Abstract
The author surveyed the Quaternary system and topography, and compiled the chronology of topographic evolution around Lake Shinji and Nakaumi area. About the sub-surface conditions of alluvial plain in Japan, it is said that the upper bed consists of fan or deltaic deposits of the present time, the lower of the marine deposits of Post-glacial epoch and that at the base, fossil valley is buried under these deposits.
The author recognized some details in addition to the above. They are considered to have occured as follows:
1. The sea level stayed on -10m--20m level in some period in the course of regression after the last Inter-glacial epoch. Consequently, the abrasion platforms were formed under that sea level.
2. The maximum fall of sea level in the last Glacial epoch is extimated to be at -80m depth in this area.
3. In the early period of Post-glacial transgression, valleys incised in the last Glacial epoch were buried under the basal fanglomerate, a part of which composes terraces on the land.
4. After that, the speed of the transgression was in excess of that of river sedimentation, and consequently, marine clay bed was deposited on the basal fanglomerate.
5. A fanglomerate bed lies between marine clay beds on the Level of -10m--30m. This suggests that the sea level stayed or regressed slightly on that level in the course of the Post-glacial transgression.
6. After that retardation, transgression began again and marine clay deposited on the fanglomerate.
7. Yumigahama sand bar began to be formed in some period about the maximum stage of Post-glacial transgression and the process still continues.
