A preliminary inventory of periglacial landforms in the Andes of La Rioja and San Juan, Argentina, at about 28°S

The Cerro El Potro and nearby mountain chains belong to the Andean Frontal Cordillera (28°S). Cerro El Potro is a glaciated mountain that is surrounded by huge valleys both on its Chilean and Argentinean flanks. Its southern limit is a steep rock wall towards the trough-shaped Río Blanco valley in Argentina, with a wide valley floor. The other sides of the mountain are characterized by well-developed Pleistocene cirques. The predominant landforms in this area have been shaped in a periglacial environment superimposed on an earlier glacial landscape. It is a region with abundant rock glaciers, a noteworthy rock glacier zone, but nevertheless, it is a relatively little known area in South America. In this preliminary inventory, the landforms surveyed were mainly gravitational in origin, including valley rock glaciers, talus rock glaciers, debris cones, landforms originated by solifluction processes and talus detrital sheets on mountain sides. Ancient moraine deposits have been found on the sides of the main rivers that cross the area form west to east, including the Blanco and Bermejo rivers. Present day fluvial activity is limited, and restricted to these main rivers. In this area of glacial valleys and small cirques, there are small lakes and other water bodies, grass covered patches and zones with high mountain vegetation. Present day glacial activity is restricted to the highest part of the area, above 5500 m a.s.l., mainly in the Cerro El Potro (5879 m) where a permanent ice field exists, as well as small mountain glaciers.     

Connectivity analyses of valley patterns indicate preservation of a preglacial fluvial valley system in the Dyfi basin, Wales

Coastal valleys in the west part of Mid-Wales, such as the Mawddach, Dysynni, Tal-y-llyn and Dyfi, acted as corridors for ice which drained the Welsh Ice Cap during the Devensian. Analyses of detailed digital elevation models, and interpretation of satellite images and aerial photographs, show the existence of large variations in the amount of glacial modification between these valleys. Although all the valleys are glacially over-deepened along late Caledonian fault lines, only the Dyfi basin exhibits a dendritic pattern, with V-shaped cross-profiles and valley spurs typical of valleys formed by fluvial processes. Connectivity analysis of the Dyfi basin shows that it exhibits an almost completely dendritic pattern with connectivity α and β values of 0.74 and 1.01, respectively, with little glacial modification of the preglacial fluvial valley pattern in the form of glacial valley breaching. Several examples of glacial meltwater incision into a well-developed pre-existing river valley system, causing river capture across watersheds, have been identified in the Dyfi basin. The degree of preservation of the preglacial fluvial valley system within the Dyfi basin indicates limited modification by glacial processes, despite the area being subjected to glacier activity during the Late Devensian at least. It is possible that major parts of the basin were covered by cold-based or slow-moving ice, close to, or under, a migrating ice-divide, with the major ice drainage occurring along the weaker zone of the Pennal Fault along which teh Dyfi valley is located, causing minor adjustments to the surrounding interfluves and uplands. It is proposed here that the general river valley morphology of the Dyfi basin is of a pre-Late Devensian age.     

A conceptual model of valley incision, planation and terrace formation during cold and arid permafrost conditions of Pleistocene southern England

Staircases of gravelly river terrace deposits in southern England occupy valleys typically underlain by frost-susceptible and brecciated bedrocks. The valleys developed during the Quaternary by alternating episodes of (1) brecciation, incision and planation through the bedrock, forming wide low-relief erosion surfaces; and (2) aggradation in braidplains of gravel a few meters thick that bury the erosion surfaces. A conceptual model to account for some of the terraces proposes that brecciation resulted from ice segregation in the ice-rich layer in the upper meters of Pleistocene permafrost, making them vulnerable to fluvial thermal erosion and therefore predisposing the bedrock to planation. The low gradients of the valleys were adjusted such that rivers transferred fine materials out of the basins but lacked the competence to remove gravel, which therefore accumulated within floodplains. The model challenges the prevailing view of incision during climate transitions. It attributes incision and planation to very cold and arid permafrost conditions, when rivers had limited discharges and hillslopes supplied limited volumes of stony debris into valley bottoms.     

