DRIFT VOLUME IN THE SOUTHERN PENINSULA OF MICHIGAN—A PRODIGIOUS PLEISTOCENE ENDOWMENT

Michigan's Southern Peninsula is by far the largest drift repository in the Great Lakes Area. Thickness of the glacial sediments averages 85 m and volume, previously unknown, exceeds 9300 km³. The distribution, however, is very uneven, as is the underlying bedrock surface. These variations and known surface characteristics provide a basis for the first qualitative and quantitative identification of distinct drift realms. Even the smallest of the eight realms contains about 35% more drift than the better known Kettle Moraine tract of Wisconsin, which encompasses 50% more area. Comparative analysis within the peninsula shows that Pleistocene glaciation completely transformed the topography to the north, whereas the southern half has palimpsest relationships with subcropping formations. [Key words: drift, drift volume, Michigan, Great Lakes, Pleistocene.]     

Steady, balanced rates of uplift and erosion of the Santa Monica Mountains, California

Topographic change in regions of active deformation is a function of rates of uplift and denudation. The rate of topographic development and change of an actively uplifting mountain range, the Santa Monica Mountains, southern California, was assessed using landscape attributes of the present topography, uplift rates and denudation rates. Landscape features were characterized through analysis of a digital elevation model (DEM). Uplift rates at time scales ranging from 10⁴ to 10⁶ years were constrained with geological cross-sections and published estimates. Denudation rate was determined from sediment yield data from debris basins in southern California and from the relief of rivers set into geomorphic surfaces of known age. First-order morphology of the Santa Monica Mountains is set by large-scale along-strike variations in structural geometry. Drainage spacing, drainage geometry and to a lesser extent relief are controlled by bedrock strength. Dissection of the range flanks and position of the principal drainage divide are modulated by structural asymmetry and differences in structural relief across the range. Topographic and catchment-scale relief are ≈300–900 m. Mean denudation rate derived from the sediment yield data and river incision is 0.5±0.3 mm yr^?1. Uplift rate across the south flank of the range is ≈0.5±0.4 mm yr^?1 and across the north flank is 0.24±0.12 mm yr^?1. At least 1.6–2.7 Myr is required to create either the present topographic or the catchment-scale relief based on either the mean rates of denudation or uplift. Although the landscape has had sufficient time to achieve a steady-state form, comparison of the time-scale of uplift and denudation rate variation with probable landscape response times implies the present topography does not represent the steady-state form.     

月表地貌起伏形态分异特征及分级标准研究

月球地貌是月球表面发生的地质和地貌过程的结果,月球地貌单元的划分和等级分类体系的构建是月球地貌学研究的基础,也是月球地貌图制图的基础和关键科学问题。地貌学是研究形态和成因的科学,高程和起伏度是最基本的地貌指标。本文基于LOLA（Lunar Orbiter Laser Altimeter） DEM数据以及LOLA和SELENE TC（Terrain Camera）融合的DEM数据（SLDEM2015,文中简称SLDEM）,利用均值变点法确定月表起伏度计算的最佳窗口,并以起伏度100 m、200 m、300 m、700 m、1500 m及2500 m为阈值将月球表面分为微起伏平原（< 100 m）、微起伏台地[100 m, 200 m）、微小起伏丘陵[200 m, 300 m）、小起伏山地[300 m, 700 m）、中起伏山地[700 m, 1500 m）、大起伏山地[1500 m, 2500 m）及极大起伏山地（≥ 2500 m）地貌7个类型。划分结果显示：微起伏平原主要分布在月海平原区域、部分有玄武岩充填的撞击盆地的盆底区域以及撞击坑坑底区域;微起伏台地主要分布在月海和月陆区域的交界区域;微小起伏丘陵主要分布在月溪和皱脊等构造单元区域;小起伏山地主要分布在撞击坑中央峰及坑底断裂区域;中起伏山地主要分布在撞击坑坑底和坑壁过渡区域、撞击坑坑壁和坑缘过渡区域、撞击盆地盆底与盆壁过渡区域以及盆壁与盆缘过渡区域;大起伏和极大起伏山地主要分布在撞击坑坑壁区域及撞击盆地盆壁区域。本文确定的月表起伏度分级标准可以对月表数字地貌分类体系的构建和月球地貌图集的编研提供定量标准和重要参考。     

