In the Konsengenya Plain, Pleistocene formations are extensively distributed. The Lower to Middle Pleistocene formations yield fossils of mollusca, pollen, diatom, and are divided into the Furenko Formatoin, the Bekanbeushigawa Formation of the Kushiro Group and the Konsen Formation in ascending order. The Furenko Formation is mainly composed of silt and silty sand. The Formation yields mollusca of Acila nakazimai, reported in the Pliocene to Lower Pleistocene. Diatom of Denticulopsis seminae var. fossilis is found in this formation. KOIZUMI (1985) reported the diatom zone of this species indicating the age of 1.89-2.50 Ma. The pollen fossils of Cryptomeria, Picea and Betula are dominantly included; the composition indicates the warm humid climate. The Bekanbeushigawa Formation consists mainly of pumiceous gravel, sand, tuffaceous silt and peat. Some species of mollusca are regarded as the cold current type. The pollen fossils of Picea, Abies, Tsuga and Larix are dominantly included; the composition indicates the vegetation of the coniferous forest in subpolar zone. The fossil diatoms regarded as fresh water species are dominantly included in this formation. The Konsen Formation is composed mainly of sand, silt and gravel. The fossils of mollusca of warmer species are remarkably included. The composition of the pollen fossils indicates the vegetation of broad-leaved forest in temperates zone. The diatomaceous fossils are composed of brackish〜sea water species and fresh water ones. According to the above results, the geological age of those formations is as follows; the Furenko Formation belongs to Upper Pliocene to Lower Pleistocene age, the Bekanbeushigawa Formation Lower to Middle Pleistocene age, and the Konsen Formation Middle Pleistocene age.

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The Pliocene to Pleistocene Kobiwako Group in the Iga and Omi basins is composed of fluviolacustrine loose sediments with more than 100 layers of volcanic ash. It is lithologically divided into five formations, i. e. the Iga, the Koka, the Gamo, the Kusatsu and the Katata Formations in ascending order. The Iga Formation in Hananoki Hills, west of Ueno City, is composed mainly of gravels and silts. It is lithologically divided into three members, namely the Yono, the Ishiuchi and the Hokke Members in ascending order. The Yono Member is composed of alternating beds of sand, silt and clay, and intercalates gravels. The Ishiuchi Member is composed mainly of gravels with silt lens, and the Hokke Member is composed of alternating beds of silt, sand and gravels. Strata ranging from the base of the Ishiuchi Member up to the top of the Hokke Member show an fining upward sequence of large scale. The Kobiwako Group in this area generally abuts unconformablly on the basement rocks on the south margin of the basin, while it is in fault contact with them on the north margin. The strike of unconformity plain has NNW-SSE and ENE-WSW directions, and that of fault plains in the basement rocks underlying the group also shows NNW-SSE and ENE-WSW directions. These facts suggest that the basin-forming movement of Kobiwako involved the development of fault systems in the basement. Gravels in the Yono Member are of acidic tuff of Miocene Muroo Group south of this area, and of chert, sandstone, and shale which were derived from Paleozoic to Mesozoic Tamba Group, and granite and gneiss of Ryoke Complex. On the contrary, gravels of the Ishiuchi and Hokke Members are of acidic pyroclastic rocks probably of Mesozoic Koto Rhyolitic Rocks around Lake Biwa, and of Tamba Group and Ryoke Complex origins. Therefore it is assumed that at the time of beginning of the sedimentation of the Ishiuchi Member, the direction of source area of these gravels was changed from south to north, causing of upheaval of the surrounding area of Lake Biwa.