Structural control on landscape development of Barak Valley, northeast India

The Barak Valley, comprising a contiguous region of three south Assam districts of Cachar, Hailakandi and Karimganj, represents a ridge and valley province with meridional to sub-meridional anticlinal hills and synclinal valleys. Thin skinned tectonics resulted in the deformation of Neogene clastics which is manifested in the form of a series of anticlines and synclines. These structural elements have profound control on the development of present-day landscape in Barak valley. Structurally controlled fluvial erosion produced a series of cuesta ridges and strike valleys which were further subjected to erosional dissection leading to development of numerous topographic highs. Along the northern part of Barak valley adjoining Barail range, three terrace levels stand out with topographic offsets of upto ～10 m. These terraces are linked to tectonic adjustments along Haflong-Disang Thrust that traverse along the Barail foothills. Numerous wetlands linked to fluvial activities and also isolated from the fluvial domain form a significant element of the Barak valley landscape. Some of these wetlands have their origin to tectonic activities of the region. While the trunk channel of the region follows a nearly E-W trend, all the major tributaries join it at nearly orthogonal position following the prevailing structural trend.     

Pleistocene river systems in the southern peribaltic area as indication of interglacial sea level changes in the Baltic Basin

The modern drainage system of central Poland developed during the Holsteinian, but it originated from the Elsterian glacial tunnel valleys and deglacial residual overflow lakes. In spite of occupation of this area by the Wartanian ice sheet and the following formation of the landscape during deglaciation, a similar river network was renewed during the Eemian. During the Weichselian the Middle Vistula valley was subjected to widespread ice-dam deposition. This resulted in rise of the base level of erosion and in westward deflection of the runoff, connected with development of the Central European spillways. The presented reconstruction of the Middle and Late Pleistocene fluvial network shows that the Holstein and Eemian sea levels were the driving force for river system development in central Poland. The Holstein and Eemian sea levels were very close to the present water level of the Baltic Sea. They made interglacial fluvial patterns roughly similar to the contemporary one, and therefore the main watersheds have been only slightly modified since that time. However, due to the considerable southward extension of the sea during the Eemian and presumably also during the Holsteinian, buried interglacial river deposits in central Poland occur at present well beneath the Holocene alluvia.     

Quaternary geology in Zanskar,NW Indian Himalaya: evidence for restricted glaciation and preglacial topography

Research into the Quaternary geology of the NW Himalaya has concentrated on the elucidation of the glacial sequence. However, whilst the main ranges of the Himalaya have been subjected to numerous glaciations and are now an obvious alpine glaciated terrain, much of the landscape in Zanskar and Ladakh is more equivocal and does not appear to have been glaciated during this time. These landscape facets may therefore have a much older origin and relate to preglacial events.In Zanskar, the main ice source in all glaciations was the strongly glaciated and still glacierized north slope of the main Himalaya. This ice then flowed generally northwards in the valleys of the Zanskar river and its tributaries leaving between them a landscape supporting only a few and scattered minor local glaciers. Evidence of early glaciation has been found on isolated valley-side remnants >200 m above the present rivers. Reconstruction of these preglacial valley cross profiles show them to be generally broad and shallow, with gentle slopes. This is in distinct contrast to the present major valley systems which can usually be divided into two parts—a lower unglaciated fluvially eroded section, such as the Lungnak (Tsarap Lingti Chu) Gorge and an upper broad glacial section, such as the Stod (Doda) valley.Down-valley extent of glaciation is defined by the upper ends of unglaciated fluvial gorges. Laterally, the glaciers were confined progressively to their valleys. Inevitably there is only evidence of successively smaller subsequent glaciations, but the tectonic uplift of the southern ranges may have been a factor in this forming an increasing barrier to the snow-bearing monsoon winds.     

Natural hazards in the Ghaghara River area, Ganga Plain, India

The Ganga Plain is one of the most densely populated regions of the world due to its fertile soil and availability of water. The rivers of this plain are the lifeline for millions of people of this vast alluvial plain. All rivers of this plain are characterized by narrow channel confined within wide valley. Continuously increasing pressure of population on this plain has led to the intensification of settlement even into the valley of the river. This unplanned expansion has enhanced the damage due to flooding during high-discharge period and lateral erosion during low-discharge period. Flooding and lateral erosion are identified as fluvial hazards in the Ghaghara River area. Extensive studies have been carried out on flooding, but not much attention has been paid to the phenomenon of lateral erosion. However, it has been observed that lateral erosion is an independent fluvial hazard that operates during low-discharge period. Low degree of compaction due to the presence of sandy and silty facies in the river valley deposits, mass movement, palaeocurrent pattern, and fractures initiates and enhances the lateral erosion. The present paper deals with the fluvial hazards in the Ghaghara River area.     

