Abstract
Along the lower course of the Tokachi River, Hokkaido, one can find six terraces, as well as a buried terrace and a fossil valley by means of the electric resistivity method and test boring. These terraces are named I, II, III, IV, V, the buried terrace, and VI from upper to lower. Terrace I to V are covered by Pleistocene (or older) volcanic ashes, but terrace VI is not. It is estimated from the observations of terrace deposits that terrace V was formed at about the time of the fall of the “Erimo Volcanic Ash, ” one of Pleistocene volcanic ashes, and that terrace VI was formed at the beginning of the Holocene. The time of formation of the buried terrace is not ascertained, because data could not be obtained in the buried terrace area. Nevertheless, it seems that the buried terrace was formed soon after the formation of terrace V.
It is certain from the results of electric resistivity methods and test borings that the depth of the fossil valley reached 60m. below sea level, and that it is filled with sand and gravel beds 15m. thick.
The composition of terrace gravels shows that the ratio of andesite is much higher in the gravels of terraces IV, V and VI, than in those of terraces I, II and III, and that the ratio of andesite in the buried gravels in the fossil valley is also higher.
Accordingly, after the formation of terrace III, volcanic activity near the upper coerce of the Tokachi River became stronger.
The profile of the fossil valley bottom seems to continue to intersect the submarine shelf surface, the margin of which exists at 60m. below sea level, 14km. off the sea shore. So, one can imagine that the old shore-line was there.
