“Obanazawa pumice bed” is extensively distributed in Obanazawa basin and its vicinity along the middle-stream of the Mogami river. This deposit was first described by Prof. Y. Tomita as the deposit under the fluvial agency of the Niu river at the time of the explosion of Funagata volcano (in Ou mountains), at the source of this river.
The writers exmained about 160 exposures and made the isopach map of the deposit. From this map, the volume of Obanazawa pumice was roughly estimated 0.19km. The deposit attains a maximum exposed thickness in the western border of its distribution area, namely in the vicinity of Hijiori spa. Further, Obanazawa pumcie bed is found on the surfaces of hill-land and higher-middle terraces (Sabane-Obanazawa surfaces).
These observations about the distribution of Obanazawa pumcie and some other evidences indicate that Obanazawa pumcie was deposited as a fallen tephra originated from Hijiori cladera (Dewa range). In the writers' opinion taking the result of pollen analyses and archeological sruvey into consideration, Obanazawa pumice bed belongs to the upper Pleistocene or lowest Holocene age.
