礫の分布より見た渡良瀬川中流地域の地形

特に扇状地面と河岸段丘面の形成の時代と機構について

抄録

At the region of the middle course of the River Watarase, gravel beds are classified as follows: (1) Godo Gravel Bed (oldest gravels) Their angularity and composition show that they are not the deposits of the River Watarase. (2) Upper terraces and the initial surface of Omama Fan They were formed before the deposition of Kanto Loam. (3) Lower terraces and the erosion surface Omama Fan It seems that they were formed during the deposition of Kanto Loam. (4) Flood plains of the River Watarase
The writer researched the ratio of composition of gravel samples on each of terrace plains, from which he tried to interpret the mechanism of. formation of terrace plains. In this region the writer chose 22 stations for sampling. Samples were taken by roughly outlining a small area of about one meter in diameter. From this area 100 gravels were chosen at, random. Fig. 4 and Table 1 show the composition of the grayel samples collected.
In Godo Gravel Bed, Pal ozoic rocks and Tertiary tuffs are common, white andesites only 11%. So the deposition of such gravels have not much relation to Akag. Volcano. Gravel beds on the initial surface of Omama Fan and upper terraces of Shiobara and Kambal, however, are mainly composed of andesites involved about 80%, which were chiefly derived from Akagi Volcano. It seems that those plains were formed by the increase of materials from Akagi Volcano, and the deposition of volcanic detritus from Akagi was most active during the formation of those upper terraces.
On the lower terrace plains and the erosion surface of Omama Fan, percentages of andesites in gravel samples show 50_??_60% and that of the flood plains about 30%. Therefore, the writer's conclusion is that the deposition, of materials from Akagi Volcano decreased during the formation of lower terraces and such tendency of decrease has continued up to the present.