This is one that I missed -- from about five years ago. A huge amount of effort has gone into the creation of models of BIIS behaviour, and this one has clearly fed into the latest BRITICE - CHRONO effort. This article is interesting in that it examines the creation, activity and decline of ice streams within the overall ice sheet mass, and tries to define the factors involved in their survival.
I was particularly interested in the work put in by the authors on ice surface gradients, and I was intrigued to see that they have reconstructed the ice surface at the time of operation of the Ice Stream in the Celtic Sea, south of St Georges Channel.
I am intrigued by the fact that the modelling throws up an ice surface altitude of c 1000m in Pembrokeshire when the snout is positioned to the north of the Isles of Scilly. Thus there was a predicted 1000m drop from there to the postulated snout position 150 km away. That's a much shallower profile than that of the Greenland ot Antarctic Ice Sheet, but much steeper than the shallowest recored gradients on the Laurentide Ice Sheet -- with falls of only 500m over 500 km -- a gradient of one metre per kilometre. Why the discrepancy? Well, a model is a model, and has to be validated through "ground truthing" -- but for what its worth, I have repeatedly suggested that the high summits of Preseli were overridden by Late Devensian (LGM) ice that was flowing broadly NW-SE. I have also suggested that the ice surface gradient might have been so low that the drop was from c 550m near Mynydd Preseli down to c 200m on the western chalk scarp of Wiltshire. The suggestion is that an ice surface at 1000m (in any of the main glacial episodes) could have resulted in a significant increase in glacier dynamics, this enhancing the erosional and transport transport of the ice mass flowing up the Bristol Channel. Again, this suggests that active ice carrying an assortment of west Wales erratics could well have flowed over the chalk scarp and transgressed at least as far eastwards as Stonehenge.
https://brian-mountainman.blogspot.com/2011/02/glaciological-dilemma.html
This additional piece of glaciological work again shows that the glaciation of Salisbury Plain was not just possible, but indeed probable..........
Details:
Exploring the ingredients required to successfully model the placement, generation, and evolution of ice streams in the British-Irish Ice Sheet
Niall Gandy, Lauren J. Gregoire, Jeremy C. Ely, Stephen L. Cornford, Christopher D. Clark, David M. Hodgson. Quaternary Science Reviews 223 (2019) 105915
Ice stream evolution is a major uncertainty in projections of the future of the Greenland and Antarctic Ice sheets. Accurate simulation of ice stream evolution requires an understanding of a number of “in- gredients” that control the location and behaviour of ice stream flow. Here, we test the influence of geothermal heat flux, grid resolution, and bed hydrology on simulated ice streaming. The palaeo-record provides snapshots of ice stream evolution, with a particularly well constrained ice sheet being the British-Irish Ice Sheet (BIIS). We implement a new basal sliding scheme coupled with thermo-mechanics into the BISICLES ice sheet model, to simulate the evolution of the BIIS ice streams. We find that the simulated location and spacing of ice streams matches well with the empirical reconstructions of ice stream flow in terms of position and direction when simple bed hydrology is included. We show that the new basal sliding scheme allows the accurate simulation for the majority of BIIS ice streams. The extensive empirical record of the BIIS has allowed the testing of model inputs, and has helped demonstrate the skill of the ice sheet model in simulating the evolution of the location, spacing, and migration of ice streams through millennia. Simulated ice streams also prompt new empirical mapping of features indicative of streaming in the North Channel region. Ice sheet model development has allowed accurate simulation of the palaeo record, and allows for improved modelling of future ice stream behaviour.
Very interesting article. I've only glanced through it, but in Appendix A there is an animation of the development, spread and retreat of the ice. This shows a maximum extent to the margins of Salisbury Plain.
ReplyDeleteNow the question that has probably been asked before and I'm sure answered; is erratic material more likely to be found at the maximum extent of an ice sheet, or more evenly spread throughout its length?
Dave
I am rather keen to visit part of Codford parish to the south - west of the two Codfords. It is to the south - east of the Codford Circle (or Wilsbury Ring) which itself is SSW of Clay Pit Hill with its clay - with - flints. My walk would hopefully enable me to access STONY HILL at NGR ST 994402.
ReplyDeleteWe know that by far the biggest proportion of any ICEBERG is below the water. Similar principle probably applies to the bluestones brought to the general area of Salisbury Plain, because they probably arrived courtesy of glaciers nigh on half a million years ago. Consequently, further glaciation events and general more recent geomorphological landform activities have no doubt made finding evidence of so - called "bluestones" somewhat difficult. Then, of course, there's the introspective self - absorbed methodology of ye olde antiquarians and then "modern" archaeologists who have been unable to get their heads around this thought process: that evidence within the general area of The Greater Stonehenge Landscape for pieces of a great variety of types of "bluestone" DO NOT have to be chips taken from the orthostats that were erected at Stonehenge - they can just as logically be remnants of the train of glacial erratics deposited after a 150 - plus miles journey!
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