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1. In the northeastern area of the Ashigara district, the beds belonging to the Seto Formation, a lower member of the Ashigara Group, contain abundant pebbles of quartz diorite which had been distributed around the Hatano basin on the side of Tanzawa side. Since this quartz diorite had intruded into the middle Tanzawa Group, the existence of its pebbles indicates that, in the Tanzawa district at the time of deposition of the lower Ashigra Group, the middle part of the Tanzawa Group was exposed on the ground surface. 2. In the Shiozawa Formation, an upper member of the Ashigara Group, the pebbles of crystalline schist and quartz diorite that constituted the lower Tanzawa Group in the central area of the Tanzawa mountainland begin to occur, and their amount increases toward upper horizons. This fact means that the lower part of the Tanzawa Group was exposed on the ground surface in the Tanzawa district at the time of deposition of the upper Ashigara Group. 3. Conglomerate of debris flow type was formed during the depositional period of the Doyama〜Seto Formations of the lower Ashigara Group. Therefore, the Ashigara sedimentary basin of this period was probably bounded by steep slopes of mountainland. 4. The debris flow type conglomerate is abundant also in the Shiozawa Formation of the upper Ashigara Group. It is considered, therefore, that the sedimentary basin at this stage was fringed by the steep slopes of mountainland. 5. From the Doyama Formation to the Tsuburano Formation of the lower Ashigara Group, the ratio of conglomerate to other rock types becomes very high, with a peak in the Seto Formation. This may indicate that the hinterland on Tanzawa side was intensely uplifted at the depositional stage of the lower Ashigara Group. In the Hatazawa Formation of the middle Ashigara Group, however, the ratio of conglomerate is the lowest and the ratio of fine-grained rocks is the highest of all formations of the Ashigara Group. It suggests that the uplift of the hinterland on Tanzawa side became very small. And the Shiozawa Formation of the upper Ashigara Group shows the highest ratio of conglomerate, indicating that the hinterland was strongly uplifted again. 6. The Doyama Formation in the lowermost horizon of the Ashigara Group is composed also of elastics derived from the hinterland on Tanzawa side. The strata underlying the Doyama Formation are not observed on the ground. Accordingly, the existence of the Izu-side block, which is insisted in the Izu collision hypothesis (SUGIMURA, 1972, etc.) to have advanced to the immediate south of the Ashigara sedimentary basin at the basin's developmental stage, cannot be confirmed except by means of boring underneath, or on the south side of the Ashigara Group.

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In the Sado Island, Neogene marine strata are widely exposed and unconformably overlie the Palaeogene and Early Miocene pyroclastic deposits which are underlain by Palaeozoic Group and Granites. The Neogene marine nonpyroclastic deposits in the southern part of Kosado Mountains are divided into the Orito, Tsurushi, Notayama and Yamadagawa Formations in ascending order. The Orito Formation consists of conglomerates, sandstones, siltstones and calcareous sandstones, with basal conglomerates, consisting frequently of poor-sorted and large size gravels, and also contains some intercalations of dacitic welded tuffs and pumice tuffs. It is underlain by Early Miocene Sanze and Kyozukayama Formations composed of andesite and perlitic dacite lavas with abut and overlap unconformity. Large foraminiferas, Operculina and Miogypsma, and molluscan fossils which are correlative with the Kadonosawa fauna have been obtained from the Orito Formation. The formation represents as typical transgressive facies in Late Early Miocene-Early Middle Miocene. The Tsurushi Formation consists of hard shales and tuffaceous mudstones which interfinger with thick pillow lavas and hyaloclastites of basalt (the Ogi Basalt Member) exposed in Ogi Peninsula. The mudstone bed under the basalt lavas contains Denticulopsis hyalina. The depositional environment changed from the shallow sea (littoral-sublittoral zones) to deeper sea (sublittoral-upper abyssal zones). The Notayama Formation consists of laminated diatomaceous mudstones and the Yamada-gawa Formation also consists of massive greenish grey diatomaceous mudstones. The two formations are Middle-Late Miocene in age according to diatom biostratigraphical analyses. The depositional environment of the Natayama Formation was more deeper than one of the Tsurushi Formation. The Quaternary deposits unconformably rest upon the Neogene. Especially, the Upper Pleistocene Ootani Formation which is composed of gravels, sands and muds is widely distributed in the central part of this area.

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The Miocene of the study area is divided into the lower non-marine Shiomachi Formation and the upper marine Bihoku Group. The Shiomachi Formation, which is about 15 meters thick, is mainly composed of tuffaceous conglomerates with thin layers of lignite. The Bihoku Group is sub-divided lithologically into the Korematsu and the Itabashi Formations in ascending order. The former formation is about 60 meters in thickness, and it is composed of conglomerates, sandstones and mudstones. The latter formation is estimated to be 65 meters thick, and it is composed of shales and alternating beds of very fine grained sandstone and shale. Many species of molluscan fossils have been obtained from this group. They are discriminated into five assemblages as follows; the Vicarya-Crassostrea-Anadara, Tateiwaia-Crassostrea, Phacosoma-"Vasticardium"-Siratoria, Macoma-Mizuhopecten and Limopsis-Fissidentalium assemblages. Among them, the first four assemblages occur in the Korematsu and the last one in the Itabashi Formations as shown in Fig. 4. The early stage of the sedimentation of the group is separated into two areas by N-S trending basement ridge. The Vicarya-Crassostrea-Anadara and the Tateiwaia-Crassostrea assemblages were distributed in the eastern area associated with brackish fauna. The Phacosoma-"Vasticardium"-Siratoria assemblage prospered in the western and inner part of the eastern areas, and the Macoma-Mizuhopecten assemblage was distributed in the muddy facies of the most central part of the eastern area. On the other hand, the Limopsis-Fissidentalium assemblage prospered and indicates a depth of more than 200 metere at that time.

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