Shap Pink Quarry -- in the centre of the original cliff exposure which appears to have been the main source for Shap Granite erratics. This is on the down-glacier (plucked) face of a large roche moutonnee which seems to coincide with the granite pluton.
The source of the erratics is a little way to the west of the M6 motorway, and c 10 kn south of Shap; and the erratics are dispersed to the south, south-east and east of the source. Some of them have travelled hundreds of kilometres.
This is interesting, from thr Abstract: The degree of edge rounding for individual blocks increases in a punctuated fashion with the distance from the outcrop as blocks fracture repeatedly to introduce new fresh unrounded edges. In contrast, block shape is conservative, with parent blocks fracturing to produce self-similar “child” shapes with distance.
This matches with my own observations on far-travelled erratics, indicating that the "joint controlled blocks" which are entrained as a result of tensile stress on the glacier bed are very rapidly modified, assuming characteristics which are largely the result of the processes operating on the glacier bed. In other words, geomorphology takes over from geology...........
And as I have observed, large parent blocks break down into smaller "child" blocks, large slabs break down into smaller slabs, and large pillars tend to break down into smaller pillars.
I would have liked to see more consideration of the processes operating in englacial erratic tramnsport, as distinct from subglacial transport. It is highly likely that many of the Shap erratics have spent at least a part of their time in transport within the body of a flowing ice stream, rather than on the bed, given the complexities of ice flow in an area of shifting ice divides. Over and again, dominant ice streams must have dominated weaker ice streams, flowing over more sluggish ice masses and inducing striking changes in ice flow directions. Indeed. this is demonstrated over and again by references to "basal ice flow directions" and "surface ice flow directions" or "high level ice". Nevertheless, this work is very relevant to our understanding of the shapes and surface characteristics of the Stonehenge bluestones -- a subject to which I shall no doubt return.......
Thanks to Carling et al (2023) for this map, showing the multiple pathways that Shap erratics must have followed during the late Devensian glaciation prior to final emplacement
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The conclusions:
The hypothesis that granite blocks would display an increase in edge rounding with distance from the source outcrop is confirmed, whilst the hypothesis that shape would evolve with distance is refuted. Although the increase in the mean radius of edge rounding for the whole block population increases
exponentially with distance, edge rounding on individual blocks is an irregular function mediated by block fracture mechanics, as block size is reduced (with shapes fluctuating between cuboids, slabs, and rods) with distance and new sharp edges are provided to partially edge-rounded blocks. Thus, edge rounding and shape coevolve as block size is reduced. Fracture transverse to block orientation is in
accord with the application of tensile stress, which controls the process by which block form is conserved as block size is reduced. Consideration of the orientation of the tensile fractures on blocks in the field might be used to approximate the direction of ice flow at the time of fracture.
Overall, the results indicate that edge rounding is unlikely to be advanced if blocks continue to fracture. Wellrounded blocks must represent blocks that have resisted splitting. In the case of exceptionally large, rounded blocks, the rock mass likely is unusually homogeneous, lacking potential fracture lines. However, smaller blocks are less likely to contain potential fracture lines, so fracture should become less prevalent as blocks are reduced in size, which then promotes edge rounding.
Future work should consider developing mathematical models that represent the function of edge rounding as predicated by a model (e.g. silver ratio) describing block size reduction. Similar studies considering other lithologies (e.g. stratified sedimentary rocks) would likely find different shape evolution patterns in contrast to the cuboid central tendency displayed by the homogeneous granite, with concomitant implications for edge rounding trends with time and distance.
exponentially with distance, edge rounding on individual blocks is an irregular function mediated by block fracture mechanics, as block size is reduced (with shapes fluctuating between cuboids, slabs, and rods) with distance and new sharp edges are provided to partially edge-rounded blocks. Thus, edge rounding and shape coevolve as block size is reduced. Fracture transverse to block orientation is in
accord with the application of tensile stress, which controls the process by which block form is conserved as block size is reduced. Consideration of the orientation of the tensile fractures on blocks in the field might be used to approximate the direction of ice flow at the time of fracture.
