Network Of Valleys Research Articles

A 3 Ga old polythermal ice sheet in Isidis Planitia, Mars: Dynamics and thermal regime inferred from numerical modeling

Isidis Planitia is a 1350 km wide impact crater located close to the martian equator. To test the hypothesis that the 2.8 to 3.4 Ga old Thumbprint Terrain preserved on the floor of this basin is a glacial landform assemblage, we perform a numerical simulation of glaciation with a thermo-mechanically coupled model of ice sheet dynamics. As model inputs, we use surface temperatures and ice accumulation patterns predicted by a General Circulation Model based on the present-day atmospheric characteristics, and values of the geothermal heat flux provided by a global model of planetary thermal evolution. We find that, under favorable orbital conditions, an ice sheet covering the entire basin can develop in 2 to 5 Ma, with a maximum thickness of 4.9 km. The modeled ice sheet is polythermal: it is permanently cold-based in the periphery and, due to a negative heat-flux anomaly, also in the center, while the pressure melting point is reached in an intermediate ring. Our simulation is consistent with the interpretation that the Thumbprint Terrain is a martian equivalent of terrestrial ribbed moraines and has formed below a wet-based ice sheet. It supports also the interpretation that sinuous ridges and linear valleys observed at the periphery of the basin are parts of a subglacial network of eskers and tunnel valleys that drained the glacial meltwater outwards, across the cold-based periphery of the ice sheet. This work strengthens the hypothesis that glaciers thick as much as several km may have existed on Mars several Ga ago and that glacial basal melting may have contributed to the production and flow of surface liquid water at that time, under an atmosphere no thicker than the present-day one.

Soil cover patterns in the northern part of the area of aspen-fir taiga in the southeast of Western Siberia

Soil cover patterns in the northern part of the area of aspen-fir taiga on the Tom’-Yaya interfluve at 170–270 m a.s.l. are analyzed. Landscapes of the subtaiga piedmont province are found at somewhat lower heights. The three major forms of the local mesotopography include virtually flat interfluve surfaces, slopes (that predominate in area), and the network of ravines and small river valleys. Modal soil combinations on the slopes consist of the typical soddy-podzolic soils with very deep bleached eluvial horizons and dark gray (or gray) residual-humus gleyic soils with dark humus coatings. With an increase in the degree of drainage of the territory (toward the local erosional network), the portion of gleyic soil subtypes decreases from nearly 100% on the flat interfluves to 10–15% on the slopes; the portion of soils with residual humus features decreases from 80–90 to 10–15%, respectively. These two soil subtypes can be considered intergrades between typical soils of the aspen-fir taiga (soddy-podzolic soils with very deep bleached horizons) and dark gray and gray residual-humus soils characteristic of the subtaiga zone in the south of Western Siberia.

Mineralogy and fluvial history of the watersheds of Gale, Knobel, and Sharp craters: A regional context for the Mars Science Laboratory Curiosity's exploration

AbstractA 500 km long network of valleys extends from Herschel crater to Gale, Knobel, and Sharp craters. The mineralogy and timing of fluvial activity in these watersheds provide a regional framework for deciphering the origin of sediments of Gale crater's Mount Sharp, an exploration target for the Curiosity rover. Olivine‐bearing bedrock is exposed throughout the region, and its erosion contributed to widespread olivine‐bearing sand dunes. Fe/Mg phyllosilicates are found in both bedrock and sediments, implying that materials deposited in Gale crater may have inherited clay minerals, transported from the watershed. While some topographic lows of the Sharp‐Knobel watershed host chloride salts, the only salts detected in the Gale watershed are sulfates within Mount Sharp, implying regional or temporal differences in water chemistry. Crater counts indicate progressively more spatially localized aqueous activity: large‐scale valley network activity ceased by the early Hesperian, though later Hesperian/Amazonian fluvial activity continued near Gale and Sharp craters.