The statistical relationship between unconfined compressive strengths and the frequency distributions of slope gradients — A case study in northern Hungary

This paper focuses on the question of whether there is a deterministic connection between the slope gradient and unconfined compression strength (UCS) as a lithological factor and on describing the nature of this connection. Moreover, the authors determined the sensitive statistical parameters in the statistical surface analyses. The surface analyses were carried out in an area extending over 1500 km² in northern Hungary and containing parts of the uplifted Palaeozoic and Mesozoic basement and the semi-consolidated material of the Palaeogene and Neogene molasse sediments.The 67 geological formations of the area were grouped into 10 petrophysical categories characterised by unconfined compressive strength as a petrophysical parameter. The geological database was the digitalized geological map of North Hungarian Region (1 : 100 000). The digital topographic database was based on 10 m contour lines of 1 : 50 000 maps, the digital elevation model was generated by kriging interpolation. Three topographic models were created with resolutions of 25 × 25, 50 × 50 and 100 × 100 m per pixels.Evident correlation can be shown between the UCS and the relative frequency of the slope gradient. The adequate regression procedure is the power regression for low slope category values while logarithmic regression is applicable at high slope angles. Based on the characteristic of the relationship and the value of r², slope category intervals can be identified the relative frequency of which is proven to be determined by the UCS. These intervals are found to be between 4–10%, 10–16%, 16–22%, 22–44% and over 44%.Using the determination equations of slope gradient between 4–10% and over 44%, the UCS of the bedrock can be calculated approximately as the average value of the two calculated results. So the quotient of the frequency of these two category intervals can be regarded as an important morphometric index for a given bedrock.From the aspect of petrophysical characteristics, the rock with UCS between 6 and 86 MPa were proven to be deterministic for the slope development i.e. in the determination of the slope category frequencies. Applying the relative frequency of slope gradients a relative erosion resistance of the petrophysical categories can be calculated.Considering the determination coefficients, among the statistical parameters of the distribution of slope category values the standard deviation, mode, mean and median proved to be determined by the UCS of the bedrock.     

Morphological differentiation characteristics and classification criteria of lunar surface relief amplitude

Lunar landforms are the results of geological and geomorphic processes on the lunar surface. It is very important to identify the types of lunar landforms. Geomorphology is the scientific study of the origin and evolution of morphological landforms on planetary surfaces. Elevation and relief amplitude are the most commonly used geomorphic indices in geomorphological classification studies. Previous studies have determined the elevation classification criteria of the lunar surface. In this paper,...     

Geomorphic inferences from regolith thickness, chemical denudation and CRN erosion rates near the glacial limit, Boulder Creek catchment and vicinity, Colorado

Granitic regolith, developed in the Boulder Creek catchment and adjacent areas, records a history of deep weathering, some of which may predate Quaternary time. Field and well-log measurements of weathering, chemical denudation and rates of erosion derived from ¹⁰Be cosmogenic radionuclide (CRN) data help to quantify rates of landscape change in the post-orogenic Rocky Mountains. The density of oxidized, fractured bedrock ranges from 2.7 to about 2.2 g cm^− 3, saprolite and grus have densities between 2.0 and 1.8 g cm^− 3, and 30 soil samples averaged 1.6 ± 0.2 g cm^− 3. Highly weathered regolith in 540 wells averages 3.3 m thick, mean depth to bedrock in 1661 wells is 7 m, and the weathered thickness exceeds 10 m in relatively large local areas east of the late Pleistocene glacial limit. Thickness of regolith shows no simple relationship to rock type or structure, local slope, or distance from channels. Catchments in the vicinity of the Boulder Creek have an average CRN erosion rate of 2.2 ± 0.7 cm kyr^− 1 for the past 10,000 to 40,000 yr. Annual losses of cations and SiO₂ vary from about 2 to 5 g m^− 2 over a runoff range of 10 to nearly 160 cm.Using measured rates in simple box models shows that if a substantial fraction of void space is created by volume expansion in the weathering rock materials, 7 m of weathered rock materials could form in as little as 230 kyr. If density loss results mainly from chemical denudation and some volume expansion, however, the same weathering profile would take > 1340 kyr to form. Rates of erosion measured by CRN could be balanced by the rate of soil formation from saprolite if the annual solute loss from soil is 2.0 g m^− 2 and 70% of the density decrease from saprolite to grus and soil results from strain. Saprolite, however, forms from oxidized bedrock at a far slower rate and rates of saprolite formation cannot balance soil and grus losses to erosion. The zone of thick weathered regolith is likely an eroding relict landscape. The undulating surface marked by relatively low relief and tors is not literally a topographic surface of Eocene, Oligocene or Miocene age unless it was covered with deposits that were removed in Pliocene or Quaternary time.     

