凍結融解作用の物理的機構に基づいて,周氷河地形の形成過程を論ずるうえでの問題点について述べる.地温が0C°直下に達したときに生じる凍結膨張によって,岩石は破砕され,土は凍上する.凍結膨張した地盤が融解するときに,沈下ないし物性の変化を起こすことによって物質が移動し,地形が変化する.その地形変化速度は,凍結融解の頻度,水分条件,地盤の物性など種々の因子に支配される.日周期性と年周期性の凍結融解作用の凍結(融解)深度の違いは,岩壁から落下する礫の粒径や斜面物質の移動限界深度に影響を及ぼす.気候条件(寒さ・湿度・永久凍土の有無など)と凍結融解作用(地形変化速度・変化様式・生成物質など)との関係を議論するたあには,凍結融解作用を支配する個々の因子を定量的に評価する必要がある.

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谷川連峰仙ノ倉山～平標山付近の標高1,895～1,990mの強風砂礫地6地点で,斜面物質の移動などに関する調査を,約5年間実施した.その結果,6地点での表面礫の年平均移動量は2.1～5.4cmと測定された.日本の高山では気温,地温が0°Cを超える期間が長いので,これらの数値のすべてを周氷河作用に帰することはできない.調査地付近での連続地温観測をもとに検討すると,それらの移動量の約60%が凍結・融解の繰り返しによる周氷河作用に,また,残りの約40%がslope washなどの非周氷河作用によっていることがわかった.晩秋～初冬の凍結進行期と春季の融解進行期の平均移動距離を比較すると,前者で1.1cm,後者で2.2cmと,後者は前者の約2倍である.それは,谷川連峰では降雪が早く,また春季は晩秋～初冬に比べ土壌水分も豊富で,凍結・融解の生じる期間が長いためであろうと考えられる.

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In this study the author attempts to discuss slow mass-movement processes caused by freezing and thawing in relation to the compositions of slope materials and fluctuations of ground temperature, in Spitsbergen and in the Japanese high mountains.
Study sites in Spitsbergen are located in Adventdalen and Reindalen, Nordensiöldland, central Spitsbergen (Fig. 1). These sites are debris slopes with gradients ranging from 6° to 32°, mainly covered with Jurassic, Cretaceous, and Tertiary shale fragments.
The rates and deformation patterns of downslope movement of surface rubble are measured by eighteen painted stone-lines. Many grass fibre-tubes, furthermore, were inserted vertically into the ground in order to detect downslope movement in the active layer.
Year-round ground temperature measurement was achieved at a flat surface (480 m a.s.l.) of the Adventdalen site using data logged at 3-hour intervals in 1988-1989. Sensors were installed at 5, 15, 30, 50 and 80 cm depths.
Studies to slow mass-movement in the Japanese high mountains were performed in Mt. Shirouma, the Kitakami Mountains, and Mt. Hoh-o. Long-term ground temperature records were observed on the Kitakami Mountains, Mt. Hoh-o and Mt. Akaishi.
The results of this study can be summarized as follows.
The average movement rate of painted stones from 1988 to 1990 was 3.3 cm/yr. in Spitsbergen (Fig. 2). Deformation features of all painted stone-lines showed patterns parallel to the basement lines independently of the compositions of slope materials (Figs. 3. 4. 5. 6). The movement rates at the tops of grass fibre-tubes were similar to the rate of movement measured by painted stone-lines (Fig. 8). These imply that deformations of painted stone-lines and grass fibre-tubes occurred by the same processes, that is, frost creep and gelifluction. Furthermore, movement of slope materials by diurnal freeze-thaw cycles is negligible in Spitsbergen, because of the very low frequency of this cycle (Fig. 10; Table 3). Therefore it is considered that annual freeze-thaw cycles mostly contribute to the movement of the slope materials in this area.
Average movement rates of painted stones measured in the Japanese high mountains are 26.1 cm/yr. in Mt. Shirouma, 21.7 cm/yr. in the Kitakami Mountains, and 39.7 cm/yr. in Mt. Hoh-o (Fig. 2). These rates are one figure faster than those of Spitsbergen. Deformation features of painted stone-lines are classified mostly into two patterns, one a festoon type and the other a parallel type (Fig. 7). The parallel type occurs on slopes mantled with thick surface rubble of coarse size. The festoon type is recognized on slopes mantled with thin surface rubble and fine materials.
The vertical profiles of many flexible tubes from the above-mentioned mountains on which they were inserted to become almost negligible at about 30-40 cm in depth (Fig. 8). This depth is about one half as shallow as that of Spitsbergen.
Freeze-thaw cycles in the Japanese high mountains overlap with diurnal ones and annual ones, especially those occurring in very high frequency (Fig. 9; Table 3). Movements on the slopes covered with thin rubble and fine materials are caused by diurnal and annual cycles. On these slopes deformations of painted stone-lines show the festoon pattern, and the movement rates are faster than those on slopes consisting of coarse rubble, which almost all move in annualfreeze-thaw cycles.
Considering the above mentioned results, in Spitsbergen differences incompositions of slope materials do not reflect movement processes and the rate of movement, because movement of slope materials occurs in almost annual freeze-thaw cycles. On the other hand, in the Japanese high mountains movement of slope materials occurs in two different freeze-thaw cycles, which are annual and diurnal ones.