Evolution of an incised valley system in the southern Adriatic Sea (Apulian margin): an onshore–offshore correlation

Seismic surveys with sub‐bottom profiler were carried out in the Manfredonia Gulf in the southern Adriatic Sea. Here, a buried surface was recognized on which three valleys, located about 80 km from the shelf edge, were deeply incised. Beneath this surface, a pre‐upper Würm seismic unit (PW) was identified. Above, two seismic units were recognized: the transgressive system tract (TST) and highstand system tract (g₂)_. On the basis of regional correlation with onshore and offshore data, these units and their boundaries were dated and correlated with phases of the last glacial–interglacial cycle. The incised valley system was attributed to the Marine Isotopic Stage (MIS) 2. The TST and g₂ units fill the valleys and were attributed to the post‐glacial sea‐level rise and highstand. The incised valleys are anomalous with respect to published models; despite having many characteristics that would have limited the fluvial incision (the lowstand shoreline that remained on the shelf, the low gradient of the shelf, the subsidence that affected the study area since MIS 5), the valleys appear to be deeply incised on the shelf, with valley flanks that can exceed 40 m in height. The model to explain the formation of the valleys comprises enhanced river discharge as the key factor in increasing river energy and promoting erosion across the low gradient shelf. Copyright © 2014 John Wiley & Sons, Ltd.     

黄河源地区的河谷地貌特征及其对黄河上游形成和演化的启示

黄河的形成与演化对于认识我国宏观地貌格局的形成、青藏高原及黄土高原的区域构造活动历史、华北平原及黄渤海陆架的形成和演化等问题具有重要意义。目前对黄河演化历史的研究主要集中在龙羊峡以下的河段,对于黄河源段的关注较少。文章基于黄河源地区河谷地貌的实地考察,并利用SRTM1-DEM数据,分析了黄河源段干流及支流河谷橫剖面的地貌特征,并与该区典型的冰川谷和兰州附近黄河的河谷横剖面进行了对比。结果表明:黄河源地区的河谷规模巨大,并呈现出谷底开阔、河床窄小、阶地不明显、谷坡陡立、河谷横剖面左右对称的U型谷特征。这些特征与该区冰蚀谷的特征相似,但与兰州段黄河成型河谷的特征相差甚远,且其河谷规模更大。我们推断,黄河源地区的河谷可能主要为冰期时的冰蚀作用所塑造,而非单纯的流水侵蚀形成。由于冰蚀作用的存在,该区早期的河流阶地可能被随后冰期的冰蚀作用所破坏,当前基于黄河源地区现存河流阶地年代的研究很可能低估了该区水系的发育历史。此外,反复的冰川进退也可能导致黄河源水系自上而下贯通,而非溯源侵蚀形成。     

OSL‐based chronostratigraphy of river terraces in mountainous areas,Dunajec basin,West Carpathians: a revision of the climatostratigraphical approach

The technique of optically stimulated luminescence (OSL) dating applied to fluvial sediments provided a geochronological framework of river terrace formation in the middle part of the Dunajec River basin – a reference area for studies of evolution of river valleys in the northern part of the Carpathians (West Carpathians). Fluvial sediments at 18–90 m above valley bottoms were dated in the valleys of the Dunajec River and one of its tributaries. The resulting ages range from 158.9±8.3 to 12.2±1.3 ka. This indicates that some of the terrace sediments were deposited much later than previously assumed on the grounds of a combined morphostratigraphical and climatostratigraphical approach. The OSL‐based chronostratigraphy of terrace formation consists of seven separate phases of fluvial aggradation, separated by periods of incision and lateral erosion. Some of the ages determined correspond to warm stages of the Pleistocene – Marine Isotope Stage 3 (MIS 3) and MIS 5 – demonstrating that some terraces were formed during interstadial or interglacial periods. The results provide a key for evaluating rates of neotectonic uplift, allowing us to decipher the response of a fluvial system to climate change within the context of the glacial–interglacial scheme.     

