The strata distributed in the lower Agatsuma River, northwestern part of the Gunma Prefecture, Central Japan, consist of the Miocene Yago Formation and the Pleistocene Onogami Formation. The marine middle Miocene Yago Formation is composed mainly of coarse-grained tuffs, accompanied partly with pumice tuffs and mudstones. The aqueous sediments of the Onogami Formation deposited in early Pleistocene covers the Yago Formation with anguler unconformity and is divided into four members in ascending order: the Shimoichishiro Breccia Member composed mainly of debris flow deposits, the Sakaizawa Pumice Tuff and Mudstone Member, the Iwaido Tuff Breccia Member composed of andesitic hyaloclastites and its fragmental deposits, and the Horinouchi Sandstone and Mudstone Member characterized by slump beds. After the deposition of the Onogami Formation, several andesitic and dacitic dykes intruded with direction of WNW-ESE; the lower Omogami Formation had fold axes of WNW-ESE and NNE-SSW. The Onogami basin in which the Onogami Formation was deposited is a structual collapsed one recognized as a type of the Island Arc disturbance advocated by Fujita in 1982. It was proved that the Onogami basin had been restricted by several tectonic lines estimated below the surface of the northwestern margin of the Kanto Plain.