The Seto Inland Sea formed during a rise in sea level following the Last Glacial Maximum. There are various studies of geological and geographical aspects of the formation of the Seto Inland Sea. This paper deals with temporal variations in sea conditions deduced from acoustic profiles around Nakajima Island, in the eastern part of the Houyo Islands, to the western side of Takanawa Peninsula within the western Seto Inland Sea. Our results indicate that seawater invaded this area approximately 11,000 years ago, when there were small bays to the north and southeast of Nakajima Island. Subsequent to this, the Imoko and Sekido Straits formed with caldrons. Following the formation of Takahama Strait, the shallow Ozu Bank began to form at the eastern part of the Tsurushima Strait.

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Based on the study of stacked Rosselia socialis, impregnated in the fine sand of the lower part of the middle Pleistocene Yokaichiba Formation in the northeastern part of Chiba Prefecture, new idea were proposed on the mechanism of the formation for these burrows as follows. Examined specimens are crowed abundantly in fine sand bioturbated heavily intercalating hummocky stratified fine sand layer. These sediments might be originally the hummocky sequence in the inner shelf, however changed into the present facies with tiering structures due to the bioturbation by various animals. As to the shape of segment of stacked burrows, funnel-type and spindle-type had been recognized, however many "cone-type" segments piling upwards with interval of 5cm〜10cm were marked in this study. The difference of these three shapes of segments might be produced by burrowing action of animal in Rosselia socialis affected by the cyclic event of erosion and deposition occurred during the course of sedimentation. Piles of cone-type segments were formed by the repeat of the sedimentation in miniature. The repeat of the sedimentation in miniature resulted the increase of thickness of bioturbated portion, occurring amalgamation of the hummocky sequence.

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Middle Pleistocene to Holocene fluvial terraces formed by the Shinano River, are distributed in the southwestern part of the Echigo Plain. This study area is one of the most tectonically active parts of Japan. Despite of largely displaced terrace surfaces, previous studies have mainly focused only on geomorphologic terrace classification. Therefore past results have left many unresolved issues regarding terrace formation. Based on the tephro-stratigraphic study of the overlying Shinanogawa Loam Formation, the terraces have been re-classified according to their age. The Shinanogawa Loam Formation include tephras of M2 (In-Kt(a)), M6 (MC-G1), K-Tz, M6.5 (Aso-4), M7 (On-Ng), K0 (Tt-E), K1 (DKP), K4 (AT) and K5 (As-K) in ascending order. The chronologic subdivision of the terraces are; the Middle Pleistocene terace (Koshijippara I), Late Pleistocene terraces (Katakai, koshijippara III, Tyoonji and Takanashi), and Holocene terraces. It became clear that previouly classified terraces, thought to be of different ages due to their varying height, were in fact the same aged Late Pleistocene terrace, implying a tectonic element. There is over 100m of vertical displacement in the Katakai terrace sueface. The cause of displacement is thought to be differential uplift across block boundaries due to block segmentation. Active faults and flexural zones can been seen across these boundaries.

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In the northeastern area of the Kamakura city, the Miura and Kazusa Groups are subdivided into the Zushi, Ikego, Urago and Nojima Formations in ascending order. Garnet-bearing Pumice bed, named KGP in this study, and some tephra beds are important key beds for the subdivision of the upper Miura Group and the lower Kazusa Group in this area. The characteristic tephra, KGP, are confirmed at more than 10 localities in the area from Kamakura city to north of Yokosuka city. On the basis of petrographical study and chemical analyses of garnet, it is concluded that the KGP tephra is correlated with the garnet-bearing tephra bed, Mk19, in the Otsuka Formation of the upper Pliocene Nakatu Group occurring in the Aikawa town, Kanagawa prefecture.

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Andesite rocks of the Yamamoto Formation unconformably overlying pre-Miocene basement rocks in the western part of "the Itoigawa-Shizuoka Tectonic Line" in the Northern Fossa Magna were dated by the K-Ar method. Aphyric andesite of the lower member of the Yamamoto Formation is estimated to be a lava flow erupted at 16.0 Ma on the land surface. Andesite of the upper member is dated at 15.5 Ma, whereas the andesite occasionally occurs as hyaloclastite. These results and geological setting suggest that tectonic movements related to the Itoigawa-Shizuoka Tectonic Line might have continued along at least several hundred meters even after 16 Ma.

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