(a) Photo of the developed land used to make the large sport square in Susai town of Akaiwa city, situated at the eastern part of the Kibi Plateau; its location is the site (a) shown in Fig. 1a. The outcrop of the Eocene fluvial deposit (conglomerate here) appeared during the development. The squared area in red was sketched to show the occurrence of the tuff. (b) Photo showing the site used to analyze the rock types of the gravels. The width of the photo is ca. 1.5 m. (c) The gravels (174 samples) collected from the area (b). The width of each photo is ca. 50 cm. The gravels are variable in their size, shape (angular to rounded gravel) and rock type. (d) Photo showing the granodiorite gravel that was dated by the K-Ar method. The pick hammer by the granodiorite is ca. 40 cm in length. (e,f) Photomicrographs of the granodiorite. Photo e is under plane polarized light, and photo f is under crossed polarized light. The sample contains fresh hornblende and plagioclase without any alterations. Hbl: hornblende, Pl: plagioclase, Kfs: K-feldspar, Qz: quartz, Ap: apatite, Chl: chlorite.