Bedrock Configuration Research Articles

Accuracy and Feasibility of Bedrock Configuration Utilizing Mazor X Navigation System

<h3>Research Objectives</h3> To prove accuracy and feasibility of utilizing navigation for implantation of S2AI screws and triangular titanium rods in the bedrock configuration during open sacro-pelvic fixation in multilevel spinal fusion for adult spinal deformity. <h3>Design</h3> Retrospective chart review. <h3>Setting</h3> Single center, ambulatory care. <h3>Participants</h3> 18 adult multilevel spinal deformity patients. <h3>Interventions</h3> Reviewed consecutive cases where bilateral S2AI and TTRs were placed in an open approach from 10/2019-9/2020. Patients had preop CT scan for implant planning utilizing Mazor X. S2AI screws were planned into the caudal pelvic teardrop with TTRs placed 1cm proximal and cephalad. <h3>Main Outcome Measures</h3> Preop implant trajectory vs. postop implant placement. <h3>Results</h3> There was no significant difference for the caudal angle in the sagittal plane between pre- and post-operative TTRs and S2AI screws (31.33° ± 3.87° vs. 30.27° ± 3.40°, P = 0.24 and 39.34° ± 2.67° vs 39.32° ± 2.92°, P = 0.98, respectively). For the horizontal angle in the coronal plane there were no significant differences between the pre- and post-operative measurements for the TTRs placed on either the left or right side (38.44° ± 2.49° vs. 40.28° ± 2.88°, P = 0.09 and 40.03° ± 2.45° vs. 38.83° ± 1.85°, P = 0.23; respectively) or for the S2AI screws (36.92° ± 2.76° vs. 38.65° ± 3.19°, P = 0.15 and 39.06° ± 2.35° vs. 38.48° ± 2.24°, P = 0.62; respectively). There were no intraoperative complications and no implants had to be re-positioned. <h3>Conclusions</h3> Placing concomitant bilateral S2AI screws and TTRs in an open approach during multilevel deformity surgery is both feasible and accurate using the Mazor X navigation system. <h3>Author(s) Disclosures</h3> None.

Exploring the role of bedrock representation on plant transpiration response during dry periods at four forested sites in Europe

Abstract. Forest transpiration is controlled by the atmospheric water demand, potentially constrained by soil moisture availability, and regulated by plant physiological properties. During summer periods, soil moisture availability at sites with thin soils can be limited, forcing the plants to access moisture stored in the weathered bedrock. Land surface models (LSMs) have considerably evolved in the description of the physical processes related to vegetation water use, but the effects of bedrock position and water uptake from fractured bedrock have not received much attention. In this study, the Community Land Model version 5.0 (CLM 5) is implemented at four forested sites with relatively shallow bedrock and located across an environmental gradient in Europe. Three different bedrock configurations (i.e., default, deeper, and fractured) are applied to evaluate if the omission of water uptake from weathered bedrock could explain some model deficiencies with respect to the simulation of seasonal transpiration patterns. Sap flow measurements are used to benchmark the response of these three bedrock configurations. It was found that the simulated transpiration response of the default model configuration is strongly limited by soil moisture availability at sites with extended dry seasons. Under these climate conditions, the implementation of an alternative (i.e., deeper and fractured) bedrock configuration resulted in a better agreement between modeled and measured transpiration. At the site with a continental climate, the default model configuration accurately reproduced the magnitude and temporal patterns of the measured transpiration. The implementation of the alternative bedrock configurations at this site provided more realistic water potentials in plant tissues but negatively affected the modeled transpiration during the summer period. Finally, all three bedrock configurations did not show differences in terms of water potentials, fluxes, and performances on the more northern and colder site exhibiting a transition between oceanic and continental climate. Model performances at this site are low, with a clear overestimation of transpiration compared to sap flow data. The results of this study call for increased efforts into better representing lithological controls on plant water uptake in LSMs.