Spatial distribution of sediment storage types and quantification of valley fill deposits in an alpine basin, Reintal, Bavarian Alps, Germany

Spatial patterns of sediment storage types and associated volumes using a novel approach for quantifying valley fill deposits are presented for a small alpine catchment (17 km²) in the Bavarian Alps. The different sediment storage types were analysed with respect to geomorphic coupling and sediment flux activity. The most landforms in the valley in terms of surface area were found to be talus slopes (sheets and cones) followed by rockfall deposits and alluvial fans and plains. More than two-thirds of the talus slopes are relict landforms, completely decoupled from the geomorphic system. Notable sediment transport is limited to avalanche tracks, debris flows, and along floodplains. Sediment volumes were calculated using a combination of polynomial functions of cross sections, seismic refraction, and GIS modelling. A total of, 66 seismic refraction profiles were carried out throughout the valley for a more precise determination of sediment thicknesses and to check the bedrock data generated from geomorphometric analysis. We calculated the overall sediment volume of the valley fill deposits to be 0.07 km³. This corresponds to a mean sediment thickness of 23.3 m. The seismic refraction data showed that large floodplains and sedimentation areas, which have been developed through damming effects from large rockfalls, are in general characterised by shallow sediment thicknesses (<20 m). By contrast, the thickness of several talus slopes is more than twice as much. For some locations (e.g., narrow sections of valley), the polynomial-generated cross sections resulted in overestimations of up to one order of magnitude; whereas in sections with a moderate valley shape, the modelled cross sections are in good accordance with the obtained seismic data. For the quantification of valley fill deposits, a combined application of bedrock data derived from polynomials and geophysical prospecting is highly recommended.     

Quaternary bedrock erosion and landscape evolution in the Sør Rondane Mountains, East Antarctica: Reevaluating rates and processes

Rates and processes of rock weathering, soil formation, and mountain erosion during the Quaternary were evaluated in an inland Antarctic cold desert. The fieldwork involved investigations of weathering features and soil profiles for different stages after deglaciation. Laboratory analyses addressed chemistry of rock coatings and soils, as well as ¹⁰Be and ²⁶Al exposure ages of the bedrock. Less resistant gneiss bedrock exposed over 1 Ma shows stone pavements underlain by in situ produced silty soils thinner than 40 cm and rich in sulfates, which reflect the active layer thickness, the absence of cryoturbation, and the predominance of salt weathering. During the same exposure period, more resistant granite bedrock has undergone long-lasting cavernous weathering that produces rootless mushroom-like boulders with a strongly Fe-oxidized coating. The red coating protects the upper surface from weathering while very slow microcracking progresses by the growth of sulfates. Geomorphological evidence and cosmogenic exposure ages combine to provide contrasting average erosion rates. No erosion during the Quaternary is suggested by a striated roche moutonnée exposed more than 2 Ma ago. Differential erosion between granite and gneiss suggests a significant lowering rate of desert pavements in excess of 10 m Ma^− 1. The landscape has been (on the whole) stable, but the erosion rate varies spatially according to microclimate, geology, and surface composition.     

