A number of ancient sand dunes are developed in the plain to the southeast of Lake Mashu. The plain is also covered with thick layers of volcanic ejecta derived from the Mashu Volcano. Two layers of ancient dune sand are recognized; one is above the layer of Kamuinupuri-a volcanic ejecta and the other above the layer of Mashu-f volcanic ejecta. Very few investigations have been made about the origin of these sand dunes.
After the investigations on volcanic ejecta, distribution of dunes, and mineralogical and mechanical compositions of dune sand, it is considered that the dunes consist of eolian deposits derived from channel deposits, consisting of coarse sand fraction of tephra sorted out by running water.