Geomorfología del río Marañón central (sector de Balsas)

El área de estudio presenta características geomorfológicas variadas. Presididas por un actor natural principal que es el río Marañón,entallando y construyendo su propio valle, a expensas de la naturaleza litológica y oponiéndose a las fuerzas tectónicas; presentaformas de relieve espectacular y procesos geomorfológicos peculiares y variados. Al mismo tiempo y de manera interrelacionada, haocurrido lo propio en sus numerosos tributarios, tanto por la margen derecha como por la margen izquierda, aquellos conforman enconjunto una red jerarquizada de valles proporcionados a su tamaño de cuenca, ajustados al eje principal, abarcando una vasta redhidrográca que con sus cuencas en este sector ocupan, gran parte de los departamentos de Amazonas al Este y Cajamarca al Oeste.El río Marañón ha construido su eje principal entallando su lecho, tal como lo han hecho, con sus propias peculiaridades, algunos otrosgrandes ríos peruanos encajados en grandes valles longitudinales, tales como el río Ucayali o Huallaga por ejemplo; el río Marañón conrumbo dominante S-N, es decir, cortado meridionalmente a las exuras de la cordillera de los Andes a lo largo de sus ejes anticlinales,hasta profundidades que superan los 2 000 m y alcanzando con frecuencia el basamento rocoso peruano, ha creado espectacularesformas del relieve y de procesos morfodinámicos complejos, cuya variedad se ve favorecido por la presencia de condiciones climáticaseventualmente potentes acondicionadas por factores de latitud, exposición, topografía, acciones antrópicas, etc., que como se verámarcan diferenciaciones importantes en la dinámica geomorfológica regional que nos proponemos clasicar, describir y explicar.

Taxonomical revision of Sorbus pseudosemiincisa (Rosaceae), a stenoendemic whitebeam from the Vértes Mts (Hungary), with the description of a new species, Sorbus pyricarpa

In this paper taxonomical revisions and treatments of Sorbus pseudosemiincisa during the last 80 years since its description until the present are discussed and summarised. As a result of these revisions the name and the interpretation of S. pseudosemiincisa were fi nally narrowed only to its lecto- and isolectotypes. According to our recent knowledge apart from the type specimens neither further herbarium materials nor living representatives of the species are known. All the remaining specimens deposited in BP and BPU and formerly identifi ed as S. pseudosemiincisa proved to belong to other taxa, such as S. adami, S. simonkaiana, S. karpatii, S. vertesensis, S. pseudolatifolia, S. cf. degenii as well as S. pyricarpa, a new species described here. Illustrations of the type specimens of S. pyricarpa, leaves, fruits and in situ photographs of its fl owering and fruiting shoots, in addition a detailed map of its actual distribution are also provided. Th e distribution of the stenoendemic S. pyricarpa is restricted to a narrow range, a network of dolomite valleys in the southwestern part of the Vertes Mts above the village of Csakbereny.

Distribution, morphology and habitats of saline wetlands: a case study from Monegros, Spain

Wetlands in semiarid regions have received less attention than wetlands in humidtemperate areas, and the limited amount of information has resulted in little regulatory recognition. A comprehensive map of the saline wetlands that occur in karstic depressions in the semiarid region of Monegros, NE Spain, was developed from historical data, topography, and surveys of vascular flora. Playa-lakes and other saline depressions are expressions of solution dolines largely founded on groundwater dynamics and favored by the limestone and gypsum-rich substrate. Substrate composition, groundwater dynamics, and the network of infilled valleys are key factors in the distribution of the wetlands. In spite of the anthropogenic imprint, wetlands morphometrics are the expression of geological processes. Significant correlations were found between basin area and depth, and between elongation and substrate composition. The predominantly subelongated shape of the Monegros saline wetlands (MSW) reflects their origin and a geometry strongly influenced by fractures. Grouping the MSW based on geological and vegetation features, provide a predictable relationship of surficial processes with the occurrence of otherwise complex and undetectable hydrological connectivity. Our ten geologybased Groups showed a high intra-group variation in depth, elongation, and vegetation cover. The eight vegetation-based categories mirror the gradation in flooding frequency and the soil salinity of MSW. The significant contrasts existing in-between the groups of wetlands and the disclosure of their causal factors provides a functional perspective at the landscape scale. This approach will help to monitor the ongoing environmental alterations associated with new on-farm irrigation developments.