Quaternary stratigraphy of the Koratallaiyar-Cooum basin,Chennai

In this paper we present Quaternary stratigraphy of the area around Chennai based on archaeological findings on the ferricrete surface, geomorphological observations supplemented by radiocarbon dating. The coastal landscape around Chennai, Tamil Nadu, has preserved ferruginised boulder gravel deposits, ferricretes and fluvial deposits of varying thickness. The area studied is approximately 150 km east to west and 180 km north to south with a broad continental shelf towards the seaward. Several rivers enter the Bay of Bengal along its shores like the Koratallaiyar, Cooum and the Adyar. Precambrian charnockite and Upper Gondwana sandstone and shale bedrock rim the shelf margin. For the most part, the Upper Pleistocene-Holocene fluvial sediments overlie an erosion surface that has cut into older Pleistocene sediments and ferricrete surface. Incised valleys that cut into this erosion surface are up to 5–6 km wide and have a relief of at least 30 m. The largest valley is that cut by the Koratallaiyar River. Holocene sediments deposited in the incised valleys include fluvial gravels, early transgressive channel sands and floodplain silts. Older Pleistocene sediments are deposited before and during the 120-ka high stand (Marine isotope stage 5). They consist of ferricretes and ferricrete gravel formed in nearshore humid environments. Muddy and sandy clastic sediments dated to the ca. 5 ka highstand suggest that the climate was semi arid at this time with less fluvial transport. The coarsening up sequence indicates deposition by high intensity channel processes. Pedogenic mottled, clayey silt unit represents an important tectonic event when the channel was temporarily drained and sediment were sub aerially exposed. Uplift of the region has caused the local rivers to incise into the landscape, forming degradation terraces.     

Geomorphic expression of the southern Central Andes forebulge (37°S,Argentina)

We present a geomorphologic analysis of an east‐west transect located east of the southern Andes of Argentina (~37°S). We observe a succession of zones that underwent erosion and deposition during the Pleistocene. If the proximal Andean foothills are incised, a proximal depozone receives sediments feeding the megafan of the Rio Colorado on the Chadileuvú plain. More distally, the abandoned palaeo‐valleys and bending of the valley floors reflect a localized uplift. Further to the east, another depozone corresponds to the Pampa Deprimida lowland. This pattern is consistent with the presence of a classical flexural geometry of the lithosphere. The distal uplift of the foreland corresponds in terms of location, length (150 km) and amplitude (240 m) to the Andean forebulge modelled by a geophysical approach. In this study, we identify the morphological imprint of this bulge and show its effect on the fluvial activity.     

藏东南帕隆藏布现今河流地貌特征 及其晚第四纪演化

河流沉积与地貌对构造与气候的变化极为敏感,可记录区域构造活动、气候变化和环境演变等多方面的丰富信息。由于独特的构造背景与气候条件,帕隆藏布不仅成为雅鲁藏布水系水量最大的支流,而且其流域在藏东南地区占有重要的地位。帕隆藏布流域内地表过程活跃且河流地貌演化过程快速,是揭示青藏高原东南部构造地貌演化的重要载体。通过对该河流地貌的形态学和沉积学分析发现,帕隆藏布河流形态具有明显的线状特征,其干流近似直线展布,而主要支流呈羽状分布,两者多呈直角交汇,表明河流形态明显受到嘉黎断裂带的构造形迹控制。进一步利用光释光和¹⁴C定年方法,对帕隆藏布的晚第四纪河流地貌演化,尤其是干流和东久河支流的晚第四纪河流阶地进行研究后发现,末次冰期以来的气候变化导致帕隆藏布的晚第四纪河流地貌呈现出典型的分段式特征,根据海拔高度主要可划分为3段：1）海拔2 600 m以下的河谷地貌呈V形峡谷,河谷比降大,阶地沉积年龄均在9.0～2.0 kaBP间,沉积属性以河流相和坡积相为主,表明是全新世以来气候变暖条件下形成的;2）海拔2 600～3 300 m的中游段河谷呈冰蚀围谷盆地、U形槽谷等,河谷比降小,河岸谷坡坡度小,主谷两岸冰碛垄发育,存留了古冰缘地貌遗迹,阶地沉积属性以古湖相、冰水相及河流相为主,测年结果在29.8～10.9 kaBP和50.9～39.8 kaBP间,显示其曾经为末次冰期和冰消期冰缘湖泊体系,后被现今的帕隆藏布所贯通;3）海拔3 300 m以上河流地貌为典型的冰川U形槽谷,谷底平坦,发育现代冰湖,仅发育Ⅰ级阶地并上面覆有冰碛物堆积体,有末次冰期的冰缘地貌遗迹,但主要受周围海洋性冰川作用,呈现现代冰缘地貌特征。整体上看,帕隆藏布的现今河流地貌上、下游两端年轻,主要形成于全新世期间;中游的河流地貌出现较早,残留了末次冰期和冰消期的冰缘地貌特征,并保留了广泛的古冰湖相沉积物。因此,帕隆藏布现今的河流形态主要出现在末次冰期以来。     

