Arakawa lowland is situated in the western part of the Kanto Plain. The alluvial deposits of this area are divided into two formations. The lower one, Nanagochi formation, is mainly composed of organic silt and sand, and includes a basal gravel bed. The upper one, Yurakucho formation, is subdivided into Lower and Upper members. The Lower Yurakucho formation is mainly composed of marine silt with sand and peaty silt in the basal horizon. The Upper Yurakucho formation consists of brackish sand, sandy silt and fluvial deposits, in ascending order.<br>Based on the characteristics of the diatom assemblages, the alluvial deposits can be divided into the following five.<br>1) The first diatom zone: This zone roughly corresponds to the basal gravel bed. Most of the diatoms are Euhalobous species. The dominant species of these samples are Coscinodiscus sp., Melosira sulcata and Cymatotheca weissflogii.<br>2) The second diatom zone: This zone roughly corresponds to the Nanagochi formation. Most of the diatoms are Oligohalobous species. The dominant species of these samples are Synedra ulna, Cocconeis placentula and Hantzschia amphioxys.<br>3) The third diatom zone roughly corresponds to the Lower Yurakucho formation and is subdivided into the two subzones A and B, in ascending order. Subzone A is dominated by Oligohalobous species. Euhalobous species, however, increase upwards in the subzone A. The dominant species of these samples are Thalassiosira bramaputrae and Actinocyclus normanii. Subzone B is dominated by Euhalobous species. The dominant species of these samples are Melosira sulcata, Cyclotella stylorum and Coscinodiscus marginatus.<br>4) The fourth diatom zone: This zone and the fifth diatom zone roughly correspond to the Upper Yurakucho formation. Most of the diatoms in the fourth zone are Oligohalobous species, but this zone is contaminated by Mesohalobous species and Euhalobous species. The dominant species of these samples are Diploneis smithii, Cymbella turgidula, Navicula viridula and Amphora arenicola var. oculata.<br>5) The fifth diatom zone: This zone is characterized by Oligohalobous species.<br>Based on the diaton-flora of each zone, the sedimentary environments are summarized as follows:<br>The basal gravel bed of the Nanagouchi formation yields marine diatoms, which however are believed to be washed out the Shimousa Group.<br>The main part of the Nanagouchi formation is very irregular and variable in its sediment characters and in its diatom assemblages. However, it is thought to have been mainly formed in fresh-water environments.<br>The Lower Yurakucho formation is subdivided into subzone A and subzone B based on the diatom assemblages. Subzone A appears to be characteristic, with an increase of salinity. Subzone B comprises marine sediments settled during the Jomon Transgression. These suggest that the earlier half of the Jomon Transgression is characterized by rapid rising of the sea level and the latter half by a stable inner bay environment.<br>In the Upper Yurakucho formation, the upper sand bed repreasents a transition from marine to fresh water facies, while the uppermost alluvial deposit shows the terrestrial enviromnents. Therefore, the whole of the Yurakucho formation can be seen as having been formed by one sedimentary cycle caused by the sea level change.