Timing, rates and geomorphic controls on holocene loess and aeolian sand deposition using multiple chronometric methods, southeastern Cordillera, Alberta, Canada

Glacier-fed river systems represent potentially significant sediment sources for aeolian processes owing to sediment production, flow seasonality, and winds within mountain settings. We investigate the timing and rates of Holocene loess and aeolian sand deposition in three major river valleys (Bow, North Saskatchewan and Athabasca) of the southeastern Canadian Cordillera through aeolian stratigraphy and dating of sedimentary materials. Source sediments are glacially-derived, seasonally-exposed alluvial deposits. Variation in river morphology promotes source-sediment deposition influenced by bedrock configuration, tributary stream deposition and alluvial fans. Two main settings for aeolian deposition are source-proximal valley-bottom sites and more distal lower-valley sites in sub-alpine and montane forest ecozones. Dating is well-constrained by radiocarbon ages and tephrochronology. Quartz optical dating is complicated, likely owing to minimal recycling of grains and quartz provenance. Mass accumulation rates (MARs) indicate that aeolian deposition was initiated soon after deglaciation and has occurred nearly continuously for at least 10000 years. Loess MARs ranged between 100 and 2500 g·m−2·a−1 during the Holocene. Rates in the last 3000 years are generally two to three times higher than those observed for earlier periods, and are highest in most recent times. We attribute Holocene accumulation at these sites in general to fluvial source sediment availability and the efficacy of forested valleys as sediment traps. The late Holocene increase in MARs is primarily attributed to increased glacial silt and alluvial sediment supply during neoglacial fluctuations including the Little Ice Age.

Evolution assessment of structurally-controlled differential subsidence using SBAS and PS interferometry in an emblematic case in Central Mexico

ABSTRACT The city of Morelia has been affected by Structurally-Controlled Differential Subsidence (SCDS) since at least 1983, modifying civil structures and infrastructure through the appearance of ground failures and differential ground subsidence. The resulting damage to streets, homes, hydraulic lines, and government facilities has caused economic losses worth millions of dollars. An increase in population has led to the overexploitation of groundwater, which is the main trigger for SCDS, while the subsoil's geology and bedrock configuration are considered conditioning factors of this phenomenon. This paper offers an analysis of the SCDS evolution over the last fourteen years and its relationship with groundwater extraction and thickness of unconsolidated sediments. The former was carried out through SAR Interferometry techniques to detect and monitor land subsidence, using Small Baseline Subset (SBAS) from 2003 to 2010 and Persistent Scatterer Interferometry (PSI) from 2014 to 2017. Additionally, groundwater pumping well data and lithological information from boreholes enabled a spatial analysis to evaluate the role of these factors in the development and acceleration of SCDS. The SBAS results show maximum sinking rates of 2.2 cm/yr. with a spatial distribution that is clearly controlled by buried geological structures. In this period, the maximum sinking rates were induced by high groundwater extraction rates in focused areas, while intermediate rates were linked to notable depletion cones. The PSI results indicate a maximum sinking rate of 2.74 cm/yr. and an accelerated subsidence migration to the west, owing to the construction of new wells and persistent high groundwater extraction rates. For both periods, the larger compressible sediment package reveals a good correlation with high subsidence rates, mainly in the hanging wall blocks of the buried faults. The identification of differential subsidence accompanied by high sinking rates in novel areas of Morelia, as well as the detection of the subsidence migration, will provide scientific support to decision-makers for the proposal of engineering solutions and urban planning capable of reducing hydrogeological hazards associated with SCDS.

Influence of the structural framework on peat bog distribution in the tropical highlands of Minas Gerais, Brazil

Peat bogs are ecosystems characterized by high levels of organic matter (OM), which accumulates in environments of low biological activity. The peat bogs present in the tropical highlands of Serra do Espinhaço Meridional (SdEM), Brazil, are associated with hydromorphic environments formed under specific conditions and the strong influence of the quartzite basement, whereby a complex pattern of faults, fractures and folds has deeply affected the drainage network and geomorphological framework. This study investigated the influence of the structural framework on the distribution and morphology of these ecosystems within a hydrological and geomorphological context. The physical features of the landscape were analyzed using geological maps, a digital elevation model and by mapping the main geomorphological feature patterns obtained from geographic information systems (GIS). Ground Penetrating Radar transects were taken in representative areas, as the best strategy for detection of buried peat bogs, as well as to analyze bedrock configuration and infer its relationship with water flow. Four peat bog typologies were identified: “entrenched bogs”, “subsurface bogs”, “structural bogs”, and “hanging bogs”. The distribution and morphology of these typologies display a strong relationship either with alluvial sediments and floodplains or with structural/lithological features which caused water retention.

