The author intends in this study to clarify the formation of the Inland Sea in Plio-Pleistocene time in relation to the activity of the Median Tectonic Line (M. T. L.). The M. T. L. is believed to have been active through several stages, but details of its activity during Plio-Pleistocene time have not been known well. He investigated the Plio-Pleistocene sediments, the Osaka Group on the northern area of the Izumi Range which is bordered, on the south, by fault scarps of the M. T. L. formed in Plio-Pleistocene stages. This study was made by relating the change of facies of the Osaka Group with the uplift of the Izumi Range. As the result, the followings could be concluded: 1. The Osaka Group in this area is divided into Uchihata members (the Pliocene), Amano members (the Plio-Pleistocene) and Semboku members (the late Pleistocene) in ascending order. 2. The Uchihata members contain the gravels derived from the area to the south of the M. T. L. In this stage, the Izumi Range had not uplifted yet and the rivers from the southern area crossed the M. T. L. 3. The M. T. L. became active after the sedimentation of Uchihata members. Along the M. T. L. was formed the structural valley of the Kino river on the south and the Izumi Range on the north. The latter supplied the surrounding area with coarse gravels. The Amano members were formed in this stage. 4. In late Pleistocene time has developed the structure of the Osaka Group accompanied with faults, flexures and anticlines of N-S or NE-SW trend by E-W compression. The Semboku members deposited in this stage.
The vegetational history and the paleogeography of the Kawachi Plain for the last 13, 000 years are summarized as follows; 1) For the period between 13, 000 and 10, 200yr.BP, the vegetation in the Kawachi Plain was dominated by the subalpine forest including Pinus (Haploxylon), Picea, Abies and Tsuga. The climate was colder and drier than that of the present. However, around the period between 11, 800 and 11, 000yr.BP, there was a short warm duration characterized by the increase of the temperate trees such as Quercus, Carpinus, Ulmus, zelkova and Corylus. The sea-level was nearly more than 22m lower than the present one. The flood plain was spreaded over the Kawachi Plain and Menyanthes, Cyperaceae and Sphagnum grew on the marshy lowland. 2) Around 10, 200yr.BP, the subalpine trees were abruptly reduced and the temperate trees (Quercus, Ulmus, Zelkova and Carpinus) began to increase. This change of vegetation was perhaps caused by the sudden climatic warming. Warm temperate plants which had been thrusted into the marginal part of the southern Kyushu and Shikoku migrated to the northward and upward. The period between 10, 200 and 9, 000 or 8, 500yr.BP was characterized by the prominant of grass pollen (Artemisia, Compositae, Persicaria and Lythrum) indicating the low density of the forest. 3) Around 9, 000 or 8, 500yr.BP, the temperate trees such as Quercus, Celtis, Aphananthe, Ulmus, Zelkova, Carpinus and Fagus rapidly increased indicating the climatic amelioration. After that time, sea-level rised fast and the shoreline transgressed into the Kawachi Plain. 4) Around 6, 500yr.BP, the migration of the plants was almost completed and the warm temperate species (mainly Cyclobalanopsis and Castanopsis) were able to colonize in the hilly district of the Kawachi Plain. At this period, the embayment area was spreaded over the lowland of the Kawachi Plain. 5) Around 3, 000yr.BP, the shoreline regressed and the lobate delta was spread over the southern part of the Kawachi Plain. The microrelief of the lobate delta was chracterized by two landform types, i. e. dry highland and marshy lowland. Yayoi man took the dry highland as a residential place and the marshy lowland as a rice field. The vegetation around the residential highland was covered with grasses. The climate was slightly cooler and wetter than that of the present. In the late Yayoi period, the great flood occurred and the residential area was submerged. After this catastrophic submergence, the Kawachi Plain was covered by the lagoonal deposit. 6) At the beginning of the Kofun period, the strandlinie of the Kawachi lagoon regressed and the flood plain of the Yamato R. was spreaded over the southern part. Man readvanced into the flood plain which was composed of natural levee deposit. From this period, man's impact on vegetation was enlarged and the broad leaved forest (Cyclobalanopsis, Castanopsis, Celtis and Aphananthe) was deforested by Kofun man. The pine forest rapidly expanded.
The Musashino Upland, a dissected fan of the Tama River in the western suburbs of Tokyo, is displaced by the Tachikawa fault, which runs in a NW-SE direction with long-continued flexure scarps. Using the tephrochronological method and data of water well logs, the author investigated geomorphic features and movement history of the fault in detail. It is thus revealed that the fault has displaced the terrace surfaces of various ages, including the Tc3 surface of 14, 000 years B.P. and the Holocene old stream channels (Fig. 1, Fig. 10). The Plio-Pleistocene sediments, buried under the Musashino Upland, have been vertically displaced about 70m at Hakonegasaki (Fig. 5, Fig. 6). This fault has only vertical displacement with no strike-slip component, and the upthrown side of the fault is northest. The long-term average slip-rate, derived from the displacement of terrace surfaces, is the largest in the central part of the fault and gradually decreases toward the both ends. The maximum average slip-rate is 36cm/1000 years. The total length of the fault is 21km. If examined in detail, it may be divided into two segments from discontinuity of the geomorphic expression and difference in the long-term slip-rate (Fig. 11). No sign of fault movement has been found from precise levelling across this fault for 40 years. So it is thought that fault movements had been recurred on this fault with large earthquakes in the late Quaternary period. The fault displacement in an earthquake, the earthquake magnitude, and its recurrence interval are estimated at about 1.8m, M. 7.1, and 5, 000 years, respectively. There are two types of surface deformations along the Tachikawa fault. One is a flexure scarp of 100-300 meters in width (Fig. 8). This flexure scarp is thought to have been formed by the existence of thick unconsolidated fluvial gravel deposits overlying the faulted Plio-Pleistocene sediments. The other is a broad undulation of a few kilometers in a half-wave length across the fault trace (Fig. 12). This undulation seems to reflect the elastic dislocation of the crust due to the movements of the Tachikawa fault.