Opal phytolith analysis of the Kanto Loam Formation (the tephra layers) composed of the Shimosueyoshi Loam, the Musashino Loam, the Tachikawa Loam and the Kuroboku Soil (in ascending order) at Narimasu Geological Exposure, Itabashi, Tokyo, was carried out. Five local opal phytolith zones can be roughly distinguished. The Kuroboku Soil (A zone) was characterized by the dominance of Pleioblastus with Paniceae, suggesting a warm temperate climate. The Tachikawa Loam (B zone) was dominated by Sasa, indicating a cool temperate climate. This zone was subdivided into two subzones, B1 and B2. Zone B1, the upper part, contained more panicoid phytoliths than Zone B2, the lower part. The Musashino Loam was divided into two zones, C and D, with the boundary between the two situated over the tephra TP (4.9×104F.T.y.B.P.). Zone C was characterized by Pleioblastus, indicating a warm temperate climate, which may correspond to the Middle Pleniglacial in Europe. An increased number of Festuceae and Paniceae were found in zone D which probably means that zone D was in the cooltemperate to subarctic climate. This cold age is correlated with the Murodo Glacial Substage of Mt. Tateyama and the Lower Pleniglacial in Europe. The Shimosueyoshi Loam (zone E) contained much more weathered phytoliths than the other zones. The vegetation at E zone, therefore, unfortunately could not be reconstructed.
Surface water temperature during the Holocene, estimated from diatoms in two pistoncores-KH-79-3, L-3 from the southern part of the Japan Sea and KH-84-3, St. 33 from the western part of the Tsugaru Strait-shows good correlation with pollen flora, oxygen isotope, and sea level change curves on a time scale of 103 years. Rhythmic fluctuations of diatom temperature (Td=Xw/(Xw+Xc)) values recognized in both pistoncores indicate the extent of influx of the Tsushima warm current into the Japan Sea. Four peaks of Td values, indicating the strong influx of the Tsushima warm current (warm period), occur within a interval from the Akahoya ash (14C date, 6, 300Y.B.P.) at 86-94cm level to 0cm level in piston-core L-3.
In this paper mass movement and slope formation in the central Kitakami Mountains are discussed with special reference to their periods. Many tephra layers of the late Quaternary found in the study area enable to study the periods of mass movement and slope formation tephrochronologically (Fig. 2). The slopes are classified as follows (Fig. 3). (1) gentle slopes on the summits, (2) piedmont gentle slopes, (3) fan-like gentle slopes, (4) smooth crest slopes, (5) upper head hollow slopes (continuing smoothly from the surrounding slopes), (6) lower head hollow slopes (smaller than (5), surrounded by clear breaks in slopes), (7) talus and alluvial cone, (8) other slopes. In the study area two periods of mass movement, chiefly by solifluction, are confirmed during the Last Glacial. The first period was in the early Last Glacial Stage, perhaps around 50, 000y.B.P., and the second was in the late Glacial Stage, between 30, 000 and 10, 000y.B.P.. These periods of mass movement correspond to those of the involutions under periglacial climate in the Northern Kitakami Lowland area (Endo, 1977) (Fig. 7). In the study area not only were well-jointed bedrocks such as shale, slate, and schist susceptible to frost shatterin, bnt also fallen volcanic ash and soil produced from deep weathered bedrock were also susceptible to solifluction. Gentle slopes on the summits and smooth crest slopes have been formed by bedrock frost shattering and solifluction in these periods of mass movement by surface processes. Gentle piedmont slopes have been formed by solifluction. Upper head hollow slopes were developed as smoothly concave profiles by debris accumulation. Fan-like gentle slopes were developed chiefly by slope wash (partly by solifluction in the Last Glacial Stage) at the same time. Most of the piedmont gentle slopes and fan-like gentle slopes began to form in the early Last Glacial Stage or in the cold period before the Last Interglacial Stage, and the deposits of the late last Glacial Stage, which is generally recognized as the maximum period of the last Glacial Stage in Japan, are only 0.5-2.0m thick. During the warming period from the latest Pleistocene to Holocene, landslides have formed lower head hollow slopes and alluvial cones have been formed at the outlet of valleys with small river basins.