Sedimentology and preservation of aeolian sediments on steep terrains: Incipient sand ramps on the Atacama coast (northern Chile)

The origin of topographically controlled aeolian landforms in high-relief settings is difficult to synthesize under general models, given the dependence of such accumulations on local morphology. Quaternary sand ramps have been linked to palaeoclimate, regional geomorphology and wind patterns; however, controls on the early development and preservation of such landforms are poorly known. This study describes the morphology and sedimentology of complex sedimentary aprons along steep coastal slopes in the Atacama Desert (Chile). Direct slope accessibility and continuous stratigraphic exposures enable comparisons between active processes and stratigraphic signatures. Stratigraphic facies distribution and its links with patterns of aeolian deposition show that the preservation of wind-laid sediments depends on the morphology and processes of specific slope sectors. The spatial organization of runoff depends on bedrock configuration and directly controls the permanence or erosion of aeolian sediment. The occurrence of either water or mass flows depends on the role of aeolian fines in the rheology of flash floods. In turn, the establishment of a rugged surface topography controlled by patterns of mass-flow deposition creates local accommodation for aeolian fines, sustaining the initial aggradation of a colluvial-aeolian system. By contrast, slopes subject to runoff develop a thin, extensive aeolian mantle whose featureless surface is subject mostly to sediment bypass down- and across-slope; the corresponding stratigraphic record comprises almost exclusively thin debris-flow and sheetflood deposits. Slope morphology and processes are fundamental in promoting or inhibiting aeolian aggradation in mountain settings. Long-term sand-ramp construction depends on climate and regional topography, but the initial development is probably controlled by local geomorphic factors. The observed interactions between wind and topography in the study area may also represent a process analogue for the interpretation of similar geomorphic features on Mars.

Investigating Ancient Maya Agricultural Adaptation Through Ground Penetrating Radar (GPR) Analysis of Karst Terrain, Northern Yucatán, Mexico

Landscape adaptation on the Northern Yucatán Peninsula, Mexico, is particularly difficult, as soils are thin and the terrain is devoid of any surface water other than the occasional sinkhole (cenote) that connects directly to the groundwater system. Despite this, ancient Maya cities, including Xuenkal, emerged and thrived, likely because of their proximity to natural sinkholes. In the case of Xuenkal, these sinkholes, known locally as rejolladas, have bases above the local water table and, as such, do not provide direct access to the underlying water, but they provide closer access. Recognizing that the presence of rejolladas was likely important to the ancient Maya the purpose of this study is to characterize the rejolladas in terms of their subsurface characteristics, specifically bedrock configuration and soil. Ground penetrating radar analysis, as well as the results of a test pit excavation, confirm the presence of deep soils in the rejollada bases. It seems that the smaller deeper rejolladas have the thickest soils and sediment. The ancient city of Xuenkal is constructed amidst a particularly dense cluster of rejolladas which may have contributed to its location. Rejolladas, containing significantly thicker soils than the surrounding karst surface, and the ability to sustain dense healthy vegetation would have been particularly desirable for the Maya to capitalize on.

Geomorphic and Sedimentologic Heterogeneity Along a Holocene Shelf Margin: Caicos Platform

