Geomorphic change of alluvial plains and coasts in the Holocene Epoch has been significant for human beings as an environmental change in Chiba Prefecture as well as in the Kanto District. The successive changes of these landforms are described for the past 6000 years, that is the relatively stable time in sea level after the Yurakucho (Flandrian) transgression. Some sea cliffs facing the Pacific Ocean have been continuously eroded by waves from 6000 years ago to the present; some ones facing Tokyo Bay had been eroded to the historic time; while some others near delta plains along the coast of Tokyo Bay had been formed by the historic time and the abrasion platforms in front of the cliffs have been buried by deltaic sand. Rivers such as Yoro and Obitsu have protruded deltas into the drowned valleys formed by the Yurakucho transgression, and then to Tokyo Bay in the same 6000 years. Much smaller rivers like Miyako in Chiba City could not have extended deltas to Tokyo Bay, but drowned valleys were closed by baymouth bars, and peat has accumulated in thus built lagoons from about 3500 years ago to the historic time. Retrogradation of sea cliffs, progradation of deltas, and the processes of baymouth bar or lagoonal deposition are generally understood, when we assume, as a first approximation, a stable sea level, uniform rate of uplift or subsidence, and continuing processes of waves and rivers under given local circumstances in that term. However, for features diverged from those made by uniform processes, such as three definite groups of sand ridges in the Kujukuri coastal plain, we need remodelling the first approximation, in this case for example, adding three small fluctuations to the stable sea level. It is known through this study that both the first and the second approximative approaches are necessary to understand the geomorphic development in the past 6000 years and also to predict the future geomorphic change. In the note attached, diatom assemblages found in 17 samples from a boring to Holocene deposits are described. The data obtained were useful to reconstruct the process of alluviation in the small valley of Miyako River.
It is generally accepted that the slight unconformity or unconformable relation in the tephras indicates immediately the existence of period of erosion preceding the stabilization of the slopes, followed by the deposition of the next tephra layers. But, generally speaking, typical unconformable horizons in the Pleistocene tephra formations of the southern Kanto districts coincide with boundaries of marine sediments and also with the boundaries of the volcanic activities of both western volcanoes, Mt. Hakone and Mt. Fuji (UESUGI, 1976; KIKUCHI and KANTO QUATERNATY RESEARCH GROUP, 1977). Therefore, the authors are of the opinion that not only in the Pleistocene tephra formations but also in the Holocene tephra formation, considerable number of unconformable relations may be true unconformities. In this paper, the unconformable relations in the Holocene tephra formation are described around the northern and eastern flanks of the Fuji volcano, located in the uppermost reaches of the two drainage basins of the Sagami and the Sakawa. The localities of the typical outcrops are shown in Figure 1, and the schematic profiles of several typical outcrops where the unconformable relations are observed, are shown in Figure 2. Standard sequence of the Holocene tephra formation of this area is illustrated on the left column of Figure 3. On the right column, all the unconformable relations and other data are recorded. The results are summarized as follows: 1) These unconformable relations are recognized in nine horizons as shown in Figure 3. 2) The unconformable horizons which are recognized in the both drainage basins are only three, that is, beneath the base of the Holocene tephra formation, and beneath the Yu-1 and Yu-2 tephras. In these horizons, buried soils underlain by the stabilized slope develops conspicuously and the tephra layers or lava flows overlying these soils are the greatest in scale among the Holocene volcanic activities of the Fuji volcano. 3) There exists no other unconformable horizons in the northern flank, and, on the other hand, in the eastern flank six horizons are recognized in addition. Those are beneath the S-0, S-1, S-5, S-15, S-24 and S-25 tephras. The reason for this difference between the two drainage basins, is one of the problems for the further investigation.
It is well known that the tectonic basins called the Kanto Tectonic Basin in the central part of the Kanto Plain has been subsiding during the Quaternary Period. Many previous workers have a firm belief that the rivers of Tonegawa and Arakawa had been running along the Tonegawa Lowland situated in the center of the Basin and flowing into Tokyo Bay before their water-courses were changed artificially into the present courses about 300 years ago. The author pictured a paleogeographic map (Fig. 2) about the central part of the Kanto Plain in the Late Pleistocene basing upon the pattern of valleys dissecting the uplands composed of fluvial and coastal deposits during the Middle Pleistocene, and assumed that the rivers of Tonegawa and Arakawa was flowing along the Arakawa Lowland at that time. The same result was already published by MATSUDA (1974) basing on the subsurface data, who figured some geologic sections traversing the Tonegawa, the Nakagawa, and the Arakawa Lowlands. That is to say, the rivers of Tonegawa and Arakawa took their courses along the Arakawa Lowland from the Late Pleistocene till the Early Holocene. On account of the deposition in the floodplain and the subsidence of the Kanto Tectonic Basin, these rivers changed their courses to the east about 4, 000 years ago. Rapid deposition began in the Tonegawa Lowland area and uplands were buried, and some sites of the Middle Jomon stage on the uplands, too.