Regional relief characteristics and denudation pattern of the western Southern Alps, New Zealand

The Southern Alps of New Zealand are the topographic expression of active oblique continental convergence of the Australian and Pacific plates. Despite inferred high rates of tectonic and climatic forcing, the pattern of differential uplift and erosion remains uncertain. We use a 25-m DEM to conduct a regional-scale relief analysis of a 250-km long strip of the western Southern Alps (WSA). We present a preliminary map of regional erosion and denudation by overlaying mean basin relief, a modelled stream-power erosion index, river incision rates, historic landslide denudation rates, and landslide density. The interplay between strong tectonic and climatic forcing has led to relief production that locally attains 2 km in major catchments, with mean values of 0.65–0.68 km. Interpolation between elevations of major catchment divides indicates potential removal of l0¹–10³ km³, or a mean basin relief of 0.51–0.85 km in the larger catchments. Local relief and inferred river incision rates into bedrock are highest about 50–67% of the distance between the Alpine fault and the main divide. The mean regional relief variability is ± 0.5 km.Local relief, valley cross-sectional area, and catchment width correlate moderately with catchment area, and also reach maximum values between the range front and the divide. Hypsometric integrals show scale dependence, and together with hypsometric curves, are insufficient to clearly differentiate between glacial and fluvial dominated basins. Mean slope angle in the WSA (ψ = 30°) is lower where major longitudinal valleys and extensive ice cover occur, and may be an insensitive measure of regional relief. Modal slope angle is strikingly uniform throughout the WSA (φ = 38–40°), and may record adjustment to runoff and landsliding. Both ψ and φ show non-linear relationships with elevation, which we attribute to dominant geomorphic process domains, such as fluvial processes in low-altitude valley trains, surface runoff and frequent landsliding on montane hillslopes, “relief dampening” by glaciers, and rock fall/avalanching on steep main-divide slopes.     

Landforms and landscape evolution in the Mylliem Granite Area,Meghalaya Plateau,Northeast India

The Mylliem granite is one of many igneous bodies within the basement complex of the Meghalaya Plateau, northeast India. Although relatively small in size at c. 90 km², it is very diverse geomorphologically and shows a range of distinctive landscapes within its limits. Relict flat watershed ridges and topographic basins characterize the northern and eastern part of the pluton, whereas to the southwest the relief becomes higher, with steeper hillslopes and deeply incised valleys. Deep weathering and thick saprolites are abundant, as are residual landforms resulting from stripping of the saprolite: domes, tors and boulders. The major reason behind the diversity of granite landscape of the Mylliem pluton is the progress of headward erosion, initiated at the Dauki fault in the south of the Meghalaya. Headward erosion enhances local relief and hence, weathering systems. Multi‐concave morphology is gradually transformed into multi‐convex one, which is hypothesized to be the specific mode of plateau evolution and scarp retreat in granite bedrock.     

三江源地区冰雹灾害分布特征及其成因

青藏高原腹地的雹灾形成与其它地区有一定的差别。根据三江源地区126个乡整理的近60 a的雹灾数据,分析了青海南部高原雹灾的空间和时间变化规律,并通过对雹灾次数与海拔高程、地形起伏度、居民点密度的相关分析,结果表明：三江源地区雹灾的高发区域为东北部的兴海、同德、贵南等地及南部的称多、玉树、囊谦等2个地区,雹灾主要发生在5～9月份,在6、8月份形成明显的双峰性特点,地形起伏度、居民点密度与雹灾频次具有明显的正相关,海拔高程与雹灾频次呈负相关,雨带推移和生产方式不同是三江源地区雹灾产生空间分异的主要原因。     

Validation of WRF model on simulating forcing data for Heihe River Basin

The research of coupling WRF (Weather Research and Forecasting Model) with a land surface model is enhanced to explore the interaction of the atmosphere and land surface; however, regional applicability of WRF model is questioned. In order to do the validation of WRF model on simulating forcing data for the Heihe River Basin, daily meteorological observation data from 15 stations of CMA (China Meteorological Administration) and hourly meteorological observation data from seven sites of WATER (Watershed Airborne Telemetry Experimental Research) are used to compare with WRF simulations, with a time range of a whole year for 2008. Results show that the average MBE (Mean Bias Error) of daily 2-m surface temperature, surface pressure, 2-m relative humidity and 10-m wind speed were ?0.19 °C, ?4.49 hPa, 4.08% and 0.92 m/s, the average RMSE (Root Mean Square Error) of them were 2.11 °C, 5.37 hPa, 9.55% and 1.73 m/s, and the average R (correlation coefficient) of them were 0.99, 0.98, 0.80 and 0.55, respectively. The average MBE of hourly 2-m surface temperature, surface pressure, 2-m relative humidity, 10-m wind speed, downward shortwave radiation and downward longwave were ?0.16 °C, ?6.62 hPa, ?5.14%, 0.26 m/s, 33.0 W/m² and ?6.44 W/m², the average RMSE of them were 2.62 °C, 17.10 hPa, 20.71%, 2.46 m/s, 152.9 W/m² and 53.5 W/m², and the average R of them were 0.96, 0.97, 0.70, 0.26, 0.91 and 0.60, respectively. Thus, the following conclusions were obtained: (1) regardless of daily or hourly validation, WRF model simulations of 2-m surface temperature, surface pressure and relative humidity are more reliable, especially for 2-m surface air temperature and surface pressure, the values of MBE were small and R were more than 0.96; (2) the WRF simulating downward shortwave radiation was relatively good, the average R between WRF simulation and hourly observation data was above 0.9, and the average R of downward longwave radiation was 0.6; (3) both wind speed and rainfall simulated from WRF model did not agree well with observation data.     

