One of the characteristics of the marine Middle Pleistocene Shimosa Group, the Boso Peninsula, is a continuous sequence of several formations controlled by glacio-eustasy, and the other characteristics is so-called sedimentary imbricate structure. It was supposed that these structures were formed under continuous subsidence of the Kanto Tectonic Basin and remarkable uplift of the south end of the Peninsula. However, since the basin is situated on the wide uplift belt of the Izu-Mariana arc and original feature of the basin was one of the fore arc basins related to the South-western Japan arc, the tectonic subsidence is a doubtful process. It seems that the Kanto Basin have been uplifted during the Pleistocene Period.
Owing to interpretation of the paradoxical broblem, a hypothetical idea was presented namely tectono-eustatic sea-level rise. The idea is supported by geological data of some other Quaternary tectonic basins in Japanese Islands, the Osaka basin, the Niigata basin and other. The process of sea-level rise during the Pleistocene is presumed based on the geological data of the south-Kanto district as follows : the sea-level stagnated at the level about -500m or more around 1ma, consequently it rapidly rised and stagnated gradually, and reached to present sea-level till about 200ka. After then tectonic uplift of land area standed out and marine terraces were formed along these coastal regions.