Tertiary deposits are distributed in the Etsumi Mountains where many landslides occur. Lithofacies correlative to the Ito-o Formation are composed mainly of volcaniclastic sandstone and conglomerate with andesite lavas in a route at the north of Heike-daira, Fukui Prefecture. These rocks are recognized as a sequence of four grading units which start from andesite lava and grade upward into conglomerate to sandstone by reworking. Most of the rocks suffer weak alteration and sandstone and conglomerate can easily be weathered. Formerly, the Ito-o Formation around this area is thought to consist mainly of andesite to basalt lavas. However main lithology of this route is sandstone and conglomerate. In addition to heavy snow and rain, weakness of the lithofacies such as weathered sandstone and conglomerate may cause landslides which show dominative distribution in this area.