The age of accumulation of the Onikobe lacustrine sediments, and the ages of formations of fluvial terraces and alluvial fans in the Onikobe Basin, were estimated using field investigations and radiocarbon datings carried out by the present author. The results are summarized as followings.
1. The first wood sample collected from the Onikobe lacustrine sediments was radiocarbon dated to be 27, 100±2, 270 1, 745y. B. P. (TH-032). But ten further materials collected later from the Onikobe lacustrine sediments were older than 30, 740-35, 810y. B. P.. Judging from this evidence, the Onikobe lacustrine sediments began accumulating before 35, 810y. B. P..
2. In the Onikobe Basin, fluvial terraces have developed at four levels along the Araogawa river. Based on the radiocarbon dates of materials collected from some terrace deposits, the third river terrace surface, named Hara surface, was formed during 1, 100-1, 400y. B. P., while the lowest Kuze surface was formed about 800y. B. P.
3. In the southern and the western parts of the Onikobe Basin, many dissected valleys have formed on the alluvial fans. In order to estimate formative ages of the alluvial fans, a black humic soil sample was collected from one of the bottom of such dissected shallow valleys. The materials were radiocarbon dated, and showed 5, 170±90y. B. P. (NU-086). The age indicates that the alluvial fans were formed earlier than 5, 200y. B. P..