Abstract Reef-rimmed margins on isolated carbonate platforms are heterogeneous, and studying Holocene platforms provides a valuable means for characterizing the spectrum of facies variability and understanding the processes behind facies changes. In this study, insights from petrographic data, granulometric measurements, bottom observations, and remote sensing and subbottom profiler data from the northwestern part of Caicos Platform make this one of the more intensely studied shallow-marine carbonate systems in the Caribbean. Collectively, analyses of the thicknesses and heterogeneity of shallow-water Holocene carbonates and the platform margin reveal details of the considerable geomorphic variability present on this part of the margin, on which the orientation changes markedly. In this area, the shelf margin arcs from northwest- to north- to northeast-facing, accompanied by changes in morphology as well as the distribution, thickness and type of sediments. The NW-facing margin has discontinuous reefs near the margin, flanked platformward by coarse sands in poorly developed aprons that transition to a deeper rocky bottom (interpreted as Pleistocene) with patches of sand to fine gravel sized sediment. Locally, lobate-to-arcuate shoals with medium sands ( 500 μm mean size) form a reef apron that is continuous along strike and over 1000 m wide. Because the interpreted top-Pleistocene surface is gently dipping seaward, Holocene sediment thicknesses vary with changes in bathymetry and generally thin away from the reefs. Sediments in most areas are between 1.5 and 3 m thick but can reach over 6 m locally. Although the interpretation of the base of the Holocene can be ambiguous under shelf-margin reefs, subbottom profiles illustrate that backreef shelf patch reefs do not necessarily nucleate on Pleistocene bedrock highs. Facies trends are interpreted to reflect a combination of physical oceanographic, sedimentologic, and biologic processes acting within the framework of Pleistocene bedrock configuration. The NE-facing margin, with the continuous reef and expansive sand apron, is interpreted to be more wave-dominated, influenced by swells from the open Atlantic, and by locally derived wind waves. Comparison with Bahamian platforms suggests that this trend is not unique; margins facing open Atlantic swells from the north–northeast have the best-developed reefs, not east-facing (windward) margins. In contrast, the beaches and tidal deltas on the NW-facing margin suggest tidal currents and longshore transport are more pronounced here, consistent with the nature of geomorphic changes through time. The enhanced tidal influence reflects the inlets open to the open bank interior, a factor which also may discourage growth of reefs on this part of the margin. Nonetheless, other Bahamian platforms with fewer tidal passes have comparable discontinuous reefs, suggesting that island continuity is not the most influential parameter on leeward reef development. Collectively, these results illustrate the nature, scale, and causes of along-strike heterogeneity and provide a conceptual model for ancient carbonate platform margins, some of which may be equally complex in their facies architecture.

Glacial erosion in the abisko mountains: kärkevagge and vassivagge, northern sweden

This paper discusses direct current resistivity soundings and geomorphological studies of Quaternary deposits in two glacial troughs in the Abisko Mountains of northern Sweden. The subject of the fieldwork is the depth of Pleistocene glacial erosion. Studies were carried out in 1998 and 2003 in the Kärkevagge and Vassivagge. The estimated thickness of Quaternary deposits and bedrock properties are discussed in the broader context of glacial erosion studies in the Abisko area. Geophysical and geomorphological studies suggest that the depth of glacial erosion was highly differentiated from –190 m in the Torneträsk basin to the metric overall erosion on the upland plateau. In medium‐sized valleys several kilometres long, erosion depth measures 30–50 m. Present‐day stream channel patterns reveal a strong relation to the bedrock configuration in valley floors

余震, 微動, 重力を用いた弓ヶ浜半島における2次元基盤構造の推定

The 2000 Tottori-ken Seibu Earthquake brought serious damage to the western area of Tottori prefecture. Anomalous distribution of severe damage of wooden structures was observed in the the Yumigahama Peninsula: that is, the most severely damaged area in the peninsula was located in the most northern part, although it seems that the ground structure of this peninsula is homogeneous from the view of ground surface. To find the cause of such the damage distribution, it is important to understand the ground structure around this area. However, there is little information about the deep structure to the bedrock.Our objective of this study is to investigate the subsurface structure of Yumigahama Peninsula and to make a two-dimensional detailed model of the bedrock configuration. For this purpose, combined analyses of aftershocks, gravity, and microtremors were carried out on the basis of (1) travel time analysis of after shocks by using ray tracing, (2) Bouguer anomaly from the gravity measurement, (3) spatial auto-correlation (SPAC) method for phase velocity of Rayleigh waves, (4) horizontalto-vertical spectral ratios (H/V) of aftershocks and microtremors. As a result, it was found that the bedrock subsides steeply from the southern part and the depth to bedrock is about 900m in the central part of the peninsula.