Architecture of fluvial-dominated valley-fill deposits in the Cretaceous Fall River Formation

The Fall River Formation is a 45 m thick layer of fluvial-dominated valley-fills and shore-zone strata deposited on the stable cratonic margin of the Cretaceous Western Interior Seaway. Fall River deposits in Red Canyon, in the south-west corner of South Dakota (USA), expose a cross-section of a 3.5 km wide valley-fill sandstone and laterally adjacent marine deposits. The marine deposits comprise three 10 m thick upward-shoaling sequences; each composed of multiple metres-thick upward-coarsening successions. The lower two of these sequences are laterally cut by the valley-fill sandstone, and are capped by metres-thick muddy palaeosols. The upper sequence spans the top of the valley-fill sandstone, and is overlain by the Skull Creek Shale. The 30 m thick valley sandstone is partitioned into four distinct fills by major erosion surfaces, and each of these fills contain many metres-thick channel-form bodies. Deposits in the lower parts of these fills are sheet-like, top-truncated channel bodies, whereas deposits in the upper parts of fills are upward-concave, laterally amalgamated channel bodies, more completely preserved heterolithic channel bodies, or wave-deposited sheets. Each valley-fill basal erosion surface records an episode of valley incision and relative sea-level fall, and the gradual progression from fluvial to more estuarine deposits upwards within each fill records relative sea-level rise. All fills are dominantly channel deposits and are capped by marine flooding surfaces. The dominance of channel deposits, the gradual change to more estuarine facies in the upper parts of fills, and the location of flooding surfaces at valley-fill tops all suggest that sediment supply initially kept pace with relative sea-level rise and valleys filled during late marine lowstand and transgression, not during subsequent highstands. Recently proposed facies models have focused on variations in the relative strength of tide, wave and river currents as controls on valley-fill deposits. However, relative rates of sediment supply and basin accommodation change, and the shift in this ratio along the depositional profile during multiple-scale cycles in relative sea-level, are equally important controls on the style of valley-fill deposits.     

Pleistocene subglacial tunnel valleys in the central North Sea basin: 3‐D morphology and evolution