1. Present distribution of vegetation Lucidophyllous forests in Japan belonging to Camellietea japonica are called “Laurisilvae” by RÜBEL (1930), however, they are very unique and characteristic to the humid warm-temperate Asian region. The horizontal zones of evergreen broad-leaved climaxes or subclimaxes are Machilus thunbergii, Castanopsis cuspidata var. sieboldii and Cyclobalanopsis spp. zones from the coast inland on the Boso Peninsula. In the representative association of lucidophyllous forests there is Castanopsietum sieboldii which is divided into two associations: Bladhio-Castanopsietum sieboldii in the north and Rumohro-Castanopsietum sieboldii in the south. Even in hilly lowlands below 400m in alt, on the Boso Peninsula, there are three vertical zones: 1) the lower hilly zone with Castanopsion, 2) the upper hilly zone with Cyclobalanopsion including Abietum firmae, and 3) the lower montane zone with Tsugion. This phenomenon is based on the shrinkage of the vertical zones. The northern border lines of the climax species remarkably overlap one another, however, those of seral species are diversified due to the difference in temperature response. There are three types of secondary forest: 1) Pinus thunbergii forests along the sea coast, 2) Pinus densiflora forests on sterile, dry habitats, and 3) Quercus serrata forests on fertile, moist habitats. There are also three main types of semi-natural grassland: 1) Imperata cylindrica var. koenigii grasslands along the sea coast, 2) Miscanthus sinensis and 3) Pleioblastus chino grasslands inland. Recently, pioneer communities of naturalized plants are extending their areas. Japan is divided into four climatic zones (subarctic, cool-temperate, warm-temperate and subtropical), but its coastal area is divided into only three zones (cool, warm and subtropical). This is due to 1) the oceanic climate, and 2) coastal species being seral species. 2. Present flora The Boso Peninsula is divided into three floristic areas: Nouth-Boso, South-Boso and Kujukuri areas. Northern plants come into the North-Boso area, southern plants occupy the South-Boso area, and both of them live in the Kujukuri area. The inner zone (North-Boso) and the outer zone (South-Boso and Kujukuri) are separated by the Wakana's zone (Fig. 6). The Crinum line passes through the southernmost part of the Boso Peninsula which is the nothern limit of frostless area and is considered to be the periphery of the subtropical zone. Southern species of tropical and subtropical regions, such as Cynodon dactylon, are distributed along the coast in Kyushu because of its oceanic climate and scarce interspecific competition. Among the 2, 500 species in the flora of higher plants in the Boso Peninsula, about 300 are aliens because man-made or man-modified vegetation covers a very wide area there. 3. Past flora and vegetation The southern border of Fagus crenata forests is the isotherm of 10°C in annual mean temperature. If the annual mean temperature had dropped 8°C during the coldest period of the last ice age, Fagus crenata forests would have covered Kyushu to its southernmost part. Even if the annual mean temperature had dropped 5°C, the Boso Peninsula would have been covered with Fagus crenata forests. Elements of the cool-temperate zone are included in the flora of the Boso Peninsula as relics of the cold past during the Tertiary and the Quaternary. Fossils of those cool floras have been found there. Pinus parviflora and Tsuga sieboldii forests on the ridges of hills are also relics of the cold age due to the shrinkage of vertical zones. During the cold age when Fagus crenata was distributed to the south, the Zoysia japonica pasture grazed by wild animals under the cool-temperate climate was distributed southward simultaneously.
First, the author reviews the palynological studies of the Upper Pleistocene in Kanto district, and point out the deficiency of their outcome for paleobotany. Accordingly, the author has studied the pollen assemblages from the Musashino and the Tachikawa Loams corresponding to the Würm Glacial Age. The Musashino Loam is characterized by a dominance of pollen of such conifer and deciduous broad-leaf trees as Pinus, Cryptomeria, Fagus, Quercus and Zelkova. In the lower of the Tachikawa Loam (under the 2nd Black Band), the pollen assemblages resemble to those of the Musashino Loam. An abrupt increasing of such grass pollen as Artemisia, other Garduoideae and Gramineae is certified at the lowermost of the 2nd Black Band.
