Gravitational deformation of mountains (sagging) is evidenced by a series of ridge-top depressions and uphill-facing scarps in the upper reaches of the Abe River, central Japan. The characteristics are present in an area extending from the Oya-kuzure landslide to Mt. Yambushi.
Ridges are underlain by slate, the cleavage of which trends slightly obliquely to the ridge axis. The cleavage dips steeply (from 70° to 90°) in most of the valley bottom outcrops, but dips gently toward the mountain at higher elevations. This orientation indicates that the slate bowed toward the valley at the surface of the slopes. Such bowing likely generated the ridge-top depressions and uphill-facing scarps. Some ridge-top depressions and uphill-facing scarps are parallel to the strike of the slatey cleavage, and some are parallel to the ridge. The depressions and scarps parallel to the ridges branch from those that are parallel to the strike, indicating that those that are parallel to the strike formed first. Thus sagging first started with the separation of rocks along cleavage planes, and then the separation surface propagated parallel to the ridge.
Depositional accumulations in depressions on ridge-tops (_??_10m deep) and at the foot of uphill-facing scarps (several meters deep) exhibited regional tephra and characteristic facies, indicating that the sagging has been developing since the late Pleistocene. Ridge-top depressions existed for at least 20 ka, and the lower uphill-facing scarps formed about 10 ka BP. The edges of ridge-top depressions and uphill-facing scarps at higer elevations are more rounded than those at lower elevations, suggesting that the higher features are older than those at lower elevations, i.e., sagging started near the ridge-top and propagated downslope.

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This study discusses the coastal evolution of the Nobi plain during 3, 000-2, 000 yr BP in relation to sediment supply and relative sea-level change. The plain is located on the Pacific side of central Honshu Island, extending about 50km from north to south and 35km from east to west. The landforms of the plain can be divided into three components: alluvial fans, natural levee areas, and a delta.
The author obtained core samples from 13 sites in the study area, measured their ¹⁴C ages, and analyzed their diatom assemblages. These analyses indicate the following history of marine regression and landform development during the late Holocene.
Remarkable expansion of the delta occurred during 3, 000-2, 000 yr BP. During 3, 000-2, 500 yr BP, the coastline moved about 5km toward the south. In this period, delta sedimentation was slow; peaty soil formed in the northern part of the flood plain and a thin foreset bed was deposited in the delta. During 2, 500-2, 000 yr BP, the coastline again moved about 5km toward the south. In this period, delta sedimentation became fast; the peaty soil was covered with flood deposits and the progradation of the foreset bed was rapid, especially during 2, 300-2, 100 yr BP, leading to the wide expansion of the subaerial delta. The volume of foreset bed deposits during 2, 300-2, 100 yr BP is estimated about 1.1-1.4km³. This is equivalent to the volume of foreset bed deposits during 3, 000-2, 300 yr BP.
The relative sea level during 3, 000-2, 000 yr BP was 1-2m below the present sea level, suggesting that the delta expansion during 3, 000-2, 000 yr BP was caused by both the sea level fall during 3, 000-2, 500 yr BP and increased sediment supply during 2, 300-2, 100 yr BP. The increased sediment supply around 2, 000 yr BP is also recognized in other Japanese coastal plains. Climatic change such as an increase in storm intensity may have accounted for the increased sediment supply.

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