基于GIS的土壤全氮空间分布估算——以江西省兴国县为例

运用GIS的空间分析技术和DEM,在区域范围内,可以表征基于母岩和地形因子的土壤-景观模型。本次研究根据兴国县151个样点数据,分析TN和地形因子的相关关系,建立回归模型,进行估算。结果表明:表层土壤中TN含量平均值为1.06g/kg,千枚岩发育的土壤中TN的平均含量最高,为1.35g/kg ;砂页岩发育的土壤中TN的平均含量最低,为0.88g/kg 。空间分布上:TN含量在0.5g/kg~1.0g/kg的面积最大,为1580km²,TN含量在2.0g/kg以上的面积最小,为127km²。地形变量中坡向对TN含量影响最大,TN含量和母岩、海拔、坡向存在着正相关关系,坡度和TN含量的相关关系不明显。利用回归分析模型和DEM(30m×30m),估算TN的空间分布,R²为0.637。     

New constraints on the late Cenozoic incision history of the New River, Virginia

The New River crosses three physiogeologic provinces of the ancient, tectonically quiescent Appalachian orogen and is ideally situated to record variability in fluvial erosion rates over the late Cenozoic. Active erosion features on resistant bedrock that floors the river at prominent knickpoints demonstrate that the river is currently incising toward base level. However, thick sequences of alluvial fill and fluvial terraces cut into this fill record an incision history for the river that includes several periods of stalled downcutting and aggradation. We used cosmogenic ¹⁰Be exposure dating, aided by mapping and sedimentological examination of terrace deposits, to constrain the timing of events in this history. ¹⁰Be concentration depth profiles were used to help account for variables such as cosmogenic inheritance and terrace bioturbation. Fill-cut and strath terraces at elevations 10, 20, and 50 m above the modern river yield model cosmogenic exposure ages of 130, 600, and 600–950 ka, respectively, but uncertainties on these ages are not well constrained. These results provide the first direct constraint on the history of alluvial aggradation and incision events recorded by New River terrace deposits. The exposure ages yield a long-term average incision rate of 43 m/my, which is comparable to rates measured elsewhere in the Appalachians. During specific intervals over the last 1 Ma, however, the New River's incision rate reached 100 m/my. Modern erosion rates on bedrock at a prominent knickpoint are between 28 and 87 m/my, in good agreement with rates calculated between terrace abandonment events and significantly faster than 2 m/my rates of surface erosion from ancient terrace remnants. Fluctuations between aggradation and rapid incision operate on timescales of 10⁴− 10⁵ year, similar to those of late Cenozoic climate variations, though uncertainties in model ages preclude direct correlation of these fluctuations to specific climate change events. These second-order fluctuations appear within a longer-term signal of dominant aggradation (until 2 Ma) followed by dominant incision. A similar signal is observed on other Appalachian rivers and may be the result of sediment supply fluctuations driven by the increased frequency of climate changes in the late Cenozoic.     

