The present paper deals with the age of the latest ash fall from Rishiri Volcano-from the points of view of dynamic forest ecology and shelterbelt establishement engineering-discussed from the development of natural coniferous forest on the sand dune at Hamatombetsu coast, about 90km east from the Volcano (Fig. 1). Generally, the surface soil of sand dune is the least fertile in physical and chemical properties. And it has only so-called sand dune herbs, or dune-formers, as a pre-existing vegetation of an absolutely strong competitor against new invaders. It may be that what makes Todo-fir (Abies sachalinensis) invade and grow naturally on such a bad soil is a kind of topographic changes of ash fall. An ash fall removes the pre-existing competitor and improves the naked ground physically for trees' invasion. Sarobetsu Research Group (1966) presumed that the age of the latest ash fall from the Volcano (Hotoku ash fall deposit) might be earlier than 4, 000 years ago. However, at Hamatombetsu, the deposit covers the present sand dune, which may be about 1, 000 years ago, or less (Figs. 2 and 3). Coniferous forests on the flat land are commonly even-aged, uniform type, influenced by topographic changes. The Todo-fir forest at Hamatombetsu, though with a little larger difference in diameter grade (Table 1), may be an uniform type and a first generation on the sand dune immediately after the ash fall, nearly equal to those at Wakasakinai and Mehkuma. Those three Todo-fir forests on sand dunes (within the area of Hotoku deposit from Rishiri Volcano) may be 150 to 200 years of age. It is presumed that the Hotoku volcanic ash deposited 200 (to 300) years ago.
In 1964 the senior writer reported the occurrence of beachrock from the beach exposed along the west coast of the entrance of Ogushi Bay in the western part of Narushima in the Goto Islands, and he discussed on the age and origin of the beachrock containing the pottery of the Sobata type and concluded its age to be of the early Jomon period by the occurrence of this pottery. On the other hand, Esaka published different opinion on the age of this beachrock in 1967 and he considered its age to be rather younger than the period indicated by this early Jomon pottery. In the present paper, the writers especially discussed on the age of the beachrock upon the basis of the 14C dating of the shells cemented in the beachrock and the field work as follows. 1. The shells included in the beachrock is dated as 5, 650±150y. B. P. (Gak. 2861). The age of this beachrock is therefore thought to belong to the early Jomon period. 2. It is considered that the sea level in the Goto Islands has been comparatively stable during the past about five thousand years since the early Jomon period. 3. Judging from the origin of beachrock, the climatic condition of the early Jomon period is presumed to have been warmer than that of the present day.
The purpose of the present study is to find the relationship of a geomagnetic field reversal to the changes in a foraminiferal fauna and the sedimentary environments. In the first part of this paper, quantitative estimations were made of the possible effects of direct and/or indirect radiations on the earth's surface caused by cosmic rays, solar flares and solar wind during the geomagnetic field reversal. The result of estimation suggested that the increased radiation during the geomagnetic field reversal could not be a factor significant to cause the appearance or extinction of marine species. A fossiliferous marine sedimentary section of the middle part of the Kokumoto Formation continuously exposed at Kamiyanagawa in the Boso Peninsula was chosen for the investigation of the behavior of the geomagnetic field during its reversal between the Matuyama and Brunhes Polarity Epochs which took place around 0.69 m.y. ago. The behavior of the geomagnetic field was established in detail through the measurement of the inclination, declination and intensity of the DRM and IsRM of the sedimentary rocks in the middle part of the Kokumoto Formation. Along the same section, analyses were made on the grain-size of the sediments, oxygen isotope ratio in the planktonic and benthonic foraminiferal tests, and the faunal assemblages of the planktonic and benthonic foraminifers. The intensity of the magnetic field fluctuated with a period of approximately 7000 years and did not disappear during the reversal. The migration of the virtual magnetic north pole occurred at the magnetic field reversal between the Matuyama and Brunhes Polarity Epochs roughly along the meridian of 120°E Long. in 4700 years. In the same duration, the depth of the water, under which the sediments were accummulated, fluctuated within the range of 200-300m with a period of approximately 7000 years. No particular relation between the magnetic field reversal and fluctuation of the water depth was recognized. The result of paleotemperature and faunal analyses of the foraminifers showed that the magnetic field reversal at the Matuyama-Brunhes Polarity Epoch boundary may have been related to the changes in the circulation pattern of the upper water-masses which caused more influx of cold water into this area in the northwestern Pacific as compared with the preceding period.
The clay mineralogical composition of the recent and buried soils derived from the upper column of the Quaternary tephras were analysed by means of the DTA and X-ray diffraction. The uppermost humified scoriaceous soils were characterized by allophane and Al-vermiculite. No gibbsite was detected. The middle part ranging from the uppermost Pleistocene to the recent was composed of fine and coarse (scoriaceous) tephras of various petrographic compositions and humified horizons on them. Al-vermiculite and kaolinite were dominant components of the -2μ fractions of these horizons. Gibbsite and 14Å/10Å intergrades were accompanied. The clay mineralogy of the lower part (upper Pleistocene) was dominated by allophane. These results suggest that the weathering sequence of primary minerals of these tephras may follow the following stages. Volcanic glass→crystalline clay minerals (Al-vermiculite and kaolinite) with gibbsite →allophane.