The northeastern part of the Kantô plain, which is the largest plain in Japan, has many terraces as shown in the geomorphological map (fig. 1). In this paper, the landdforms and the geological structures of these terraces are described as below. (1) General characteristics of the terraces. The above area is divi ed into two topographic regions by the Yamizo mountain range running from north to south. The estern part is practically made up of marine Pleistocene strata. On the other hand, the western part has many river terraces which are formed by fluvial gravels and volcanic ashes. (2) Dating of the terraces by tephrochronology (tehro means volcanic ashes). During the Quaternary period, the terraces had been covered by volcanic ash layers called Kanto Loam. Presently the older terraces and the newer ones both are covered by thick and thin Loams respectively. This relationship between terrace and Loam makes it possible to date the land surfaces chronologically. Through this the author obtained the following chronological table. (3) The river terraces built by eolian volcanic ashes. The western area is chiseled off by many river terraces. In the northern part of this terraces region, the relative heights of terrace surfaces from the preseat flood plain are higher than that of the southern part. But, if the volcanic ashes on the whole terraces are stripped off, the surfaces of the terraces have practically no significant heights, In other words, the higher the terraces, the thicker the volcanic ashes are as shown in Fig. 4, In conclusion, these river terraces would be called “Eolian volcanic ash terraces”.
As the result of the writer's survey on the erosion surfaces in the Haku-san mountain ranges and its adjacent areas, 19481954, the following main facts are known : 1) The Bessan erosion surface having about 2200 m. to 2300 m. in height lies near the peak of Bessen (2399.4 m. high) which is composed mainly of coarse grained sandstone and conglomeratic sandstone of the Tetori group of Mesozoic. Bessan, which lies to the south of the Haku-san volcanoes, and protrudes about 100 m. from the Bessan erosion surface, is seemed to be a monadnock on this surface. 2) The basement of the Haku-san volcanoes shows from 2200 m. to 2300 m. in height. Therefore it is thought that the surface on this basement is the northern extention of the Bessan erosion surface. 3) The Shira-kawa erosion surfaces having 1700 m. to 1800 m. in height are distinctly observed at Sarugababa-yama, Gozen-dake, Hideri-dake and other places, and these surface are about 300500 m. lower than those of Bessan district. 4) The flat plane west of Gankyoji mountain in the Koike area was formerly considered as an erosion surface by predecessors but it is now regarded as a deposited plane due to the volcanic activities of the Gankyoji mountain. 5) The Bessan and Shira-kawa erosion surfaces have been formed during from post-Miocene to the pre-Haku-san volcanic activities in Quaternary, and the former erosion surface seems to be more or less older than the latter in age.
On making ground water table map, it is needed to have the value of ground height. On a preliminary survey, one sometimes uses the value from map reading instead of that from leveling. The author wants to know the accuracy of the value from map reading. He carried on his survey on Kisogawa fan-like delta plain, Oomama fan-like upland, Kinugawa alluvial plain and Yumihama sand bar areas. He selected many points on these districts and measured the heights by leveling (G. H.). Then he checked these heights on topographic maps having the scale of 1 : 25, 000 and that of 1 : 50, 000 (G. H. 2.5 G. H. 5.0) respectively. Reading the differences between the value of leveling and that of map reading on each scale of maps (X2.5, X5.0), he calculated the mean X2.5 and R5.0. Then he estimated the range of mean taking confidence factor as 95 %. As to the absolute value of mean, there is no signifigant difference between the value from the map of 1 : 25, 000 and that of 1 : 50, 000. The range of the distribution of mean value from the map of 1 : 25, 000 is 1.0 meter and less than that from the map of 150, 000 amount to 1.5 meters. Generally speaking, ground height from map reading can be used for making water table map with contour interval of 2 meters on flat areas.