Interpretation of Bedrock Topography within the Port Jackson (Sydney Harbour) Region using Marine Seismic Reflection

An interpretation of the bedrock topography of virtually the whole of Sydney Harbour has been undertaken to estimate the bedrock configuration, identify paleochannels, and determine the nature and thickness of overlying sediment layers. The interpretations are based on an extensive series of marine seismic reflection data recorded in 1975 covering the harbour entrance and tributaries, and data recorded in August 2000 covering the harbour entrance and adjacent areas. Sydney Harbour has a bedrock configuration defined as a meandering channel that follows a generally deepening path as it reaches the harbour entrance with the bedrock configuration deepening to 85 m below sea level (b.s.l.) at the harbour entrance. Within the harbour, the paleochannel maintains a depth of about 30-40 m b.s.l. in the southern region near Point Piper and in Middle Harbour, and a depth of about 40-70 m b.s.l. in the middle region around the Sow and Pigs reef. In the northern survey area, paleochannel depths of 40-50 m b.s.l. were found in the Manly region. In Parramatta River, bedrock depths occur between 20-30 m b.s.l. in upstream sections of the channel and 40-45 m b.s.l. in downstream sections of the channel. In Lane Cove River, the depth of the paleochannel extends to 20-35 m b.s.l. Meandering paleochannels occur throughout Sydney Harbour with tributary paleochannels clearly defined. Sydney Harbour sediments interpreted as Holocene marine sand were found to be thickest (65 m) near the harbour entrance. Average estimated sediment thicknesses were 17.4, 10.8, 14.3 and 13.6 m in Port Jackson, Parramatta River, Middle Harbour and Lane Cove River respectively.

Study on the Horizontal Transport of Some Radionuclides in the Wieprz River Valley

A horizontal transport of radionuclides was studied by the analysis of the radioactivity of the surface soil samples from valleys of Wieprz river and its bottom sediments. Natural gamma-isotopes (40K, 238U and 232Th series) antropogenic (134Cs and 137Cs) and alpha-isotopes 238Pu and 239,240Pu were measured. The different kind of bed rock and terrain configuration, influenced the radionuclide transportation from the soil to river bottom sediments. Radioactivity of the sediment samples is definitely lower than the soils. Very strong adsorption of isotopes in soil hinders their horizontal migration. Calculated 238Pu/239,240Pu ratio is characteristic for global fallout and about 90% of the 137Cs comes from Chernobyl.

Weertman regelation, multiple refreezing events and the isotopic evolution of the basal ice layer

AbstractA simple model is developed to simulate the isotopic fractionation which accompanies Weertman regelation at the bed of temperate-based glaciers. The fractionation equations of Jouzel and Souchez (1982) are applied to multiple refreezing events over measured glacier-bed profiles, and mass balance is maintained as the basal ice and meltwater produced at one bedrock hummock enter the next. Simulation results indicate that undeformed regelation ice layers are on the order of millimetres to centimetres thick, often being completely melted at the stoss face of certain hummocks and exceptionally reaching a thickness in excess of 10 cm. Neither the internal morphology nor the isotopic composition of these layers is constant, but both vary down-glacier in accordance with bedrock configuration. A glacier-wide fractionation process is identified whereby heavy isotopes are preferentially removed from the basal meltwater film and incorporated into the basal ice. This process might go some way to explaining the anomalously “light” isotopic composition measured in base-flow waters leaving some glaciers. Vertical isotope profiles through undeformed basal ice layers are reconstructed and show that significant isotopic excursions can occur at a scale of millimeteres, while the range of isotopic compositions within such multi-layered regelation ice is greater than that which would occur in ice produced by a single refreezing event. In circumstances where the regelation system is disrupted by removal of film waters into a network of linked cavities, it is found that the remaining basal ice may be significantly enriched in heavy isotopes relative to the composition of the initial mass inputs to the system. Heavy isotope enrichment of this magnitude and consideration of the thickness of the basal ice layers concerned may explain the absence of recorded basal ice samples heavy enough to have been formed in equilibrium with subglacial precipitates sampled at one of the sites (Glacier de Tsanfleuron) and reported in an earlier paper.

