A Geomorphological Study of Marine Terraces near Ube City, Yamaguchi Prefecture

Abstract

Near Ube City in the southern part of Yamaguchi Prefecture, at least, three levels of marine terraces are recognizable. The surface of the upper terrace is 30 to 40 meters above the present sea level, and the contemporary strandline corresponding to the terrace is presumed to have somewhat higher than 40 meters. The middle terrace preserves its flat surface between 10 and 15 meters. The presumable strandline is generally about 20 meters above present sea level. The lower terrace is preserved at the height of 10 meters or less. In the cases of the upper and middle terraces these surfaces are composed of either the wave-cut bedrocks or sands, pebbles and gravels (1-6 meters thick), and in the latter case the facies are of beach deposits. The middle terrace, in particular, strikingly bears beach gravels which are deposited along a horizontal line at some places. These two terraces are covered with reddish loam 1-3 meters thick, seemingly derived from volcanic ash.The feature of these terraces may be interpreted as the evidence of the process outlined below. When the upper terrace was formed, the sea level was almost stagnant for a period, then a gradual regression of the sea-level followed to form the abrasion slope. After that, the sea-level went into a stand-still again to form the middle terrace. The formation of these two terraces apparently goes back to the age before the climax of the eustatic regression in the last glacial stage. The lower terrace is composed of sand bar deposits and its surface was formed in a post-glacial period.