RIVER TERRACES AND ROCK FLOORS ALONG THE UPPER STREAM OF THE YÛBETSU, HOKKAIDO

Abstract

Having made a reconnaisance in this area, the writer classified it into several geomorpholoical surfaces, concerning about the nature and the time of the formation of each surface. (1) Those surfaces are termed, from the lower to the upper, A to G, and mountains; and B is subdivided into Ba and Bb. The relative height of A to G is about 250 meters. (2) A•B•C•D are river terraces, while E has characters of the rock bench or the partial pediment (by Ben A. Tator, 1952-1953). F may be the same with E, and a part of D resembles E. Above identification depends on the structures of the superficial layers underlying those surfaces and on the forms of those surfaces. G is the surface of the inselberg-like isolated hill, relating to E and F. (3) The rocks forming mountains are ancient ones of Hidaka Belt, and the rock underlying G is liparite, while the rest on which the large part of each surface-A-F-lie is loose tuffaceous sandstone. (4) Among the materials (gravels or finer materials) underlying Bb and C, various paleolithic. stone implements have been discovered by Shoichi Yoshizaki, archaeologist. (5) The writer takes a view that such stone implements must be embedded in the material underlying Ba. The reason is that Ba and Bb are supporsed to be the protected terraces, owing to the presence of the gorge of the hard rock of Hidaka Belt downstream, and the time of those formation supposed to have been in the same cycle. But an alternative interpretation would be possible, that is, the time of the formation of the graded reach emerged as Ba later belongs to the alluvial epock, but the time of the accident by which Ba was formed belongs to the dilluvial epock. (6) Geologist Kiyoshi Hasegawa has studied the area and, from the gravels underlying each surface, concluded that the time of the formation of the volcano Tengudake corresponds to the time of the formation of C, and that of the present peak of the volcano is later than that of B and earlier than A. The lava flows of the volcano might affect the drainage systems of this area, and they might also affect the agencies to form those surfaces. (7) In addition to the facts above mentioned, there exist another characteristics, that is, each surface has different pedological character. So, the writer supposes that such surfaces could be classified corresponding to the geological age, from the view point of geomorphology, geology, archaeology and pedology. And the conditions, especially climatic ones, under which each surface was formed could be estimated.