The Obihiro volcanic sand (Ob) has been used as one of the key beds for the Quaternary deposits in the Tokachi Plain. Stratigraphy of this volcanic sand and its relation to the topographic surfaces were studied. Horizontal variation of its grain size, thickness and mineral composition were traced in detail. Thus, it is suggested that the volcanic sand (Ob) is equivalent to an eastern part of the Eniwa pumice-fall deposits (Ea) which was derived from the Eniwa volcano, near the Sapporo-Tomakomai Low-land. A slight difference in mineralogy between Ob and Ea is noted; a volume ratio of augite to hypersthene of Ea is larger than that of Ob. This difference, however, can be explained as a result of " air-fall differentiation " during the travel of ash-laden cloud. Augite phenocryst is usually coarser than hypersthene phenocryst. Consequently, the coarser grains must have fallen down near the source, resulting in the mineralogical difference as mentioned above. Stratigraphic sequence of Ob is between the Shikotsu pumice-fall deposit Spfa 1 (32,200+4,700,-3,100 y. B. P.) and the Tarumai pumice-fall deposit Td (early Holocene). Accordingly, this volcanic sand with its absolute age will play an important role in the Quaternary geology of the Tokachi Plain as well as the Sapporo-Tomakomai Low-land.
