In 1958, the exsistence of the land below sea level in the Tokyo lowland, so-called the Tokyo Zero Metre Region, was made clear and mapped by T. Nakano, one of the authors. Since then, he has often tried to analyse the geographical meanings of the land below sea level and he pointed out the importance of progressing ground subsidence. Applied geomor-phologically, the lowland area of Tokyo must be studied in relation with the exsistence of the land below sea level and progress of ground subsidence, and the relationship between ground subsidence and geology of Alluvium must be made clear.
From such standpoint, the authors attempt to restore the buried terraces and valleys by using the data of bore holes and the results of soil tests in the first place and then the de-tailed regional analysis of ground subsidence was carried out by using the data of the obser-vation stations of ground subsidence shown in Fig. 4. The restored buried terraces and valleys are shown in forms of geomorphological map of underground landf orms in Fig. 3 basing on the two datum profiles shown in Fig. 1 and other data concerned. The idealized schematic profile showing 5 buried terraces and several valleys dissecting the terraces mentioned above is demonstrated in Fig. 2.
Data on ground subsidence were analysed annually and regionally. Defining the area of ground subsidence more than 100mm/year as the central area of ground subsidence, it is clear that the central area of ground subsidence is limited at the central part of the Tokyo lowland, and almost coincides with the area above lowest terrace and valleys deeper than 30 metres as shown in Fig. 6. This may indicate the role of thickness of Alluvium to the ground sub-sidence. About 60-100% of the amount of ground subsidence correspond to the compaction of Alluvium or shallower deposits, shallower than 70 metres.
Among Alluvium, clayey sediments are recognized as the major compacting layers, and the relationship between the thickness of clayey sediment and the amount of ground subsidence is found. As shown in Fig. 8, the relationship is not so simple. Not only regional variations, but also annual change of ground subsidence should be investigated. Annual changes of ground subsidence are caused by the change of the amount of ground water taking up from Alluvium and Diluvium water bearing layers. Discussion on this matter will be carried out after the reinvestigation of data on ground water.
So far as the results of this study, thickness of clayey sediments may be recognized as the indicator of ground subsidence in future. At the same time, thickness of clayey sediments will be the indicator of soft ground, which is weak for earthquake damages in the Tokyo lowland. In order to prove this thought, seismological analysis of Alluvium and its top layer untill 10 metres has been carried out by an another study group. Basing on such studies, authors will try to compile the geological map of Alluvium to be used for earthquake damages prediction and prevention.
Not only the application to seismological geological mapping, but also to the urban renewal planning must be considered. Because the area weakest for natural disasters and public nuisance such as ground subsidence are simply bordered by the lines which are decided by the buried landforms and the amount of ground subsidence. This is our future study problem deriving from the study of buried terraces and valleys.

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利根川下流域の沖積平野における地下水中の塩素含有量の分布を調べ,塩素の起源について若干の考察を行ない,次の結果を得た. 1) 利根川下流域の地下水中の塩素含有量は,局部的に非常に多いところがある. 2) 小見川,佐原,鹿島地域で,利根川の右岸と左岸とを較べると,その量は非常に異なっている. 3) 右岸の佐原地域においては, 1,000mgcl^-/L以上の大きい値を示す所が数ヶ所見られ,これらは,ある帯状の分布を示している.小見川地域の場合にも帯状を示すが,これは,明らかに井戸の深い地域と一致し,基盤上の化石谷と一致している. 4) 右岸の帯状を示す地域の塩素含有量は,利根河口堰工事現場の河床に湧出する地下水のそれに類似している. 5) 調査地域の最近世の地史等から考えて,調査地域の地下水中に塩素含有量の多い帯状を示す地域の塩素は,化石海水に由来するものと考えた. 6) 化石海水の絶対年代は,帯水層の堆積の時代から判断して, 1～1.5万年であると推定される. 7) この化石海水は,中性ないしは微酸性を呈する.

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