Four phases of cross‐cutting tunnel valleys imaged on 3‐D seismic datasets are mapped within the Middle–Late Pleistocene succession of the central North Sea basin (Witch Ground area). In plan the tunnel valleys form complex anastomosing networks, with tributary valleys joining main valleys at high angles. The valleys have widths ranging from 250 to 2300 m, and base to shoulder relief varying between 30 and 155 m, with irregular long‐axis profiles characteristic of erosion by water driven by glaciostatic pressures. The youngest phase of tunnel valleys are smaller and have a thinner infill than the older generations. The fill of the larger valleys comprises three seismic facies, the lowermost of which has high amplitudes and is discontinuous. The middle facies consists of wedge‐shaped packages of low‐angle dipping reflectors and is overlain by a facies characterised by sub‐horizontal reflectors, which onlap the valley margins. The seismic character, and comparison with lithologies identified in other northwest European Pleistocene tunnel valleys both onshore and offshore, suggests that the lower two seismic facies are most likely sand and gravel‐dominated, while the uppermost facies consists of glaciolacustrine and marine muds. The 3‐D morphology of the valley margins combined with the geometry of the infill packages suggest that episodic discharge of subglacial meltwater was responsible for incising the valleys and depositing at least some of the infill. Proglacial glaciofluvial deposits are inferred to account for some of the fill overlying the subglacial deposits. Glaciolacustrine and marine muds filled remaining valley topography as the ice sheet retreated. The preserved valley margins are shown to be time‐transgressive erosion surfaces that record changes in geometry of the tunnel valley system as it evolved through time, implying that valleys associated with each ice‐sheet advance/retreat cycle were dynamic and probably long‐lived. Within the constraints of the existing stratigraphy the oldest tunnel valleys in the Witch Ground area of the central North Sea are most likely to be Marine Isotope Stage (MIS) 12 (Elsterian, ca. 470 ka) in age and the youngest pre‐MIS 5e (last interglacial, ca. 120 ka). If each tunnel valley phase was formed during the retreat of a major ice sheet then four glaciations with ice coverage of the central North Sea are recorded in the pre‐Weichselian, Middle–Late Pleistocene stratigraphy. Copyright © 2006 John Wiley & Sons, Ltd.     

湖南张家界溹水河谷与流域形态特征及其地貌意义

利用数字高程模型(DEM)数据、地形图、地质图数据结合实地踏勘,分析并计算了张家界地貌主要分布区溹水流域河谷形态特征和侵蚀积分值(HI),探讨了流域地貌侵蚀发育阶段特征。研究发现:溹水干流比降沿程基本上逐渐减小,纵剖面呈上凹形,凹度值约为0.70;主要支流比降明显大于干流,凹度值介于0.12～0.98;溹水干流河谷宽度和宽深比沿程向下表现出上升趋势、深度表现出下降趋势;溹水河谷横剖面凹度值上游较低,中游较高,近河口又降低,平均值约为0.63,都大于0.5;上中游主要支流河谷相对窄深,下游支流河谷相对宽浅;支流河谷横剖面的凹度值变化范围较大,平均值大于0.50。说明这一地区在地壳迅速抬升之间有较长时间地壳相对稳定,横向侵蚀形成一定宽度的河漫滩,河谷为U型谷,地貌发育处于壮年期或壮年期的中晚期。河谷侧蚀,河谷凹度值大于0.5,是张家界地貌独立峰柱得以形成的重要条件;溹水流域上游下段和中游HI值较高,下游HI值较低,最高值出现在天子山周围;没有发现地势和基岩特性对溹水流域HI值分布存在显著影响;张家界砂岩峰林地貌发育地区集中于溹水上游下段和中游上段的泥盆系砂岩分布区,这一地区HI平均值为0.46,处于地貌侵蚀旋回的壮年期。     

Cenozoic basin evolution of the Central Patagonian Andes: Evidence from geochronology,stratigraphy, and geochemistry

The Central Patagonian Andes is a particular segment of the Andean Cordillera that has been subjected to the subduction of two spreading ridges during Eocene and Neogene times. In order to understand the Cenozoic geologic evolution of the Central Patagonian Andes, we carried out geochronologic(U-Pb and⁴⁰Ar/³⁹Ar), provenance, stratigraphic, sedimentologic, and geochemical studies on the sedimentary and volcanic Cenozoic deposits that crop out in the Meseta Guadal and Chile Chico areas(～47°S). Our data indicate the presence of a nearly complete Cenozoic record, which refutes previous interpretations of a hiatus during the middle Eocene-late Oligocene in the Central Patagonian Andes. Our study suggests that the fluvial strata of the Ligorio Marquez Formation and the flood basalts of the Basaltos Inferiores de la Meseta Chile Chico Formation were deposited in an extensional setting related to the subduction of the Aluk-Farallon spreading ridge during the late Paleocene-Eocene. Geochemical data on volcanic rocks interbedded with fluvial strata of the San Jose Formation suggest that this unit was deposited in an extensional setting during the middle Eocene to late Oligocene. Progressive crustal thinning allowed the transgression of marine waters of Atlantic origin and deposition of the upper Oligocene-lower Miocene Guadal Formation. The fluvial synorogenic strata of the Santa Cruz Formation were deposited as a consequence of an important phase of compressive deformation and Andean uplift during the early-middle Miocene. Finally, alkali flood basalts of the late middle to late Miocene Basaltos Superiores de la Meseta Chile Chico Formation were extruded in the area in response to the suduction of the Chile Ridge under an extensional regime. Our studies indicate that the tectonic evolution of the Central Patagonian Andes is similar to that of the North Patagonian Andes and appears to differ from that of the Southern Patagonian Andes, which is thought to have been the subject of continuous compressive deformation since the late Early Cretaceous.     