Aksum's regional trade: new evidence from archaeological survey

Outstanding questions about ancient Aksum (Tigray, Ethiopia), which emerged as the capital city of a powerful kingdom between the first/second and seventh/eighth centuries AD, include the reasons for its particular location in the highlands of northern Ethiopia. Its situation at the hub of a radiating network of river valleys along which developed a complex system of communication and exchange routes linking the area with the surrounding regions was probably most significant. Analysis of recent discoveries from archaeological surveys provides an assessment of the short- and medium-range exchange circuits in which the Aksum area was involved. New evidence of the intra-regional circulation of chert pre-cores is presented, directly illustrating an aspect of the distribution of relatively low value materials within Aksum's wider economy.

Microstructures of glassy alloys: presence of hills, valleys, and veins

Amorphous ribbon surfaces, wheel and free sides are peculiar, but no microstructural description is not yet proposed. Here, we introduce a new description of the two surfaces by analogy with biological organs like a leaf as a network of hills, valleys, and veins. The venation can help understand the transport properties like heat dissipation during ribbon processing and mechanical properties like resilience or tensile strength of the amorphous ribbon. The new microstructure presentation can be useful to describe the crystallization of glassy materials

Preface: special section of papers dealing with overdeepened basins and valleys in the alpine realm

Overdeepened basins and valleys are common features in most regions that were glaciated during the Quaternary. Such structures are known from areas formerly covered by the Laurentian and Scandinavian Ice Sheet as well as for some parts of the Alps and their foreland. The phenomenon describes subsurface geomorphic features, erosionally cut into bedrock and later filled up by sediments. These structures can reach a depth of several hundred metres below the present valley floor. The sedimentary fillings of overdeepened basins and valleys are of particular interest for applied geosciences, for example in the context of groundwater resource management and subsurface constructions (i.e. tunnels). Recently, the topic of when and how overdeepening occurred in the past has gained additional attention with regard to the long-term stability of nuclear waste disposals, especially in Switzerland. In this context, it is important to note that relatively little research focussed on this issue during the past three decades. The last overview on the topic was a compilation of papers published in a special issue of the journal Eiszeitalter und Gegenwart in 1979. This special section in the present issue of the Swiss Journal of Geosciences comprises six papers related to the phenomenon of overdeepening in the Alps. The article by F. Preusser, J. Reitner and Ch. Schluchter summarises the areal distribution and geometry, reviews the present knowledge of the age of formation, and discusses possible processes responsible for overdeepened features. P. Jordan analyses the overdeepened structures in the foreland of the Swiss Alps using a Digital Elevation Model of bedrock surface based on outcrop and borehole data. It is shown that overdeepening mainly occurs within the limits of the Last Glaciation Maximum (LGM), but with a few overdeepened valleys and basin reaching beyond. M. Durst Stucki, R. Reber and F. Schlunegger interpret the buried valleys beneath the city of Bern as a network of tunnel valleys originating from subglacial high pressure melt water erosion. Valleys found in the surrounding Molasse uplands are regarded to represent proglacial melt water paths. The subsurface characteristics of three deep valleys from the Eastern Alps are investigated by E. Bruckl, J. Bruckl, W. Chwatal and Ch. Ullrich using different geophysical surveys. The authors conclude that deep erosion is not related to basal shear stress and that glacial confluence situations did in these examples not lead to important overdeepening. Rather, it is suggested that deep erosion was related to high pressured melt water at the glacier bed in settings of already tectonically weakened bedrock. The upper part of the sedimentary filling of the overdeepened Wehntal in northern Switzerland is analysed by F. Anselmetti, R. Drescher-Schneider, H. Furrer, H. R. Graf, S. Lowick, F. Preusser and M. Riedi revealing that it reaches back to at least 180,000 a. It is shown that the penultimate glaciation (pre-dating the Last Interglacial) may have reached the area M. Fiebig (&) Institute of Applied Geology, University of Natural Resources and Life sciences, Peter Jordan-Str. 70, 1190 Vienna, Austria e-mail: markus.fiebig@boku.ac.at

Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea

Passchier, S., Laban, C., Mesdag, C.S. & Rijsdijk, K.F. 2010: Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea. Boreas, Vol. 39, pp. 633–647. 10.1111/j.1502‐3885.2009.00138.x. ISSN 0300‐9483.Changes in subglacial bed conditions through multiple glaciations and their effect on ice dynamics are addressed through an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin. During Elsterian (MIS 12) ice growth, till deposition was subdued when ice became stagnant over a permeable substrate of fluvial sediments, and meltwater infiltrated into the bed. Headward erosion during glacial retreat produced a dense network of glacial valleys up to several hundreds of metres deep. A Saalian (MIS 6) glacial advance phase resulted in the deposition of a sheet of stiff sandy tills and terminal moraines. Meltwater was at least partially evacuated through the till layer, resulting in the development of a rigid bed. During the later part of the Saalian glaciation, ice‐stream inception can be related to the development of a glacial lake to the north and west of the study area. The presence of meltwater channels incised into the floors of glacial troughs is indicative of high subglacial water pressures, which may have played a role in the onset of ice streaming. We speculate that streaming ice flow in the later part of the Saalian glaciation caused the relatively early deglaciation, as recorded in the Amsterdam Terminal borehole. These results suggest that changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice‐sheet reconstructions.

Geology and hydrogeology of the Jiul de Vest–Cernişoara Basins, Romania

The Jiul de Vest, Lapusnicul Mare, and Cernisoara Rivers are in the western part of the Southern Carpathian Mountain Range. They form the natural boundary between the Retezat and Godeanu Mountains to the north and west, respectively, and the Vâlcan Mountains to the South. The Piule-Iorgovanu Mountains (Retezatul Mic Mountains) are between the Buta and Jiul de Vest Rivers, and are an alpine karstic plateau (2,000 m above sea level) of Jurassic limestones. On this plateau, an extensive network of dry valleys was developed and shaped by glaciers, which deeply eroded the carbonate (limestones) and noncarbonate rocks. Limestones are exposed from 710-m elevation at Cerna Spring (Izvorul Cernei) to 2,080-m elevation in the Piule Mountains. Between the three rivers, hydrogeological connections have developed, supported by the continuity of the limestones and by structural-tectonic features. All three rivers contribute to the recharge of the most important karst spring in Romania, Cerna Spring.

Agriculture in central Tibet: an assessment of climate, farming systems, and strategies to boost production

In the south of the Tibet Autonomous Region of China there is a network of valleys where intensive agriculture is practiced. Although considered highly productive by Tibetans, farm incomes in the region are low, leading to a range of government initiatives to boost grain and fodder production. However, there is limited information available on current farming practices, yields, and likely yield constraints. The present paper uses available data and farmer interviews to describe the agro-climate and current systems of crop and livestock production, and considers possible strategies to boost production. Although winters in Tibet are cold and dry, summer and autumn provide ideal conditions for crop growth. Cropping systems are characterised by heavy tillage, frequent irrigation, high seeding rates and fertiliser applications, some use of herbicides, and little stubble retention or mechanisation. Spring barley and winter wheat are the predominant crops, followed by rapeseed, winter barley, and minor fodder and vegetable crops. Average yields for the main grain crops are around 4.0 t/ha for spring barley and 4.5 t/ha for winter wheat, significantly lower than should be possible in the environment. Farmers typically keep five or six cattle tethered near the household. Cattle are fed diets based on crop residues but are generally malnourished and rarely produce beyond the needs of the family. It is suggested that research and extension in the areas of crop nutrition, weed control, irrigation, seeding technology, and crop varieties should enable significant increases in grain yield. Increases in cattle production will require increases in the supply of good quality fodder. Cereal/fodder intercrops or double crops sown using no-till seed drills might enable the production of useful amounts of fodder in many areas without jeopardising food grain supply, and allow more crop residues to be retained in fields for improved soil health.