Weertman regelation, multiple refreezing events and the isotopic evolution of the basal ice layer

AbstractA simple model is developed to simulate the isotopic fractionation which accompanies Weertman regelation at the bed of temperate-based glaciers. The fractionation equations of Jouzel and Souchez (1982) are applied to multiple refreezing events over measured glacier-bed profiles, and mass balance is maintained as the basal ice and meltwater produced at one bedrock hummock enter the next. Simulation results indicate that undeformed regelation ice layers are on the order of millimetres to centimetres thick, often being completely melted at the stoss face of certain hummocks and exceptionally reaching a thickness in excess of 10 cm. Neither the internal morphology nor the isotopic composition of these layers is constant, but both vary down-glacier in accordance with bedrock configuration. A glacier-wide fractionation process is identified whereby heavy isotopes are preferentially removed from the basal meltwater film and incorporated into the basal ice. This process might go some way to explaining the anomalously “light” isotopic composition measured in base-flow waters leaving some glaciers. Vertical isotope profiles through undeformed basal ice layers are reconstructed and show that significant isotopic excursions can occur at a scale of millimeteres, while the range of isotopic compositions within such multi-layered regelation ice is greater than that which would occur in ice produced by a single refreezing event. In circumstances where the regelation system is disrupted by removal of film waters into a network of linked cavities, it is found that the remaining basal ice may be significantly enriched in heavy isotopes relative to the composition of the initial mass inputs to the system. Heavy isotope enrichment of this magnitude and consideration of the thickness of the basal ice layers concerned may explain the absence of recorded basal ice samples heavy enough to have been formed in equilibrium with subglacial precipitates sampled at one of the sites (Glacier de Tsanfleuron) and reported in an earlier paper.

Seismic investigations in the vicinity of the great Sphinx of Giza, Egypt

AbstractAn integrated suite of seismic geophysical surveys including seismic refraction, refraction tomography, and high resolution reflection was utilized in the areas within the Sphinx Enclosure, the Sphinx Temple, and in the surrounding desert areas to the east and south of the Sphinx. The purposes of the investigations were to analyze the depth and distribution of weathering in the exposed limestone bedrock, search for voids/cavities around and under the Sphinx, and describe the bedrock configuration where buried by sands in the adjacent desert areas. The survey discovered that subsurface weathering below the floor of the Sphinx Enclosure is not uniform, suggesting varied periods of subaerial exposure for different portions of the excavation. A series of cavities on each side of the Sphinx, as well as between its paws, was defined. Further, the sand‐buried bedrock surface in the desert area shows evidence of several excavated, or possibly downfaulted, structures.

Origin of Land Subsidence and Earth Fissuring, Northeast Phoenix, Arizona

Differential subsidence over a buried bedrock hill caused by rapid water-level decline resulted in a 400-ft (120 m) long earth fissure in a northeast Phoenix construction site in January, 1980. This is the first occurrence in the Paradise Valley basin, and the first within the city of Phoenix. Local water levels have declined more than 300 ft (90 m) accompanied by 3.44 ft (1.05 m) of measured subsidence, with indirect evidence for 5 ft (1.5 m). The depth to ground water is as much as 580 ft in the area. The maximum subsidence rate measured was 0.44 ft. (0.13 m) annually. Water levels began dropping in the mid-19509s, and subsidence began a decade later after 100 to 150 ft (30 to 45 m) of decline in water level. The subsidence bowl has grown about 2 mi 2 (5 km 2 ) annually, first mostly westward, and more recently north and east, with further expansion expected. More than 9 ft (3 m) of subsidence is possible. Gravity survey results suggest the fissure is associated with a bedrock hill buried at 150 ft (45 m) on a large pediment. Measured differential subsidence, after Assuring, occurred in a pattern similar to the inferred subsurface bedrock configuration, indicating that the fissure was coincident with surficial horizontal tensile maxima above the crest of the buried hill. Maximum annual strain and accumulated tensile strain at the time of Assuring were calculated at 0.01 percent and 0.06 percent, respectively. These data suggest future reactivation of the filled-in fissure, and on the basis of gravity measurements, probable lengthening of the fissure. Finite element analysis indicates maximum surficial tensile stress of 3,400 psf (1.4 kg/cm 2 ) and strain of 0.19 percent when Assuring occurred. The large pediment that underlies most of the study area is divided into a shallow inner part [less than 500 ft (150 m) deep] of moderate relief; and a deeper, outer part gently sloping northeastward from 500 to 1,000 ft (150 to 300 m). The pediment is separated from thick alluvial deposits toward the center of the valley by a major NW-SE normal fault. This subsurface configuration controls the spatial pattern of land subsidence, water-level decline, and future Assuring. Fissures may be localized in three geologic settings: 1) buried bedrock hills, 2) the hinge-line of subsiding areas, and 3) buried faults.

