Limnogeomorphogy is the branch of geomorphology studying landform changes and processes in lake-catchment systems. Estimation of erosional force in the systems is closely related to limnogeomorphological studies. Earth surface processes and changes in the systems are generally printed in lacustrine sediments, which are of great use for considering erosional forces. Two time-intervals are introduced for discussion of the forces; one is the short interval during the past 10 years (instrumental observation period) for a small system, and the other is the long one during the past some 10-100 thousand years for large systems. The erosional force can be expressed as mathematical equations using dominant periods in sedimentation rate both for the short and long intervals in the systems. Erosion ratio for volcanic mountain dissection and evolution of drainage density are discussed for the long interval using mathematical equations with the erosional force estimated from sedimentation rate in Lake Biwa. The calculated results for the dissection and evolution show that the establishment of the erosional force in the equations is of value for understanding the temporal changes.

A multibeam bathymetric survey and SCUBA diving observations were conducted off a basalt sea cliff with vertical columnar joints at Keya, Itoshima City, Fukuoka Prefecture, Japan, to observe the geomorphology and environment of the abrasion platform in high resolution. The formation of abrasion platforms significant at the northern end of the cape, where the waves were concentrated. Two platforms were observed at depths of 12 m (13 m below mean sea level) and 17 m (18 m below mean sea level). The −12 m platform is covered by boulders of tens of centimeters to more than 1m in diameter, and is formed by the erosion of the basalt sea cliff. These boulders may have been moved by storm waves and formed the −12 m platform by active abrasive erosion. On the landward area of the −17 m platform, sand and gravel are deposited between rounded boulders larger than 1 m in diameter. Scarce vegetation on the rounded boulders and sediments indicates that sediments may have been moved and displaced during storm surges. The seaward area of the −17 m platform also consists of large boulders with a diameter of 1 m or more, however, the numerous organisms attached to the surface of the boulders (e.g. Aglaopheniidae and Gorgonacea) and rich biota around them implies that these boulders have remained stable for a long period of time. At depths greater than 20 m, the slope becomes smooth and gentle, with sand ansilt. The absence of vegetation on the slope surface indicates that the sediments may move during storms. Considering the formation periods of the two abrasion platforms, the −12 m platform is considered to be a recent abrasion platform. In contrast, the −17 m platform is considered to be a relict landform of the abrasion platform that formed during the last interglacial period, owing to the gradual subduction trend of the study area. Assuming that the 104-107 m wide present abrasion platform (−12 m platform) eroded after about 8,300 years BP, when the postglacial sea level reached the present depth of approximately 10 m, the average retreat rate of the basalt sea cliff was estimated to be 1.25-1.3 cm/year.