The eastern region of the Myoko volcano group, Central Japan, is covered with thick tephra formations, and is called the“tephra region of Myoko volcano group”. A catalogue of Pleistocene tephra layers in this tephra region was presented by HAYATSU and ARAI (1980). This paper presents the additional descriptions about some marker tephra layers distributed especially at the southern half part of this tephra region. Petrographic characteristics of six marker tephra layers such as pumice, scoria and ash layers are described in detail. In addition, the stratigraphic relations between marker tephras and non-volcanic sediments at the foot of volcanoes are shown. Some lake deposits (Sasagamine, Furuma, Mure and Nojiri lake deposits) are formed during the nearly same period respectively. Some of marker tephra, layers have their source in the volcanoes far away from the Myoko volcano group. They are usually found as very thin layers in the tephra formations, being well preserved only at the limited locations where settling of tephra proceeded rather rapidly.
In the present paper, a pollen analytical study was made on 55 samples obtained from the upper member of the Shimoabira Formation, and the lower and the upper member of the Shiomi Formation belonging to the late Pleistocene in SE Ishikari Plain, Hokkaido, and palaeoenvironmental changes were discussed, being based on three pollen zones or divisions recognized, which were comparable to the three geological divisions mentioned already. Successive palaeoenvironmental changes are suggested from the pollen diagrams that the climatic condition have changed from warm to cool temperature condition. This thermal change, however, was considered to be a change within the Fagus crenata=Sasa Class in a phytosociological sense. Thus, the palaeoclimatic environments were possibly corresponding to the present climatic environments of the Kuromatsunai Area which is a significant demarcation area separating the Cool Temperate vegetation with Fagus crenata from the Cold Temperate vegetation with Quercus crispula, P. glehnii and P. jezoensis but lacking F. crenata in natural habitats in Hokkaido. The conclusion mentioned above was abstracted from the following: (1) gradual increasing of the ratio Picea/Abies from older periods to recent periods or from the pollen zone III to the zone I, although more or less irregular fluctuations were recognizable often in pollen diagrams, (2) conspicuous and nearly constant mixture of Picea, Tsuga, Abies, Cryptomeria, Pterocarya, Fagus, Quercus, etc. which was believed to be a proof of long existence of the Mixed forest, not pure boreal coniferous forests like the Taiga nor exclusive Temperate Summergreen forests, and (3) drastic decrease of maritime Chenopodiaceous pollen grains in the sample number just above the lowest sample number of the upper member of the Shimoabira Formation, showing palaeoenvironmental change from the high sea-level condition to low sea-level condition. Two exceptional pollen diagrams were shown: Ka-II' and Ni-I'. The pollen diagram of Ka-II' was considerably different from diagrams of the remaining 4 diagrams and was characterized by a tendency with increased amount of pollen grains of Picea and Cryptomeria, even though a species list of pollen grains was very similar to the remainder. The diagram of the lower part of Ni-I' pollen zone was characterized by complete lack of conifer grains. Elucidation of these exceptional cases was not tried in the present discussion and remained to be solved. Through the Shiomi Formation Period, it was supposed that bog vegetations were constantly developed; one is Picea-Sphagnum bog and the other is Alnus-Cyperaceae bog. As a result of the present study, it was shown that constant and extensive spreading of bog was considered to be the most characteristic palaeoenvironmental feature of the Shiomi Formation. Thus, the authors proposed that the Shiomi Formation Period should be called“Shiomi Bog Period”in pollen diagrams. This Period was contemporary with the Biraotori Bog Period and dated a period from the Riss-Würm Interglacial to WI Subglacial Period.