The Diluvial Terraces in the Northeastern Kantô Plain, Central Japan

Abstract

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”.