Spin-SKitaev model: Classical ground states, order from disorder, and exact correlation functions

In the first part of this paper, we study the spin-S Kitaev model using spin-wave theory. We discover a remarkable geometry of the minimum-energy surface in the N-spin space. The classical ground states, called Cartesian or CN-ground states, whose number grows exponentially with the number of spins N, form a set of points in the N-spin space. These points are connected by a network of flat valleys in the N-spin space giving rise to a continuous family of classical ground states. Further, the CN-ground states have a correspondence with dimer coverings and with self-avoiding walks on a honeycomb lattice. The zero-point energy of our spin-wave theory picks out a subset from a continuous family of classically degenerate states as the quantum ground states; the number of these states also grows exponentially with N. In the second part, we present some exact results. For arbitrary spin S, we show that localized Z2 flux excitations are present by constructing plaquette operators with eigenvalues ${\pm}1$, which commute with the Hamiltonian. This set of commuting plaquette operators leads to an exact vanishing of the spin-spin correlation functions beyond nearest-neighbor separation found earlier for the spin-1/2 model [G. Baskaran et al., Phys. Rev. Lett. 98, 247201 (2007)]. We introduce a generalized Jordan-Wigner transformation for the case of general spin S and find a complete set of commuting link operators similar to the spin-1/2 model, thereby making the Z2 gauge structure more manifest. The Jordan-Wigner construction also leads, in a natural fashion, to Majorana fermion operators for half-oddinteger spin cases and hard-core boson operators for integer spin cases strongly suggesting the presence of Majorana fermion and boson excitations in the respective low-energy sectors. Finally, we present a modified Kitaev Hamiltonian, which is exactly solvable for all half-odd-integer spins; it is equivalent to an exponentially large number of copies of spin-1/2 Kitaev Hamiltonians.

Catastrophic flooding origin of shelf valley systems in the English Channel

Megaflood events involving sudden discharges of exceptionally large volumes of water are rare, but can significantly affect landscape evolution, continental-scale drainage patterns and climate change. It has been proposed that a significant flood event eroded a network of large ancient valleys on the floor of the English Channel-the narrow seaway between England and France. This hypothesis has remained untested through lack of direct evidence, and alternative non-catastrophist ideas have been entertained for valley formation. Here we analyse a new regional bathymetric map of part of the English Channel derived from high-resolution sonar data, which shows the morphology of the valley in unprecedented detail. We observe a large bedrock-floored valley that contains a distinct assemblage of landforms, including streamlined islands and longitudinal erosional grooves, which are indicative of large-scale subaerial erosion by high-magnitude water discharges. Our observations support the megaflood model, in which breaching of a rock dam at the Dover Strait instigated catastrophic drainage of a large pro-glacial lake in the southern North Sea basin. We suggest that megaflooding provides an explanation for the permanent isolation of Britain from mainland Europe during interglacial high-sea-level stands, and consequently for patterns of early human colonisation of Britain together with the large-scale reorganization of palaeodrainage in northwest Europe.

Seaward-Branching Coastal-Plain and Piedmont Incised-Valley Systems Through Multiple Sea-Level Cycles: Late Quaternary Examples from Mobile Bay and Mississippi Sound, U.S.A.

