Abstract
The Lower Terrace and this formation are distributed on the fringe of the southern part of Nara Basin, Japan. I restore this accumulation initial surface from erosional features. Geological boundaries and gentle slopes seen iin so-called Yamato-Sanzan, three small mountains stated below, consisting of the igneous rock touched off this theme. I show vertical section views of Yamato-Sanzan whom I took along ridges to the chart below. Here a little less than 120 m above sea level can find Yamato-Sanzan common level to show it in broken line. In the neighborhood of this level, are recognized borders of intruded rocks and the granite on Unebiyama and of the gabbro and granite on Amanokaguyama, and gentle slopes of the rhyolite at the Miminashiyama. The erosion of the granite by weathering slowed down in this level, I read it. In other words these bodies lower than this level had been underearth. The current local erosion reference datum ranges 60 to 80 m above sea level as Yamato-Sanzan are located on gentle slopes. I will assume a higher stable basal level of erosion approximately 40 m from the present. In addition, decomposition of the granite excels in the vicinity of the earth surface where water movement is active. Leaching of potassium and sodium constituting the mineral of granite are to the zone of saturation top surface of the subsoil water wholly. The groundwater table links local basal level of erosion, and this enlarges headwards as headward erosion proceeds. The above mentioned higher basal level is corresponding to the Lower Terrace in the Ryumon Mountains to the south of Yamato-Sanzan and in the Kongo Mountains to the West. The left Lower Terrace considerably deteriorates now, but their pieces of the accumulation initial surface can be recognized. The alluvium of Nara Basin is extremely thin, and, according to the excavation data, and it is thought that the current basin is a dissected topography of the Lower Terrace.
