日高山脈最北部の西側丘陵地には，氷期の凍結融解作用によって形成された緩傾斜の周氷河斜面が広がる．写真の緑地は放牧地であり，周氷河斜面下部の山麓緩斜面が有効に利用されている様子がうかがえる．2016年8月，北海道や東北地方を襲った台風10号の影響により，日高山脈周辺では総降水量400～500 mmという記録的な豪雨に見舞われ，その結果700か所以上の斜面崩壊が発生した．写真は，並列した2つの「まゆ型」を呈する斜面崩壊をUAVにより撮影したものである．写真中央部を流れる小河川の上流域においても多数の崩壊が発生したため，土石流対策として現在は砂防堰堤が設置されている．

 この崩壊は，山麓緩斜面下部に位置する周氷河性斜面堆積物の移動域から堆積域にかけて発生した．前面を流れる河川は災害後の対策工事により緩やかな弧を描いているが，被災前は河道が「くの字」に折れ曲がっていた．その攻撃斜面に位置する谷壁への側方侵食を契機として，背後の周氷河斜面が次々と崩壊し，斜面上方へと拡大していった．緩斜面を覆う周氷河性斜面堆積物の層厚は約4 mに達する．崩壊地内に生じた水流による下刻により，斜面堆積物の基底付近をすべり面とする多重スランプ（multiple slump）が生じたものと推定される．

 かつて緩傾斜な周氷河斜面の崩壊はきわめて稀であったが，近年の気候変動に伴う台風の進路変化や豪雨の頻発化により，これまで比較的安定していた斜面が崩壊する事例が散見されるようになった．未解明な部分が多い周氷河斜面の崩壊メカニズムや発生場の条件について，今後さらなる事例の蓄積と検討が求められている．

（写真：輿水健一　2020年6月3日撮影；解説：石丸　聡）

 A classification of the 26 taxa of fossil and recent fishes is presented together with thoughts regarding their phylogenetic relationships. In addition to myllokunmingiids, dated as middle Cambrian, cyclostomes (including present-day myxinids and lampreys) are also recognized here as having Cambrian origins, based on their divergence time estimated from a molecular analysis. Ostracoderms (agnathans characterized by exoskeletons), a paraphyletic grouping including pteraspidomorphs, anaspids, thelodonts, and cephalaspidomorphs, were dominant in the Palaeozoic Era; pteraspidomorphs, comprising arandaspids, astraspids, and Eriptychius in the Ordovician and heterostracans in the Siluro-Devonian Periods; anaspids comprising birkenids, Jamoytius, and euphaneropids in Siluro-Devonian Periods, therodonts in Ordovician and Siluro-Devonian Periods, and cepharaspidomorphs, including osteostracans, galeaspids, and pituriaspids, also being Siluro-Devonian ostracoderms. Osteostracans and gnathostomes share the perichondral bone derived from other ostracoderms. Gnathostomes (jawed fishes) consist of placoderms, maxillate placoderms, Janusiscus-Ramirosuarezia, osteichthyans, and chondrichthyans. Placoderms diversified remarkably during the Devonian, and are located here at the base of gnathostomes, due to some ostracoderm similarities; maxillate placoderms with dermal premaxillary, maxillary, and dentary-like osteichthyans in the Silurian. Janusiscus-Ramirosuarezia have no otico-occipital fissure. Osteichthyans and chondrichthyans share dental lamina and otico-occipital fissure that differ from placodermi. Osteichthyans include ancestral stem osteichthyans in the Upper Silurian and Middle Devonian which were divided into actinopterygians and sarcopterygians in the Middle Devonian. Chondrichthyans include Ordovician chondrichthyan-like scales, ancestral stem chondrichthyans, acanthodians, and chondrichthyan crown group. Stem chondrichthyans and acanthodians (previously all combined under acanthodians) are now separated on the basis of scale microstructure from each other, and are differentiated from the chondrichthyan crown group, including pucapampellids, holocephalans, and elasmobranchs. Only cyclostomes, actinopterygians, sarcopterygians, holocephalans, and elasmobranchs are now represented among recent fishes.

 A carbonate rock boulder with a cone-in-cone structure was discovered in the Youbake riverbed of Akabira-kawa at Oganomachi. This sample is the second example of a cone-in-cone structure found in Japan, following a sample from the Itsukaichi-machi Group. The cone-in-cone comprises fibrous calcite and fine quartz grains. A thin clay band covering the cone has a stepped profile on one side and is rich in chlorite and illite crystals. The carbonate has low δ¹³C values of −12.1‰ to −13.4‰ and low δ¹⁸O values of −6.8‰ to −7.4‰. These isotope data agree well with those of previous studies.

