抄録
In the eastern part of Hokaido, in one of Japan's most famous national parks is Lake Akan, situated in the Akan Caldera. The Akan Caldera is surrounded by Mounts O-Akan (1371m), Me-Akan (1503m.), Furebetsu (1097m), and Fuppeshi (1225m), All of which are believed to have been formed following subsidence of the volcono and formation of the Caldera.
During the summer of 1951, the writer was engaged in a survey of lake deposits and lacustrine terraces. Wide areas of lacu.strine terraces were found around the lake and the following conclusions were reached:
(1) Lake Akan is presumed to be a dammed lake from the fact that Tanakadate had discovered the clear shape of drowned valleys on the bottom of the lake. The writer believes that the upper stream of the Akan River flows down within the district of volcanic lava-flow.
(2) The height of lacustrine terracers is approximately 440 meters above sea level while the present lake level is 419 meters. Hence. the height of the lacustrine terraces indicate the level of the palaeo-lake and it nay be presumed that the lake once extended to the upper part of the Akan River. Supporting this idea is the writer's finding of diatomn fossils, such as Melosira italica, and Stephanodiscus astraea, both of which are characteristic of lakes. The inferred height of the palaeo-lake level has coincided with that obtained from the table work at summit level.
(3) Drs. Minato and Ishikawa consider that the form of the Akan caldera was caused by a volcanic mud-flow in the late Diluvium, as in the case of Lakes Towada, Shikotsu, Hakkoda and Kutcharo, The writer regards the thickness of the volcanic mud-flow as not very thick from observations of mudstone having N24°E, N8° in that dip and a strike near the lake shore. It appears to be a case indicated by Dr. Kuno and other geologists who consider bed rock . under volcanoes to be found frequently in high positions of the volcano.