Six different types of shallow water molluscan assemblages have been distinguished from the embayment deposits accumulated during the maximum stage of the post-glacial Jomon Transgression (ca. 6, 500-5, 000y.B.P.) in the southern Kanto, central Japan. These assemblages are also recognized in the embayment deposits of the very early stage as well as in the later stage of the transgression. In detail, however, their rise and fall are in harmony with the sedimentary history during transgression. In the early stage of the transgression, ca. 10, 000-7, 000y.B.P., the Crassostrea gigas-Anadara granosa-Batillaria zonalis assemblage and the Dosinella penicillata-Paphia undulata-Anadara broughtonii assemblage predominated in accordance with the development of muddy facies on the bottom of drowned valley. On the contrary, the sandy bottom assemblages, such as, the Meretrix lusoria-Mactra veneriformis-Umbonium moniliferum and the Meretrix lamarcki-Umbonium gigantium-Glycymeris albolineata assemblages, became dominant after the maximum stage of the transgression, from about 4, 500 to 2, 000y.B.P., during the reclamation stage of the embayment by the sandy materials provided from the adjoining rivers. The intertidal assemblage, the Crassostrea assemblage, found in the innermost part of the bay is characterized by the association of several warm water species. Two steps of invasion of the warm water species are recognizable in the southern Kanto region. At the first step, ca. 9, 500-8, 700y.B.P., Anadara granosa, Anomalocardia squamosa and some other warm water molluscs appeared following the advancement of the Crassostrea assemblage into the embayment. The second step is at the beginning of the maximum stage of the Jomon Transgression, about 6, 500 to 6, 000y.B.P.. Molluscs of the tropical nature are the invaders of this step. They are represented by Ostrea pauluciae and Tellinimactra edentula. These warm water species rapidly declined in the later stage of maximum transgression, between 5, 000 and 4, 000y.B.P., and most of the species disappeared after 4, 000 years. Cause of disappearance of the warm water species has been said to be the lowering of water temperature. But it is known that the main cause of the disappearance is most probably attributable to the disappearance of their habitats, shallow muddy environment, rather than the lowering of water temperature. There are many shell-mounds formed during the Jomon Era along the west coast of Tokyo Bay. Their situation and duration, relative abundance, and the change in species composition of shells can be explained by changes of paleogeography, sedimentary facies, and associated molluscan assemblages in and adjoining coastal areas of the embayments formed during the Jomon Transgression.
Seasonal dating by counting the daily growth lines of the shells of the bivalves Meretrix lusoria, Tapes japonicus and Corbicula japonica was applied to the materials excavated from 12 archaeological sites ranging in age from Earliest through Late Jomon, the Kofun and the Edo periods (4 shell layers from shell-midden deposits, 7 shell deposits in abandoned dwelling pits, and 8 shell deposits in other pits). 1) The stratigraphic distribution of collection seasons of the bivalves in shell deposits can be used to distinguish primary deposits from secondary deposits. These latter deposits have several seasons within one block and the seasons are scattered homogeneously throughout the shell layer. Shell-midden layers on sloping ground were found to be secondary deposits. Most of the shell deposits in dwelling pits and storage pits seemed to be primary deposits in which the particular collection seasons clustered stratigraphically. 2) The seasonal sequences of the primary deposits are illustrated in Fig. 5. In all the deposits only 5 layers were determined to contain shells from only one 1/8-year collection season. The remaining layers contained shells collected during several 1/8-year seasons, and some of them showed successive accumulation over several seasons. In a few of these layers the time span of accumulation exceeded a one-year cycle (Fig. 6). 3) The accumulation speed of the primary deposits was then calculated from the volume of the shell layers accumulated within the largest 1/8-year layer in each deposit (Fig. 7). In general, the shell deposits of the Jomon period showed rapid accumulation, for example, 1000×103cm3 in the 1/8-year shell deposit of the Kode dwelling pit 407. Those of the Kofun and Edo periods showed slower rates. 4) Each assemblage usually contained shells collected throughout the year, suggesting that these sites were used for full one-year cycles as permanent settlements. The general pattern of the seasonal assemblages of shell collecting activity examined in shell-midden layers shows about 70% of the total shells dated to spring and early summer, then this percentage decreasing gradually in late summer and autumn, reaching almost zero in winter.
The fact that the faunal remains of shell middens were originally selected from the local faunal assemblages denotes that they are able to provide the basic data; first for reconstructing the local prehistoric environment and second for clarifying the culture factors of the collecting habits of the shell midden people. To arrive at these two ultimate ends, first, we have to identify the species of shellfish, and further, examine the relative frequencies of the species and their size-age frequency distributions. But the characteristic features obtained in these ways will have to be compared with other standards, otherwise these results will have no meaning. From this viewpoint, the fossil molluscan assemblage in the Holocene deposits readily provides standards for analysis of the shell middens. The midden shellfish were selected from the local marine molluscan assemblage. The degree of selectivity is based on collecting methods, equipment, grounds and seasons. Prehistorically, it is important to clarify the nature of collecting habits. For this reason, it will be useful to compare samples before and after gathering, i. e. fossil molluscan assemblages and shell middens respectively. In the present paper, a comparison of the shell midden samples with the fossile assemblage on the basis of relative frequencies of species and their sizes has been presented in order to investigate species and size selection in the shell midden samples. That is to say, through a comparative study of both natural and midden samples, we are able to examine both the quality and quantity of exploitative selection, and further, the pattern of resource exploitation which is the cause of the selection.
As a various kind of the artificial ground movements are observed in the south Kanto district, we have described only the artificial ground movements with the developments of the ground water resources and the natural gas resources (natural gas bearing salt water) in this paper. The stratum-deformation patterns caused by the pumping action are different according to the geological age of the stratum. The Holocene deposits have shown generally the irreversible consolidation with the draw down of ground water pressure by the pumping discharge. The Pleistocene deposits, however, have been contracted under the decrease of ground water pressure by the over pumping discharge but have been rebounded slightly under the recovering of the ground water pressure by the pumping control. On the other hand, it has been observed that the pulse of the ground water pressure has been annually by the change of pumping discharge of the wells for the agricultural use and etc., and has been conveyed undulatingly to a distance. Therefore, it seems that the ground movements have been carried undulatingly by the undulative change of the ground water presures.