 First, a comprehensive review is conducted of recent studies examining the interplay between the diversity of physical geographic environments, primarily shaped by landforms, and biodiversity within mountainous regions. Global warming increases the threat of the extinction of flora and fauna in alpine zones. Meanwhile, the role of landforms and geomorphic processes as refugia for cold-adapted species has garnered attention for potentially mitigating global warming effects. The concept, termed “conserving nature's stage,” emphasizes the importance of geodiversity—the variety of abiotic factors constituting nature's stage—in maintaining biodiversity. Although broad-scale studies suggest a positive correlation between geodiversity and biodiversity, fine-scale research studies are scant. Understanding the impacts of geodiversity on biodiversity at a fine scale, particularly in individual geosites or small natural features (SNFs), is crucial. Biological distributions are influenced by both historical and ecological biogeographic processes, involving organism dispersal with local abiotic and biotic factors. Fine-scale analyses are essential to elucidate these processes, thereby enhancing an understanding of broad-scale distribution patterns and their underlying mechanisms. Second, research conducted on ponds in the mountainous areas of Japan is reviewed. This allowsa emore specific examination of the relationship between geodiversity and biodiversity identified in existing literature reviews. The relationship between geodiversity and biodiversity has been investigated in one of the SNF—mountain ponds in the Northern Japanese Alps. The findings reveal that species-level diversity of aquatic insects and diatoms is shaped by micro- and small-scale landform-induced environmental variations in ponds. In addition, genetic diversity of aquatic insects is influenced by geographic separation and elevation differences between ponds that are governed by medium-scale landforms acting as dispersal barriers. Furthermore, these dispersal barriers also influence diatom species diversity in ponds. Distinct dispersal capabilities of aquatic insects and diatoms within the same watershed—active versus passive—may explain these differences. By integrating findings from these studies on the present-day organisms of mountain ponds with those from investigations on pond sediments, methodological challenges are described in integrating geological, paleoecological, and mountain ecological biogeographic knowledge to reconcile broad-scale and fine-scale patterns of biological distributions.

 In recent years, an increasing frequency of heavy rainfall events has led to more frequent slope failures on fossil periglacial slopes in Hokkaido. These slope failures, observed in areas such as the Hidaka Mountains, can be classified into three types—deep-type, shallow-type, and gully-type—based on their scale, morphology, and depth of occurrence. Deep-type failures extend across the postglacial dissection front, from the upper sideslope to the lower sideslope. Shallow-type failures commonly occur in head hollows, whereas gully-type failures typically develop on upper sideslopes with smooth ground surfaces. At the lowermost part of the periglacial slope deposits, layers of gravel facies and the underlying heavily weathered bedrock beneath exhibit high permeability, which causes groundwater to accumulate in these zones. Deep-type slope failures are triggered when pore-water pressure increases near horizons with contrasting permeability. In contrast, the uppermost part of the periglacial slope deposits is generally composed of low-permeability massive silt facies, overlain by more permeable black soil. This stratigraphic configuration promotes the concentration of rainwater above the periglacial deposits, resulting in shallow-type slope failures mainly within the black soil layer. When piping erosion develops in the lower part of the highly permeable layer, it can cause a collapse of the overlying topsoil, resulting in gully-type slope failures. Given the increasing frequency of heavy rainfall events, understanding these failure mechanisms is crucial for disaster prevention. Accurate prediction of failure type based on the position within the periglacial slope can aid in risk assessment and mitigation efforts.

 Mass movements of gravel are investigated on periglacial smooth slopes at Mikunizakai, Mt. Hakuba, and Mt. Shakushi in the Hakuba Mountains, northern Japanese Alps, using unmanned aerial vehicle (UAV)-derived orthoimages. UAV orthoimages are generated from aerial photographs acquired with UAV between 2020 and 2022 using structure-from-motion multi-view stereo (SfM-MVS) photogrammetry. Based on field surveys and UAV-derived orthoimages, surface gravel areas are classified into three categories: matrix-filled fine-gravel (< 8 cm), matrix-free fine-gravel (< 8 cm), and matrix-free large-gravel (≥ 8 cm). Subsurface materials are examined to a depth of 50 cm, together with snow-cover conditions. From autumn 2021 to autumn 2022, annual mean distances of gravel mass movements are 11.7 cm at Mikunizakai, 7.9 cm at Mt. Hakuba, and 13.4 cm at Mt. Shakushi. Large mobility is observed in matrix-filled and matrix-free fine-gravel areas. In these areas, gravel mass movements are attributed mainly to daily freeze–thaw cycles in spring and autumn, with an additional contribution from wash processes induced by summer rainfall events.