Columbia Mountain landslide: late-glacial emplacement and indications of future failure, Northwestern Montana, USA

The well preserved and undissected Columbia Mountain landslide, which is undergoing suburban development, was studied to estimate the timing and processes of emplacement. The landslide moved westward from a bedrock interfluve of the northern Swan Range in Montana, USA onto the deglaciated floor of the Flathead Valley. The landslide covers an area of about 2 km², has a toe-to-crown height of 1100 m, a total length of 3430 m, a thickness of between 3 and 75 m, and an approximate volume of 40 million m³. Deposits and landforms define three portions of the landslide; from the toe to the head they are: (i) clast-rich diamictons made up of gravel-sized angular rock fragments with arcuate transverse ridges at the surface; (ii) silty and sandy deposits resting on diamictons in an internally drained depression behind the ridges; and (iii) diamictons containing angular and subangular pebble-to block-sized clasts (some of which are glacially striated) in an area of lumpy topography between the depression and the head of the landslide. Drilling data suggest the diamictons cover block-to-slab-sized bedrock clasts that resulted from an initial stage of the failure.The landslide moved along a surface that developed at a high angle to the NE-dipping, thinly bedded metasediments of the Proterozoic Belt Supergroup. The exposed slope of the main scarp dips 30–37°W. A hypothetical initial rotational failure of the lower part of a bedrock interfluve may have transported bedrock clasts into the valley. The morphology and deposits at the surface of the landslide indicate deposition by a rock avalanche (sturzstrom) derived from a second stage of failure along the upper part of the scarp.The toe of the Columbia Mountain landslide is convex-west in planview, except where it was deflected around areas now occupied by glacial kettles on the north and south margins. Landsliding, therefore, occurred during deglaciation of the valley while ice still filled the present-day kettles. Available chronostratigraphy suggests that the ˜1-km thick glacier in the region melted before 12,000 ¹⁴C years BP—within 3000 years of the last glacial maximum. Deglaciation and hillslope failure are likely causally linked. Failure of the faceted interfluve was likely due tensile fracturing of bedrock along a bedding-normal joint set shortly after glacial retreat from the hillslope.Open surficial tension fractures and grabens in the Swan Range are limited to an area above the crown of the landslide. Movement across these features suggests that extensional flow of bedrock (sackung) is occurring in what remains of the ridge that failed in the Columbia Mountain landslide. The fractures and grabens likely were initiated during failure, but their morphologies suggest active extension across some grabens. Continued movement of bedrock above the crown may result in future mass movements from above the previous landslide scarp. Landslides sourced from bedrock above the scarp of the late-glacial Columbia Mountain landslide, which could potentially be triggered by earthquakes, are geologic hazards in the region.     

The Jiloca karst polje-tectonic graben (Iberian Range, NE Spain)

The Jiloca depression, one of the largest morpho-structural units of the Iberian Range and traditionally considered as a neotectonic graben, is interpreted as a karst polje developed within an active halfgraben. This polje, 705 km² in area, constitutes one of the largest documented poljes. Several evidences—(1) a sequence of eight-stepped levels of corrosion surfaces, (2) the reduced thickness of the basin fill, (3) fault-controlled mountain fronts with topographic scarps much higher than the structural throws—demonstrate that great part of the topographic relief of the depression has been generated by corrosional lowering rather than by tectonic subsidence. The height difference between the highest corrosion surface and the polje bottom indicate that the depression has been deepened around 300 m by corrosion processes. The initiation of the karst polje was determined by the creation of the Jiloca halfgraben by normal faults, which deformed a Pliocene regional erosion surface. The development of the polje has been controlled largely by the asymmetric structure and the slight neotectonic activity of the graben. Changes in the position of the polje bottom inferred from the slopes of the different corrosion surfaces (polje paleotopography) may have been controlled by neotectonic movements.     

The Valley of Chin lee

Abstract

This paper is concerned with an instructional design for the teaching of landform geography. A sequence of organizational problems is examined and suggestions made from a previous study of the southern peninsula of Michigan. Considerations of grid design, selection of terrain parameters, and the problem of regional definitions are assessed from both the historic point of view and the results of the pilot study. Of three basic landform parameters, i.e., average elevations, local relief, and average slope, the single criterion of elevation is advocated for the detection of regional homogeniety over large topographic surfaces. However, the use of local relief as a measure of surface texture is proposed for the delineation of individual terrain compartments. Suggested for all age levels above the elementary school, this project is a feasible design for an oft-neglected aspect of physical geography.     