Abstract Most incised valleys become more organized seaward through tributaries merging with the main trunk valley. Late Quaternary incised valleys on the Mississippi and Alabama inner continental shelf branch seaward, although they do coalesce towards the shelf break where they feed shelf-edge deltas. To link fluvial systems with their associated previously mapped incised valleys on the shelf, and evaluate the contribution of small coastal-plain valley systems to the lowstand systems tract, high-resolution seismic data and cores were collected from Mobile Bay, eastern Mississippi Sound, and the Mobile bay-head delta. These data show four unconformity-bounded stacked units, and this study focuses on the upper two regionally mappable units. The upper two unconformities were sampled in core as exposure surfaces, and, on the basis of stratigraphic position, depth of incision, and 14C dates, are interpreted as sequence boundaries. The shallowest sequence boundary (A) formed in response to the Oxygen Isotope Stage 2 sea-level lowstand, while the underlying sequence boundary (B) formed in response to an earlier lowstand (Stage 6?). A map of Sequence Boundary B shows a network of seaward-branching valleys, 20–35 m deep and 0.6–7.7 km wide, that extend across Mobile Bay and Mississippi Sound. These valleys, which are bounded by well-developed terraces, are extensions of the piedmont Mobile fluvial system and the coastal-plain Fowl and La Batre fluvial systems. A map of the Stage 2 Sequence Boundary shows that all systems generally reoccupied their previous valley positions and are separated by interfluve plateaus. Valley fill between Sequence Boundary B and the Stage 2 Sequence Boundary is generally composed of a basal unit of alluvial sediments overlain by bay-head delta deposits; however, Stage 2 to 1 valley fill is composed entirely of central-basin sediments. The presence of bay-head delta deposits on the inner shelf indicates this depositional environment backstepped across the estuaries to modern positions. Backstepping likely occurred as the low-gradient paleovalleys were inundated, resulting in rapid rates of transgression. Fluvial gradients measured on Sequence Boundary B and the Stage 2 Sequence Boundary, below the modern Mobile bay-head delta plain and the estuaries, are very low (1.3–0.3 m/km). The Mobile bay-head delta and upper Mobile Bay are confined by steep topography, which opens basinward into the low-gradient shorelines surrounding lower Mobile Bay and Mississippi Sound. During falling sea level, channel branching likely occurred as the low-gradient fluvial systems extended basinward beyond the confined topography. These channels incised as sea level fell below the steeper shelf break during the lowstand. Stage 2 valley morphology is partially influenced by the position of the underlying Stage 6 valleys. Both coastal-plain and piedmont valley systems exhibit compound fill, indicating that differentiation between these types of incised valleys cannot be based on valley-fill architecture alone.

The western part of the Gulf of Cadiz: contour currents and turbidity currents interactions

Recent multibeam bathymetry and acoustic imagery data provide a new understanding of the morphology of the western part of the Gulf of Cadiz. The gulf is under the influence of a strong current, the Mediterranean Outflow Water (MOW). This current is at the origin of the construction of the giant Contourite Depositional System. Canyons and valleys with erosive flanks are observed. Only the Portimao Canyon is presently connected to the continental shelf. Channels occur on the continental shelf but are presently disconnected from the deeper network of channels and valleys. Slumps are localized in steep slope areas. They are caused by oversteepening and overloading, sometimes probably associated with earthquake activity. Slumps transform sharply into turbidity currents, depositing turbidites on the floor of deep valleys. Interaction of the MOW and gravity currents is suggested by the filling of the incisions located on the drifts below the present seafloor, the shifting of valleys and canyons in the direction of the MOW flow inducing an unusual phenomenon of capture of submarine valleys.

Historical deforestation as a cause of alluviation in small valleys, subcarpathian loess plateau, Poland

The undulating loess Kanczuga Plateau, 250–270 m a.s.l., is located in the temperate climatic zone of Central Europe, within the northern foreland of the Carpathians. It is dissected by a network of valleys 30–60 m deep, infilled with several metres of complex minerogenic and organic deposits which started to accumulate during the early Holocene. The southern side of one of these valleys is dissected by several dry valleys. The mouth sections of these valleys have buried surface comprising peat and/or organic silts, overlain by sandy–silty alluvium. 14C dates indicate that the alluviation of these valley floors, which proceeded from the headwaters to the mouth sections, began in the 1100 ad. The gradual intensification of soil erosion in the upper part of the catchment and associated sedimentation probably encompassed only a few centuries, with greatest alluviation during the Little Ice Age. This plateau, which is under the influence of more continental climatic conditions than the low-mountain and upland regions of Western Europe, features more climatic contrasts. Here environmental changes triggered by land use were superimposed on climatic change. The most intensive processes of soil erosion and sediment transfer as well as alluviation, coincided with the first phase of cooling during the Little Ice Age. It was the final stage of the intensification of these processes, probably more intensive than its initial phase in the eleventh century, caused primarily by deforestation resulting from the development of agriculture.

