Abstract
Since the beginning of the Holocene age, the Mashu volcano, situated in east Hokkaido, had grown into strato-volcano. About 7, 000 years ago, a caldera depression occurred as a result of the out-flowing of a great deal of gray pumice. The pumice flow deposit (Ma-f) had widely formed a pyroclastic plateau around the volcano. Then, in some places the plateau was soon dissected by rivers and was covered by reworked pyroclastics.
These processes are well known by recent researches of the volcanic geology and the tephrochronology including the 14C dating.
Till present time, from the altitude of the plain, this plateau had been considered as a terrace plain formed in Pleistocene age. The authors emphasized that it will be sometimes unreliable to consider the terrace or terrace-like plains in volcanic areas, only by the view point of topography.