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The authors discuss the development of block fields and sorted polygons on the flat surfaces at the summits of fells (monadnocks; tunturi in Finnish) in Finnish Lapland in relation to the retreat of the ice sheet during the last glaciation.
There are many fells in the study area (Fig. 1) with their summits elevated several hundred meters; the highest peak is 806 m. Block fields and inactive sorted polygons with erratic boulders in their forming materials were developed on the flat surfaces at the summits of fells above about 600 m. Only block fields with erratics were developed on the summits between about 500 m and 600 m in elevation. Furthermore, on the summits below about 500 m, neither block fields nor sorted polygons were developed but glacial striaes and erratics existed.
All of the summits in the study area were covered with ice sheet during the late glaciation. However, because the higher summits with block fields appeared above the ice sheet like nunatak forms in the early stages of deglaciation, it was cold enough to produce block field-forming materials. On the other hand, the lower summits without block fields were covered with ice sheet for a longer period than the higher ones. It was not cold enough to produce block field-forming materials on the lower summits (Fig. 3, Table 1). Therefore, in the present study area, the authors define a boundary zone, which is a transitive zone of freeze-thaw weathering with an altitudinal range below the linear weathering limit (Nesje et al., 1987).
Inactive sorted polygons are commonly developed on the summits of fells higher than 600 m in their elevation. On the other hand, active sorted polygons are developed on the river floors and lower river terraces at the bottoms of the shallow valleys, mostly about 100 m in elevation (Fig. 2). Very severe temperature inversion was observed at Kevo (Fig. 1) in one of the valleys where active sorted polygons were developed. This is a very common phenomenon in Finnish Lapland. The values for freezing index, thawing index, and freeze-thaw days and cycles (Fig. 4, Table 2) show that valley bottoms have a more suitable climatic environment for periglacial processes than mountain tops. The authors think that severe temperature inversion is one of the important factors for active sorted polygon forms.