Internal structure and evolution of the Late Quaternary sequence in a shallow embayment: The Amvrakikos Gulf, NW Greece

The Late Quaternary stratigraphic pattern of the Amvrakikos Gulf in the eastern Mediterranean has been studied on the basis of the analysis of high-resolution seismic profiles and short sediment cores. Lowstand, transgressive and highstand systems tracts have been identified that are configured in a major depositional sequence of a fifth-order sea level cycle. During the last glaciation, when the sea level was lowered by ∼55 m relative to its present position (ca. 50 ka BP), the Amvrakikos Gulf was probably emerged, giving rise to: (a) subaerial erosion and fluvial incision in the western shallow part of the Gulf; and (b) development of a paleo-lake in the eastern deepest domain. Until the early phase of the post-glacial transgression (ca. 11–50 ka BP), a paleo-river, characterized by a dense network of V-shaped valleys, was draining the western part of the Gulf and flowing into the Ionian Sea. In the isolated eastern part, lowstand lacustrine deposits (deltaic and prodeltaic units) accumulated within the paleo-lake. At the latter stage of the last post-glacial transgression (ca. 11 ka BP), seawater entered the Gulf and the transgressive systems tract was deposited, consisting of incised-channel fills, beach–shoreface deposits and delta/prodelta wedges. Buried terraces have been identified also within this transgressive tract that reveal a step-like sea level rise. Since ca. 6 ka BP, when the sea level reached its present-day position, a distal prodeltaic unit has been deposited and overlain by prograding deltaic wedges. The spatial distribution of these wedges provides evidence of a continuous shifting of the river mouths during Holocene. The recent sedimentation patterns are related to the terrigenous inputs primarily from the Arachthos River, located in the northeastern part of the Gulf, and secondarily from the Louros River, located in the northwestern part.

Fluvial origin of the valley system in northern Victoria Land (Antarctica) from quantitative geomorphic analysis

A network of sinuous valleys with typical dendritic pattern characterizes northern Victoria Land (NVL) in Antarctica. Subparallel to parallel and angular to rectangular patterns are also present. Quantitative geomorphic analysis of the valley network has been carried out utilizing GIS spatial analysis. While drainage densities and drainage frequencies show low values, segments of the NVL valley network are substantially well organized, as indicated by bifurcation ratio (R b ) and direct bifurcation ratio (R bd ) parameters. All basins faithfully adhere to Horton’s laws of drainage network composition. Quantitative geomorphic analysis suggests that the valley system can be ascribed to fl uvial origin and that consequently, a morphoclimatic system completely different from that of present day must have driven its carving. The resulting data provide indications about the origin of the valley network and this invaluable information can be used for the reconstruction of earlier phases of glacial history and climatic and tectonic evolution of this signifi cant Antarctic region. Fluvial erosion enhanced the denudation of the Transantarctic Mountains from at least 55 Ma to at least the Eocene-Oligocene boundary (ca 34 Ma). Fluvial basins adapted to the tectonic structure, following the main regional fault systems. A well-developed alpine topography postdates the fl uvial morphology. Temperate glaciers were responsible for denudation until the Late Miocene. Parasitic glaciers presently mantle the previously sculpted topography. Present-day glacial erosion is negligible and denudation has been exceedingly slow since 7.5 Ma.

サシバ（Ｂｕｔａｓｔｕｒ ｉｎｄｉｃｕｓ）の営巣場所数に影響する環境要因

Nesting locations of gray-faced buzzard eagles were studied in two areas of about 348 in Tochigi prefecture. Their food items were also monitored. From the nesting data, a multiple regression model was constructed to predict the number of nesting locations within a 2x2 mesh grid from several environmental factors such as vegetation and human population size. The resulted model could explain about 70% of the variation in the number of nests (r2×0.71). Factors used in the model were length of forest edge facing paddy fields in a mesh grid and in surrounding eight meshes grids (positive factor), and population (negative factor). In the study area gray-faced buzzard eagles were feeding mainly on frogs, and the birds were observed to perch on the tree top in the forest-paddy field edge waiting for frogs to appear. The preference on forest-paddy field edge shown by the model might reflect the importance of such habitat as the species' hunting area. In our study area, the gray-faced buzzard eagles were especially abundant in continuous 'Yatsu' area, a landscape in which a low tableland is eroded by rivers to create a fractal shaped network of valleys. Our results, along with other published data, indicate that the Yatsu landscape is an important breeding habitat for this species.