground moraines indicates that the glacial terminus retreated at least about 1.4 km in horizontal distance and 250 m in relative height for about 1, 000 years, between the maximum of Futamata stage and the fall of AT.
According to the tentative correlation of glacial stages in the high mountains of Japan (Iwata, 1986), glacial stages in the Uchikomi Valley are correlated as follows: 1) Ichinosawa stage, early stage of Last Glacial Age (about 60, 000 y.B.P.); 2) Futamata stage, first glacial advance in late stage of Last Glacial Age (about 25, 000 y.B.P.); 3) Migimata stage, second glacial advance in late stage of Last Glacial Age (about 20, 000-15, 000 y.B.P.) ; 4) Kita.kenkoku stage, Late Glacial (about 15, 000-10, 000 y.B.P.).
