A geological columnar section was compiled during the geological survey and the exploration for strong coking coal in the Shikamachi area and its surroundings in the Saseho coal field in 1950-1951. The coal deposits of the field occur in a series of Palaeogene sediments 3, 000m in thickness. They are marine to fresh water in origin and are mainly an alternation of shale and sandstone with intercalated layers of tuffaceous rocks and very poor and hardly recognizable conglomerate. The columnar section was applied to the geologically unsubdivided coal field to the north and adjoining the area and covered for the most part by olivine basalt lava flows. A strong coking coal seam 0.70m in thickness with no parting was found at the depth of 14.25m. by the No.12 test boring of Hiratayama Mining Co. (Nakao No.1).This coal seam is correlative with the northern extention of so-called Sanmaimono (Fukui-ichimai) which is thick enough and of sufficiently good quality to be mined at several pits.It makes the coke suitable for metallurgical uses.
Black shale bed containing many remains of Cyclammina tani Ishizaki, a foraminifera of Eocene-Miocene age, crops out on the surface at Nakao. The thickness of the layers between the black shale bed and the coal seam is smaller than 30m. A local parallel unconformity was already detected under the black shale bed at Kase and no coal seam is visible locally by wash-out at Kanbayashi pit.
The authors believe, there is a local unconformity between the black shale bed and the coal seam, “Sanmaimono”, at Nakao, also.