RIVER TERRACES AND EARTH MOVEMENT ALONG THE RIVER TOYO

Abstract

The River Toyo flows into Atsumi Bay, taking the northeast-southwest direction along the median line which is one of the most important tectonic lines in Japan. There is a considerable difference in the topography and geology between the areas of the right side (Inner Zone) and the left (Outer Zone) of the river (Figs. 1 and 2). The authors surveyed the terrace topography and deposits along the River Tokyo in order to make clear the process of the formation of terrace plains and the earth movement in this region. The main results are as follows: 1) The terrace plains of this region are classified into three kinds: the upper, the middle and the lower. The upper terraces are distributed widely on the right side of the river and are correlated with the surface of Takashihara. The middle terraces are most remarkable and also widely distributed on the right side of the river. The lower terraces are subdivided into two. The lower terraces I distributed locally are intermediate terraces between the middle terraces and the lower terraces II which gradually change to the present flood plain (Figs. 3 and 4). In the both terraces, the upper and the middle, in the lower course from Shinshiro City their distribution and altitudes are different from those in the upper course area, and the gradients of the former terrace plains are steeper than those of the latter's (Tab.1). So the upper and the middle terrace plains situated in the lower course have the characteristics of fan-like topography. 2) The origins of the terrace deposits are classified into two groups (Tab. 2). The one is the terrace gravels which usually contain the liparite gravels derived from the uppermost area, and are transported by the River Toyo itself. The other is the fanglomerates which consist of the angular gravels of mica schist or mica gneiss constituting the Inner Zone mountain land, and seems to be transported by the small tributaries. The distribution of the fanglomerates coincides with that of the fan-like topography above mentioned (Figs. 5, 6 and 7). 3) Small faults cutting the bedrock are observed at the mountain foot of the Inner Zone (Fig. 5). Strikes of the fault plains are mostly NE-SW, namely, it resembles to the direction of the steep scarp and the arrangement of kernbuts and kerncols. Judging from the data mentioned above, the authors have reached tentative conclusions concerning the process of the formation of the terrace plains and the earth movement as follows: It seems that during the formation of the upper terrace plain the deposition of the River Toyo took place more actively than during the middle and lower terrace plain formation. The small tributaries from the Inner Zone mountain land also deposited the great amount of the fanglomerates following the deposition of the upper terrace gravels. It means that there was a tendency to elevate more rapidly in the Inner Zone than in the Outer Zone area. Such a tendency of the earth movement is also observed during the middle terrace plain formation, but the amount of this is smaller than the case of the upper. However, this tendency is not observed during the lower terrace plain formation. Moreover, even in the Inner Zone area, this tendency is clear at the lower course from Shinshiro City. The longitudinal profile of each terrace plain is astringed toward the lower course. Consequently, it is considered that the amount of the upheaval movement connected with the formation of the terrace scarp is greater in the upper course area than in the lower course in each stage. Such a tendency is contrary with that of the earth movement observed in the region in Tempakuhara which is older than the terrace plains in question.