In the Atsumi Peninsula, Pleistocene deposits called Atsumi group are extensively distributed, and consist of Futagawa, Tahara and Toyohashi formations in ascending order. The lower or the Futagawa formation composed mainly of silt and sand is overlain by the Tahara formation with disconformity, and the latter underlies the Toyohashi formation with local disconformity. Many fossil plant remains have been obtained from various horizons of the group. In this paper the writer reports on the floras of the Futagawa and the Tahara formations. The lower part of the Futagawa formation includes some broad-leaved evergreen trees, which are found generally in the present flora of this peninsula and its vicinity. The climate at that time, therefore, may have been as today. The climate in the age of the upper part of the formation seems, however, to have become cooler, judging from the predominance of the temperate elements. The Tahara formation is divided into three members implying a submergence, the upper two of which are concerned here. The plant remain assemblage of the middle member is composed of broad-leaved deciduous and evergreen trees identical or closely related to the species now living in Central Japan. However, that of the upper lacks in deciduous trees suggesting temperate climate. Consequently, it is assumed that the former represents a little cooler condition than that of the present, whereas the latter as warm as today. It is noticed that Fagus aff. Hayatae, Abies firma and Cyclobalanopsis occur in abundance in the remains of the Tahara formation, compared with those of the Futagawa formation. On the geologic ages of these formations, the writer believes that they are assigned to the early and the middle Pleistocene respectively, judging from no characteristic elements of the Pliocene and also of the late Pleistocene in the floras.
The article is summarized as follows: (1) It has been a current view that in the eastern part of the Tama Hills extend two terrace surfaces, that is, older Tama II and younger Shimosueyoshi terraces. But the authors found another terrace called Tsuchihashi terrace which intervenes between Tama II and Shimosueyoshi terraces in age is distributed in the middle of the slopes bordering the two terraces. It is probable that a regression occurred between Tsuchihashi and Shimosueyoshi stages, but it is not yet answered whether a regression intervened between Tama II and Tsuchihashi stages. (2) The authors found consolidated dark blue part of the volcanic ash exclusively in the lower horizon of Musashino “loam” overlying these terraces in this area. The authors believe that the part is unweathered volcanic ash rapidly fallen in remarkable thickness. But it is also possible that the color of the part was resulted from reduction by underground water.
It has generally been accepted that the Manchidani formation unconformably overlies the Osaka group proper in the marginal part of the Osaka basin. But, the writers had begun to have some doubt about the erosional surface beneath the Manchidani formation. As a result of our detailed investigation, the Manchidani formation in the Senpoku Hill was confirmed to belong to the Osaka group itself. Consequently, the Manchidani formation at Manchidani and Hirakata had to be reexamined. The conclusion on the basis of observations at these fields is as follows (Fig. 6). 1. The Larix bed at Manchidani and the Pinus korciensis bed at Hirakata (Shinkori) correspond to the horizon between Ma 6 and Ma 7. 2. The Syzygium bed at Uegahara and Hirakata correspond to the Ma 8 horizon. 3. The Tomistoma bed at Machikaneyama corresponds to the horizon beneath the Syzygium bed.