Bedrock Control on Glacial Limits: Examples from the Ladakh and Zanskar Ranges, North-Western Himalaya, India

AbstractIn the north-western Himalaya, the distribution of modem glaciers and snowlines in the Ladakh and Zanskar Ranges adjacent to the Indus River valley suggests comparable climatic conditions prevail in the two ranges. Similarly, the positions of terminal moraines and reconstructed equilibrium-line altitudes (ELAs) indicate equivalent magnitudes of Neoglacial and Late Glacial advances in both ranges. However, the terminal positions and reconstructed ELAs from the late Pleistocene maximum advances are at least 400 m lower in the Ladakh Range than in the nearby Zanskar Range. These differences do not appear to reflect either climatic or tectonic controls. Rather, they are caused by an unusual bedrock configuration in the Zanskar Range, where vertical strata of indurated sandstones and conglomerates, and narrow steep-walled canyons cut through them, created a bulwark that effectively precluded significant down-valley advance. Without recognition of this physical impedance to glacial advance, uncritical reconstructions would greatly overestimate the altitude of the ELA in the Zanskar Range.

Bedrock Control on Glacial Limits: Examples from the Ladakh and Zanskar Ranges, North-Western Himalaya, India

AbstractIn the north-western Himalaya, the distribution of modem glaciers and snowlines in the Ladakh and Zanskar Ranges adjacent to the Indus River valley suggests comparable climatic conditions prevail in the two ranges. Similarly, the positions of terminal moraines and reconstructed equilibrium-line altitudes (ELAs) indicate equivalent magnitudes of Neoglacial and Late Glacial advances in both ranges. However, the terminal positions and reconstructed ELAs from the late Pleistocene maximum advances are at least 400 m lower in the Ladakh Range than in the nearby Zanskar Range. These differences do not appear to reflect either climatic or tectonic controls. Rather, they are caused by an unusual bedrock configuration in the Zanskar Range, where vertical strata of indurated sandstones and conglomerates, and narrow steep-walled canyons cut through them, created a bulwark that effectively precluded significant down-valley advance. Without recognition of this physical impedance to glacial advance, uncritical reconstructions would greatly overestimate the altitude of the ELA in the Zanskar Range.

The Margin of the Greenland Ice Sheet at Isua

AbstractField studies at a particular place at the margin of the Greenland ice sheet have provided information about the ice sheet. Ice temperatures were measured in five drill holes, two of which reached the unfrozen area of basal melting. Surface water entered these two bore holes, reaching the base in one, but remaining 59 m above the base in the other. The existence of this water conduit or fracture at 240 m depth, the calculated temperature profiles, and the local bedrock configuration suggest an area of stationary ice overridden by the ice sheet. This situation suggests creep rupture at depth in the ice sheet. Ice-fabric analysis made above 240 m depth shows patterns similar to fabrics elsewhere near the margin in zones of low deviatoric stress. Unfortunately no cores were obtained below that depth where stationary ice may exist.

The Margin of the Greenland Ice Sheet at Isua

AbstractField studies at a particular place at the margin of the Greenland ice sheet have provided information about the ice sheet. Ice temperatures were measured in five drill holes, two of which reached the unfrozen area of basal melting. Surface water entered these two bore holes, reaching the base in one, but remaining 59 m above the base in the other. The existence of this water conduit or fracture at 240 m depth, the calculated temperature profiles, and the local bedrock configuration suggest an area of stationary ice overridden by the ice sheet. This situation suggests creep rupture at depth in the ice sheet. Ice-fabric analysis made above 240 m depth shows patterns similar to fabrics elsewhere near the margin in zones of low deviatoric stress. Unfortunately no cores were obtained below that depth where stationary ice may exist.