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Three-dimensional fabric analysis was done in order to characterize the fabric of slope deposits gathered from various sources. Although the distribution of periglacial slope deposits on a logarithmic ratio plot partly overlaps those of nonperiglacial deposits such as soil creep, sediment flow plots, debris flow plots, talus, outwash and stratified slopes (in openwork beds) indicate low values of both C and K compared with those of periglacial slope deposits (Fig. 1, Table 1). This result leads to the conclusion that (1) the periglacial slope deposits have higher C and K values than non-periglacial deposits except for glacial ones; (2) the periglacial slope deposits are plotted in a zone with C ranging over 2.5, K ranging over 0.5 on the logarithmic ratio plot; and (3) it is, therefore, possible to distinguish the periglacial slope deposits from the non-periglacial ones by means of logarithmic ratio plots.
Three-dimensional fabric analysis of 2, 350 clasts in the site located on the fossil periglacial slope deposits in the Hidaka Mountains has been done to evaluate the relations of macro fabrics to both length of a-axis (Table 2) and clast shape (Table 3). The main results are that (1) clasts with a longer a-axis have macro fabrics with little deviation from mean orientation (Figs. 2 and 3); and (2) clast shape has little effect on fabric in the longer a-axis range (Fig. 4, Table 4).
Taking these results into consideration, azimuths and dips of clasts with longer a-axis have to be measured in order to gain fabrics with little deviation from mean orientation in periglacial slope deposits.

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The authors conducted a field survey related to the genesis and occurrence of permafrost in James Ross Island and Seymour Island, Antarctic Peninsula region, during the 1989-1990 Antarctic summer season. Mean annual air temperature in both islands is estimated at about-10°C.
Seymour Island is located in the Weddell Sea. There was no ice-sheet over the island during or since the last glaciation period. Marine terraces of three different levels are distributed in the is-land: an upper terrace at Meseta (about 200 m a. s. 1.), a middle terrace at Sub-Meseta (about 50 m a. s. 1.) and a lower terrace at Larsen (about 5 m a. s. 1.). In order to estimate the thickness of permafrost, geo-electric resistivities were surveyed on these terraces. In the lower terrace at Larsen, long-term monitoring of ground temperature profiles was carried out for two years. The annual mean ground temperature and the temperature gradient indicate that the permafrost base is 34 m deep. This coincides with the depth obtained by geo-electric resistivity measurements.
In James Ross Island, about 90% of the ground surface is covered with an ice sheet or glaciar. Ice-free ground spreads to the northwestern part of the island. Coastal terraces of three different levels develop around Santa Marta Point along Croft Bay: upper terrace(21-24 m, 32-35 m a. s. 1.), middle terrace (10-17 m a. s. 1.) and lower terrace (3-5 m a. s. 1.). The group of upper terraces is composed of glacial till or fluvio-glacial deposits. Shell samples were collected from the deltaic deposit which composes the middle terrace, and from marine sands and gravels which cover the surface of the lower terrace. Then ¹⁴C dating was done with the results of about 25, 000 y. B. P. and 3, 000 y. B. P. respectively. According to geo-electric resistivity measurements, the depths of permafrost bases on the upper and lower terraces are estimated at 40 m and 3-5 m, respectively. It is considered that permafrost on the upper terrace occurred prior to the last glacial maximum age. The comparison between Antarctic and Arctic permafrost suggests that the permafrost in Antarctic regions is shallower than that in Arctic regions under similar annual mean temperatures at the present time.

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わが国では1970年代に入って,周氷河地形に関する研究が急増した.これは1960年代の第四紀研究の進展の影響を受けたものであると考えられる.研究は周氷河性平滑斜面における斜面物質の粒度組成や移動速度,移動プロセス,斜面形などを明らかにすることから始まったが,これはヨーロッパにおける研究の不備を補うものであった.研究の進展に伴い,地質ごとに斜面物質の粒度組成が異なることや,斜面物質の安定度に違いのあることも明らかにされ,それらと植生分布との関係も調べられた.また最終氷期最盛期頃に加えて晩氷期やネオグラシエーション期に岩屑生産のあったことも明らかにされつつあり,周氷河性平滑斜面の研究は第四紀研究や植物生態学に対しても重要性を増しつつある.

