There are characteristic erosional gentle slopes in Aki mountains, which are considered as pediments, because the slopes are located in front of steep mountain escarpments, truncating the bedrock of granite, and are covered with veneers of cobbles, boulders or coarse sands, of which bedding and sorting are not remar-kable. Longitudinal widths of the pediments are about 300−2000 meters and the inclinations about 7°−15°. The inclinations of escarpments behind the pediments are about 26°−35°, in rare occasions 45°. The escarpment foot has not a zigzag outline but a straight one. The developmental stages of pediments are rich with varieties from young to old. The pediments are classified into three levels: 400−460 meters (upper level), 200−400 meters (middle level) and about 50 meters (lower level) in heights of each lower margin. The lower level pediments develop most broadly. In the central area of Aki mountains (that is near Hiroshima City), an area of pediments occupies 50% of the landscape.
The pediments have been formed by the parallel retreat of escarpment, which continued until the mountain mass had almost disappeared. The development of the pediments are conditioned by the following facts;
1. Fine crystalized granite or hard rock covers coarse crystalized granite (Hiroshima type granite)
2. Intermittent elevation was equal in quantity all over the region. Fault line net which accelerated vally deepening presented more favorable condition for the development of the pediments, because the development of the pediments begins after the completion of valley deepening. At recent time, however, the development of the pediments has already ceased and these surfaces are dissected by the stramlets. Therefore, the pediments must have developed under the climatic environment different from today's.
The angle between escarpment and pediment is controlled by the quality of rock, In the granite area with vertical or nearly vertical joints, it is rather large, and the escarpment is comparatively steeper and the pediments gentler.
The position of the Knickpoint is determined by the height of the base-level of erosion and the longitudinal range of the pediment. Escarpments behind the pediments are not fault scarps but erosional ones.
The period of the formation of these pediments is later than that of the fault movements of Aki mountains and earlier than the Wurm Ice age. And the middle level pediments were formed nearly at the same time as the Saijo lacustrine bed (Pleistocene age) in Saijo Basin, east of Hiroshima City deposited.