In order to determine the height and age of paleo sea level precisely, it is necessary to find suitable indicators. The tube worm Pomatoleios kraussii (BAIRD) was an excellent indicator of sea level stands on the southeast coast of Boso Peninsula. Studies of vertical distribution of P. kraussii, living and emergent at the time of the 1923 Taisho Earthquake, showed that the upper limit of the P. kraussii zone was located at 0.1±0.1m above mean sea level. At Hiraiso, the upper limit of the distribution of fossil P. kraussii emergent at the time of the 1703 Genroku Earthquake was 6.8m. Using the relationship between the upper limit of P. kraussii and mean sea level, paleo sea level prior to the Genroku Earthquake was estimated to be 6.7±0.1m. On dating radiocarbon ages of small samples such as P. kraussii tubes, the alternation of samples might lead to incorrect results. Three out of four radiocarbon ages of fossil P. kraussii tubes emergent at the time of the 1703 Genroku Earthquake were dated to be about 100-250 years older than the time of the earthquake. Observation of these tubes by scanning electron microscopy revealed that the tubes which gave the older ages had been recrystallized, and did not yield reliable ages.
The latest younger Fuji tephras, which have erupted since 100B.C., were divided into 13 units on the eastern and northern slopes of Mt. Fuji, that is, S-23-1-2, S-24-1-10 and S-25 (1707 Hohei scoria bed), in ascending order. The standard sequence is presented. Both macroscopic and microscopic characteristics of each unit are also described. Approximate ages of tephras and associated lavas are as follows: S-23-1-2, 2100y. B.P.; S-24-1, 2000y.B.P.; S-24-2 (which is surely pyroclastic flow deposits in a broad sense), 1900-1500y.B.P.; S-24-3-5, 1500y.B.P.; S-24-6, 1400-1300y.B.P.; S-24-7, 1300y.B.P.; Hinoki-Marubi lavaII, Taka-Marubi lava, Ken-Marubi Iava, 1300-1000y.B.P.; S-24-8, 1000-800y.B.P.; S-24-9-10, 500-243y.B.P.; S-25, 243y.B.P.
Three tephra horizons were recognized at Itai Archaeological Site, Hyogo Prefecture, from the quantity distribution of volcanic material (mainly volcanic glass shards) and the refractive index of three columnar sequence samples. They are named Itai-Lower, Itai-Upper and Itai-Uppermost tephra horizons, in ascending order. The three tephras are correlated with BB 55, BB 51 and BB 15 volcanic ash layers (in ascending order) in the 200m drilling core samples of Lake Biwa, and also can be correlated with the previously studied widespread tephras: Aira-Tn (AT), Daisen-hoki and Kikai-Akahoya (K-Ah) ashes, respectively. Paleolithic tools were found at the three horizons as follows. (1) Below the Itai-Lower tephra horizon, stone tools consist mainly of awl-like tools of sanukite, and include knife-shaped tools, scrapers, retouched flakes, hammerstones and cores. (2) Between the Itai-Lower and Itai-Upper tephra horizons, stone tools consist mainly of knife-shaped tools made of elongated flakes and wide-flakes, and include edge-ground ax-shaped tools, scrapers, retouched flakes, hammerstones and cores. (3) Between the Itai-Upper and Itai-Uppermost tephra horizons, a tanged point was found. Based on the tephra correlation and 14C ages obtained from the site, ages of Paleolithic tool horizons at Itai Archaeological Site are determined to be approximately 22, 000y.B.P., 21, 000-20, 000y.B.P. and 20, 000-6, 300y.B.P., in ascending order.
The ages of peaty mud samples including the Aira-Tn ashes (AT) that were identified from refractive indices and chemical compositions were determined precisely with radiocarbon dating using the benzene-liquid scintillation method. The eruption age of AT was found to be 24, 720±290y.B.P.
Gentle slopes are distributed between uplands and lowlands in the Tsukuba Upland area, Ibaraki Prefecture, Japan. These slopes are dominant on the left bank side of streams flowing to the west and on the right bank side of streams flowing to the southeast. Opposite side slopes, in contrast, are generally steep. In other words, topography along the streams in upland areas shows an asymmetrical valley pattern. Drilling samples show that the upland is occupied, from top to baseward, by the Younger Loam (equivalent to the Tachikawa and Musashino Loams) and the Joso and Narita Formations of Quaternary age. The gentle slopes are underlain by slope deposits of poorly sorted silty sands and gravels 1-3 meters thick. The deposits consist of particles essentially derived from the Joso Formation. The slope deposits lie unconformably on the Joso and Narita Formations and are overlain by the upper Younger Loam which has AT (Aira-Tn Volcanic Ash) dated at 21, 000-22, 000y. B. P. in its basal part. This stratigraphic evidence indicates that the slope deposits were formed during the last glacial age. It is assumed, from the relation between the slope direction and the frequency of slope cryoturbation in the winter season, that the slope deposits were accumulated by refrigeration in the cold climates of the late Pleistocene.