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周氷河作用の発現域は,極相帯としての周氷河帯とこれに接する推移帯とに区分できる.本稿では,現在の日本における周氷河帯およびその直下の推移帯と永久凍土帯との位置関係,さらに周氷河作用と永久凍土との因果関係について検討した.周氷河帯は日本付近で永久凍土帯と斜交する.その原因は,周氷河帯下限が最暖月平均気温の等温線で示されるのに対し,永久凍土帯下限は年平均気温の等温線で示され,しかも両等温線の南北勾配が相互に著しく異なっているたあである.このような斜交関係の存在によって,周氷河帯および推移帯はともにいくつかのタイプに分類される.周氷河作用のうち,構造土など,小規模な地形や現象の発現に関与する程度の微弱な周氷河作用は永久凍土とは本来無関係である.一方,周氷河性平滑斜面のような大規模な地形を形成しうる強力な周氷河作用は,本来永久凍土の存在を前提に発現すると考えられる.

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最終氷期の日本列島における周氷河帯-非周氷河帯の境界について,化石周氷河斜面の分布と更新世火山灰の山地斜面上における分布から検討した.一般に化石周氷河斜面は過去の周氷河環境の示相地形として用いられてきている.しかし,周氷河帯の範囲を復元するにあたって問題となる,山頂から連続的に分布する化石周氷河斜面の下限高度にはかなりの高低が認められる.また,その下限付近より低い高度帯では,点在する化石周氷河斜面として岩塊流が分布する.これらの分布からは,最終氷期の周氷河帯-非周氷河帯境界は,高度的に広がりをもち,連続的周氷河斜面帯・点在周氷河斜面帯・安定斜面帯などが錯綜した状態であったとみられる.一方,更新世火山灰の分布上限高度は,最終氷期における周氷河帯-非周氷河帯境界領域のなかで,周氷河現象を阻止するほどの植生限界線として意義づけられる.東北日本における (1) およそ3万年前以前に降下堆積した火山灰層と (2) およそ1.5万年前～1万年前に降下堆積した火山灰層の分布上限高度は,いずれも低緯度側に向かって高くなり,また最終氷期後半から晩氷期にかけて上昇している.

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赤石山地において,現在の物質移動速度と連続の式を用いて,周氷河性平滑斜面の変化速度の第一次近似的な見積もりを行なった.その結果,高山の山頂付近に点在する小起伏面は,“準平原遺物”ではなく,氷期の周氷河作用によって尾根が従順化して形成された“高位削剥面”である可能性を指摘した.

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中国山地東部の山頂・山麓にみられる堆積性地形の特徴,形成プロセス,形成年代および形成環境について検討した.これらの地域には,山麓部に麓屑面,山頂部には岩塊流と平滑斜面があり,その中間にはロックフォールテーラスが分布する.これらの地形の形成年代は,おもにテフラの降下年代からみて,麓屑面のII面・岩塊流・平滑斜面・ロックフォールテーラスが最終氷期中,I面がそれ以前の形成であることが明らかとなった.地形の性状,堆積物の層相・構造,形成年代から,上述の地形はそれぞれ寒冷な気候環境下の周氷河作用により形成されたと考えられる.本地域は最終氷期後半に寒冷で乾燥した気候状態におかれ,周氷河作用に基づく地形の形成されやすい環境が存在していたと考えられる.

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従来,関東以西の標高の低い地域から,最終氷期の周氷河地形の存在が報告されている.しかし,気温低下量から推定された氷期の森林限界は,この地域よりも高所に位置しており,周氷河地形が存在する理由は明らかではなかった.低所の周氷河地形の分布は,日浦 (1980) の植生図,千葉 (1956) による「ハゲ山」の分布,氷期に多量の火山灰が降下した地域,および冬期に降水が少ない地域と良好な対応をもつ.このことに注目して,低所の周氷河地形の形成環境を検討した.その結果, (1)ひとたび森林が破壊されると回復が困難な地域(ハゲ山密集域)および多量の火山灰降下域では,植生維持力が低いために,最終氷期に低所の広範囲で森林を欠く状況や疎林が出現した, (2) そこでは積雪の少なさも相まって地表の凍結融解が活発化し,周氷河地形が形成された,という解釈が得られた.

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