Glacial curvilineations found along the southern sector of the Laurentide Ice sheet and a hypothesis of formation involving subglacial slope failure in tunnel valleys and subglacial lakes

First discovered in Poland, glacial curvilineations (GCLs) are enigmatic landforms comprising parallel sets of sinuous ridges and troughs of metres amplitude and around 150 m wavelength, found within kilometres‐wide valleys interpreted as being produced by meltwater flowing subglacially. Their morphological and sedimentary characteristics and association with tunnel valleys has been described for some prominent Polish examples. From these observations the existing hypothesis is that they form as a consequence of erosion by longitudinal vortices that develop in subglacial floods. Here we report, for the first time, GCLs found along the southern sector of the Laurentide Ice Sheet in three northern states of the USA. Using mapping and topographic analysis from high resolution digital elevation models we report observations on their morphological properties and landform associations. We find aspects of their context and morphology difficult to explain using the existing hypothesis. We instead suggest that these glacial curvilineations are produced by subglacial bank and slope failures that locally widen tunnel valleys, or that occur near subglacial lake shorelines. Further investigation is required to test this hypothesis and to ascertain the mechanisms of proposed mass movements, which may have occurred by rotational or translational slope failure or by creep deformation. Our preferred mechanism is that such movements occurred where subglacial water was emplaced over previously perma‐frozen ground. Under such circumstances, sediment blocks thawed by the water may then easily glide over a frozen décollement at low slope angles; analogous to subaerial active‐layer glides in permafrost environments. Permafrost spring sapping may have provided lines of weakness for slope failure. If the requirement for permafrost is found to hold, then GCLs may become an important indicator of the palaeo‐distribution of permafrost. Copyright © 2017 John Wiley & Sons, Ltd.     

Rock slope instability assessment using spatially distributed structural orientation data in Darjeeling Himalaya (India)

We discuss a geographic information system (GIS)‐based methodology for rock slope instability assessment based on geometrical relationships between topographic slopes and structural discontinuities in rocks. The methodology involves (a) regionalization of point observations of orientations (azimuth and dip) of structural discontinuities in rocks in order to generate a digital structural model (DStM), (b) testing the kinematical possibility of specific modes of rock slope failures by integrating DStMs and digital elevation model (DEM)‐derived slope and aspect data and (c) computation of stability scenarios with respect to identified rock slope failure modes. We tested the methodology in an area of 90 km² in Darjeeling Himalaya (India) and in a small portion (9 km²) within this area with higher density of field structural orientation data. The results of the study show better classification of rock slope instability in the smaller area with respect to known occurrences of deep‐seated rockslides than with respect to shallow translational rockslides, implying that structural control is more important for deep‐seated rockslides than for shallow translational rockslides. Results of scenario‐based analysis show that, in rock slopes classified to be unstable, stress‐induced rock slope instability tends to increase with increasing level of water saturation. The study demonstrates the usefulness of spatially distributed data of orientations of structural discontinuities in rocks for medium‐ to small‐scale classification of rock slope instability in mountainous terrains. Copyright © 2010 John Wiley & Sons, Ltd.