In Japan most of the coastal plains are originated from the sedimentary basin of the Neogene formations. Three types of formation process have been reported to explain the accordant level of the hills around the coastal plain;
Type A: When the terrace surface of the soft rocks around the coastal plain is cut controled by the next, slightly lower baselevel, the undulating surface is made as a state of dynamic equilibrium of the unconsolidated materials.
Type B: At the thinner soft rocks around the sedimentary basin, the basement of the soft rocks reappears as undulating surface without any baselevel control.
Type C: The undulating surface emerged in early Pleistocene and subjected to erosion under the baselevel control till the formation of the Higher Terrace.
The types A and P have been reported by the author with the examples in the Kitakami valley plain, Tohoku, and in the Muroto peninsula, Shikoku. The type C has been reported by others to explain the so-called “Setouchi Surface” along the Japan inland sea.
The erosion surface of the northern Awaji island reported in this paper is correlated with the “Setouchi Surface”. The undulating surface on the horst corresponds to the basement of Miocene and Upper Pliocene sedimentary rocks on the granite bedrock. Judging from the development of paleo red soil with 10R hue, it becomes clear that this erosion surface had been formed before the Higher Terrace stage. The surface had not been covered with any sea level during the Higher Terrace stage nor with the Lower Pleistocene member of the Osaka Group.
So-called “Setouchi Surface” may have various origins and formation processes in each tectonic location. The type E is also suggestive for the explanation of the lower peneplain in the Chugoku mountains.
