Iddingsite occurs as an alteration product of olivine by weathering in the tephras which are widely distributed in South Kanto, Japan. It is a reddish-brown colored aggregate composed of goethite, lepidochrocite, and hydrous amorphous substance containing silica and magnesia. It is more abundantly found in the horizon from the base of Musashino (ca. 30, 000-60, 000y.B.P.), to the upper part of Shimosueyoshi (ca. 60, 000-130, 000y.B.P.) tephra layers in Pleistocene. Regarding iddingsite as the equivalent of olivine, the heavy mineral composition is rather constant from the upper part of Shimosueyoshi tephra layer to the Tachikawa (ca. 10, 000-30, 000y.B.P.) and Musashino tephra layers.
The Kokaigawa Lowland around Shimodate, northern end of Ibaragi Prefecture, is filled up with the Tachikawa Gravels transported by the Paleo-Kinu River. Those are more than 15 meters in thickness and divided into three units; Upper Sand & Gravel, 13, 000-19, 000y.B.P, Middle Gravel, 20, 000-26, 000, and Lower Gravel, 34, 000-, as shown in Tables 1 and 2. Prior to 26, 000y.B.P, the Paleo-Kinu River was flowing from the Kokaigawa Lowland through the Kyowa Upland to the Sakura River Lowland (IKEDA, et al., 1977; SUZUKI, 1980: SUZUKI et al., 1981). About 26, 000y.B.P, middle-stream of the Paleo-Kinu River was captured by the Present-Kokaigawa Lowland, and Middle Gravel and Upper Sand & Gravel were deposited during 26, 000 to 20, 000y.B.P. After their deposi-tion, probably in Holocene time middle-course of the Paleo-Kinu River was changed again to the Present-Kinu River.