Analysis of overdeepened valleys using the digital elevation model of the bedrock surface of Northern Switzerland

Based on surface and borehole information, together with pre-existing regional and local interpretations, a 7,150 square kilometre Raster Digital Elevation Model (DEM) of the bedrock surface of northern Switzerland was constructed using a 25 m cell size. This model represents a further important step in the understanding of Quaternary sediment distribution and is open to a broad field of application and analysis, including hydrogeological, geotechnical and geophysical studies as well as research in the field of Pleistocene landscape evolution. An analysis of the overdeepened valleys in the whole model area and, more specifically in the Reuss area, shows that, in most cases, overdeepening is restricted to the areas covered by the Last Glaciation Maximum (LGM). However, at various locations relatively narrow overdeepened valleys outreach the tongue basins and the LGM ice shield limits. Therefore, an earlier and further-reaching glacial event has probably contributed significantly to the overdeepening of these valleys. No significant overdeepening has been identified downstream of Böttstein (Aare) and Kaiserstuhl (Rhine), although the ice extended considerably further downstream, at least during the most extensive glaciation. Except for the bedrock between Brugg and Böttstein, no overdeepened valleys are found significantly north of the outcrop of Mesozoic limestone of the Folded and Tabular Jura. A detailed analysis of the Reuss area shows that the Lake and Suhre valleys are separated from the Emmen—Gisikon Reuss valley basin by a significant bedrock barrier. The individual bedrock valleys are divided into several sub-basins, indicating a multiphase evolution of the valleys. Some of the swells or barriers separating the sub-basins coincide with known late LGM retreat stages. In the Suhre valley, an old fluvial valley floor with restricted overdeepened sections is documented.     

The social impact of anthropogenic landscape modification in the Río Verde drainage basin,Oaxaca, Mexico

This article examines the effects of anthropogenic landscape modification in the upper drainage basin of the Río Verde on environments and populations of the lower Río Verde Valley, Oaxaca, Mexico. Archaeological and geomorphological research conducted by the Río Verde Formative Project indicates that Late Formative (400–100 B.C.) population growth and agricultural intensification in the Oaxaca and Nochixtlán valleys increased erosion and runoff from the Verde's upper drainage. This geomorphic change in the highland valleys altered the drainage system and led to increased flooding and alluviation in the lower Río Verde Valley. The environmental effect in the lower valley may have increased the agricultural potential of the region, leading to population growth, and at least indirectly to social change.     

Holocene valley-floor deposition and incision in a small drainage basin in western Colorado, USA

The valley floor of a 33.9 km² watershed in western Colorado experienced gradual sedimentation from before ∼ 6765 to ∼ 500 cal yr BP followed by deep incision, renewed aggradation, and secondary incision. In contrast, at least four terraces and widespread cut-and-fill architecture in the valley floor downstream indicate multiple episodes of incision and deposition occurred during the same time interval. The upper valley fill history is atypical compared to other drainages in the Colorado Plateau.One possible reason for these differences is that a bedrock canyon between the upper and lower valley prevented headward erosion from reaching the upper valley fill. Another possibility is that widespread, sand-rich, clay-poor lithologies in the upper drainage limited surface runoff and generally favored alluviation, whereas more clay-rich lithologies in the lower drainage resulted in increased surface runoff and more frequent incision. Twenty-two dates from valley fill charcoal indicate an approximate forest fire recurrence interval of several hundred years, similar to that from other studies in juniper-piñon woodlands. Results show that closely spaced vertical sampling of alluvium in headwater valleys where linkages between hillslope processes and fluvial activity are relatively direct can provide insight about the role of fires in alluvial chronologies of semi-arid watersheds.