Overall, the results indicate that edge rounding is unlikely to be advanced if blocks continue to fracture. Wellrounded blocks must represent blocks that have resisted splitting. In the case of exceptionally large, rounded blocks, the rock mass likely is unusually homogeneous, lacking potential fracture lines. However, smaller blocks are less likely to contain potential fracture lines, so fracture should become less prevalent as blocks are reduced in size, which then promotes edge rounding.
Future work should consider developing mathematical models that represent the function of edge rounding as predicated by a model (e.g. silver ratio) describing block size reduction. Similar studies considering other lithologies (e.g. stratified sedimentary rocks) would likely find different shape evolution patterns in contrast to the cuboid central tendency displayed by the homogeneous granite, with concomitant implications for edge rounding trends with time and distance.
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Coevolving edge rounding and shape of glacial erratics: the case of Shap granite, UK
Paul A. Carling
Earth Surface. Dynamics 12, pp 381–397, 2024
https://doi.org/10.5194/esurf-12-381-2024
Published: 26 February 2024
Abstract.
The size distributions and the shapes of detrital rock clasts can shed light on the environmental history
of the clast assemblages and the processes responsible for clast comminution. For example, mechanical fracture ue to the stresses imposed on a basal rock surface by a body of flowing glacial ice releases initial “parent” shapes of large blocks of rock from an outcrop, which then are modified by the mechanics of abrasion and fracture during subglacial transport. The latter processes produce subsequent generations of shapes, possibly distinct in form from the parent blocks. A complete understanding of both the processes responsible for block shape changes and the trends in shape adjustment with time and distance away from the source outcrop is lacking. Field data on edge rounding and shape changes of Shap granite blocks (dispersed by Devensian ice eastwards from the outcrop) are used herein to explore the systematic changes in block form with distance from the outcrop. The degree of edge rounding for individual blocks increases in a punctuated fashion with the distance from the outcrop as blocks fracture repeatedly to introduce new fresh unrounded edges. In contrast, block shape is conservative, with parent blocks fracturing to produce self-similar “child” shapes with distance. Measured block shapes evolve in accord with two well-known models for block fracture mechanics – (1) stochastic and (2) silver ratio models – towards one or the other of these two attractor states. Progressive reduction in block size, in accord with fracture mechanics, reflects the fact that most blocks were transported at the sole of the ice mass and were subject to the compressive and tensile forces of the ice acting on the stoss surfaces of blocks lying against a bedrock or till surface. The interpretations might apply to a range of homogeneous hard rock lithologies.
of the clast assemblages and the processes responsible for clast comminution. For example, mechanical fracture ue to the stresses imposed on a basal rock surface by a body of flowing glacial ice releases initial “parent” shapes of large blocks of rock from an outcrop, which then are modified by the mechanics of abrasion and fracture during subglacial transport. The latter processes produce subsequent generations of shapes, possibly distinct in form from the parent blocks. A complete understanding of both the processes responsible for block shape changes and the trends in shape adjustment with time and distance away from the source outcrop is lacking. Field data on edge rounding and shape changes of Shap granite blocks (dispersed by Devensian ice eastwards from the outcrop) are used herein to explore the systematic changes in block form with distance from the outcrop. The degree of edge rounding for individual blocks increases in a punctuated fashion with the distance from the outcrop as blocks fracture repeatedly to introduce new fresh unrounded edges. In contrast, block shape is conservative, with parent blocks fracturing to produce self-similar “child” shapes with distance. Measured block shapes evolve in accord with two well-known models for block fracture mechanics – (1) stochastic and (2) silver ratio models – towards one or the other of these two attractor states. Progressive reduction in block size, in accord with fracture mechanics, reflects the fact that most blocks were transported at the sole of the ice mass and were subject to the compressive and tensile forces of the ice acting on the stoss surfaces of blocks lying against a bedrock or till surface. The interpretations might apply to a range of homogeneous hard rock lithologies.
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See also:
Proceedings of the Geologists' Association, 2023.
https://doi.org/10.1016/j.pgeola.2023.01.002
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