Landscape disequilibrium on 1000–10,000 year scales Marsyandi River, Nepal, central Himalaya

In an actively deforming orogen, maintenance of a topographic steady state requires that hillslope erosion, river incision, and rock uplift rates are balanced over timescales of 10⁵–10⁷ years. Over shorter times, <10⁵ years, hillslope erosion and bedrock river incision rates fluctuate with changes in climate. On 10⁴-year timescales, the Marsyandi River in the central Nepal Himalaya has oscillated between bedrock incision and valley alluviation in response to changes in monsoon intensity and sediment flux. Stratigraphy and ¹⁴C ages of fill terrace deposits reveal a major alluviation, coincident with a monsoonal maximum, ca. 50–35 ky BP. Cosmogenic ¹⁰Be and ²⁶Al exposure ages define an alluviation and reincision event ca. 9–6 ky BP, also at a time of strong South Asian monsoons. The terrace deposits that line the Lesser Himalayan channel are largely composed of debris flows which originate in the Greater Himalayan rocks up to 40 km away. The terrace sequences contain many cubic kilometers of sediment, but probably represent only 2–8% of the sediments which flushed through the Marsyandi during the accumulation period. At 10⁴-year timescales, maximum bedrock incision rates are 7 mm/year in the Greater Himalaya and 1.5 mm/year in the Lesser Himalayan Mahabarat Range. We propose a model in which river channel erosion is temporally out-of-phase with hillslope erosion. Increased monsoonal precipitation causes an increase in hillslope-derived sediment that overwhelms the transport capacity of the river. The resulting aggradation protects the bedrock channel from erosion, allowing the river gradient to steepen as rock uplift continues. When the alluvium is later removed and the bedrock channel re-exposed, bedrock incision rates probably accelerate beyond the long-term mean as the river gradient adjusts downward toward a more “equilibrium” profile. Efforts to document dynamic equilibrium in active orogens require quantification of rates over time intervals significantly exceeding the scale of these millennial fluctuations in rate.     

Determination of ice thickness, subice topography and ice vol-ume at Glacier No. 1 in Tien Shan, China by ground penetrating radar

We describe a radio-echo sounding (RES) survey for the determination of ice thickness, subglacial topography and ice volume of Glacier No. 1 , in Tien Shan, China, using ground-penetrating radar (GPR). Radar data were collected with 100-MHz antennas that were spaced at 4 m with a step size of 8 m. The images produced from radar survey clearly show the continuity of bedrock echoes and the undulation features of the bedrock surface. Radar results show that the maximum ice thickness of Glacier No. 1 is 133 m, the thickness of the east branch of Glacier No. 1 averages at 58. 77 m while that of the west branch of Glacier No. 1 averages at 44. 84 m. Calculation on ice volume indicates that the ice volume of the east branch of Glacier No. 1 is 51. 87 × 106 m3 and that of the west branch of Glacier No. 1 is 20. 21 × 106 m3. The amplitude of the undulation of the bedrock surface topography revealed by radar profiles is larger than that of the glacier surface topography, indicating that the surface relief does     

Movement of a small slipfaceless dome dune in the Namib Sand Sea, Namibia

The migration of a small slipfaceless dome dune close to the northern edge of the Namib Sand Sea has been measured by topographic survey. The dune dimensions are 45-m wide and 1-m high with a volume of 551 m³ that has been calculated as the difference between the dune's surface elevation and an interdune surface extrapolated from measurements around the edge of the dune. The migration direction, 64°, and distance moved, 90 m, are measured against stakes set out in 1976. The dune has moved about 90 m between 1976 and 1999. This is an average linear migration rate of around 4 m year⁻¹, and is equivalent to an annual sand transport rate of about 1.2 tonnes m⁻¹ year⁻¹. The calculated total potential sand flow in this part of the Namib Desert is 119 tonnes m⁻¹ year⁻¹, and the resultant potential sand flow is 63 tonnes m⁻¹ year⁻¹. The dune migration is therefore about 1% of the total potential sandflow and 2% of the resultant indicating that dune migration is only a small part of total potential sand transport. The results suggest that small slipfaceless dome dunes are very inefficient at trapping sand, and that winds blowing across the interdune in this area are undersaturated with sand.