Abstract
In the southwestern part of Sizuoka Prefecture, along the Pacific coast, lies the coastal plain of Totomi, consisting of alluvial coastal lowlands, and diluvial uplands which are called Makino-hara, Iwatahara, Mikataga-hara and so on. Mount Ogasa, a hill 260m high, is situated south of Kakegawa city located between Makino-hara and Iwata-hara.
The area studied is the drainage area of the River Kiku, the flat hill-land of about 100m in height between Mount Ogasa and Makino-hara. The most part of this area consists of the alternation of pliocene sandstone and mudstone. The former is not yet consolidated, but the latter is consolidated sufficiently to hard rock.
The author investigated the lithologic condititions of the formation of the small flat surfaces in this area.
The results ara as follows:
1. The greater part of the small flat surfaces is the structural terrace related to the bedding planes of the sandstone.
2. The distribution of sfnall flat surfaces is chiefly restricted to places where the sandstone strata are thick (two times thicker than mudstone strata) in the area composed of alternations of the sandstone and the mudstone. Moreover, where the dip of the strata oxceeds 20 degrees, there or no small flat surfaces. It seems, therefore, that their formation and scale are determined not only by the the thickness of the strata but also by thir dip.
3. In the area studied, the sandstone is more resistant to weathering or erosion than the mudstone. This fact seems to be one of the important factors for the formation of the small flat surfaces or slope development.
4. In detail, degree or direction of slope of the small flat surfaces is different from place to place according to the condition of the rock, especially to the dip of strata, thickness rate of the sandstone to mudstone, and difference of resistance of these rocks to erosion or weathering.
5. The small flat surface formed on mudstone strata has a steeper slope than that on the sandstone. urthermore, the direction of slope of the surface is discordant with that of strata, and continuity of ;mall flat surfaces is interrupted by stream erosion.
