Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click

Friday 5 July 2019

More on ice gradients in the Celtic Sea arena

A winter photo of the Malaspina Glacier, the word's biggest piedmont glacier at the present time.

I have mulled over this topic before, because of the apparent mismatch between modelling work and "ground truthing" in which field evidence is crucial.  The diagram above shows assorted ice sheet long profiles, including some which accord reasonably well with the theoretical ice profile of an equilibrium glacier or ice sheet, and others (the red and green lines) which suggest that in some cases extremely low or "flat" profiles can be observed or deduced.  The diagram is from a 1973 article by Matthews, in which he wondered why glaciers with such low profiles could still move and be capable of eroding and transporting erratics and morainic materials.  See refs below.

The perfect piedmont glacier -- there are quite a few of them in North Greenland and Arctic Canada.  Generally the gradient of the feeder glacier is steeper, and the gradient on the piedmont part very flat..... 

The flat profiles come from the SW part of the Laurentide ice sheet and the "piedmont" glaciers flowing from the Cordilleran ice sheet. Lionel Jackson and colleagues have worked in areas affected by these glaciers, and have come to the view that gradients were often only about half as steep as those deduced by earlier researchers.  Without going into detail about shear stresses, basal sliding and internal deformation, there seems to be a rule that piedmont glaciers can have very low profiles as the ice spreads laterally across open country.  That makes sense,  and measured gradients are usually below 8m per kilometre and sometimes as low as 1m per kilometre.  

If we translate that into some crude figures for the Celtic Sea lobe, and if we assume for the moment that the long profile should be oriented from NE towards SW, as assumed by most researchers over the past 20 years, that means there could have been a grounded snout at -150m on the continental shelf edge and an ice surface at +250m about 400 km away in St George's Channel (if the gradient was 1m per km).  At the other extreme, with a gradient of 8m per km, the ice surface in St George's Channel would have been at +3050m -- more or less as I have previously assumed in earlier posts.   If we take a more modest figure of 4m per km, assuming the ice was dynamic enough to cross the Celtic Deep and still push southwards for 400 km, we get figures of -150m at the snout and +1,450m at the St George's Channel constriction point.

If the new suggestion from Scourse et al (2019) is to be taken seriously, glacially-forced isostatic depression on the shelf edge around 27,000 years ago must imply thicker rather than thinner ice, grounded over a substantial area.  No doubt the modelling people are hard at work on this issue as we speak...........

Whatever the models throw up, we have to explain the presence of a Devensian ice edge at about 300m on Carningli and about 250m on the north face of Mynydd Preseli -- and at assorted altitudes below 100m around the south Pembrokeshire coast.  Can these ice edge altitudes be accommodated in a scenario involving a very low long profile for the piedmont glacier in the Celtic Sea arena?  Watch this space......



W. H. Mathews (1974) Surface profiles of the Laurentide ice sheet in its marginal areas. Jnl of Glaciology,  Volume 13, Issue 67 1974 , pp. 37-43.  Published online: 30 January 2017.


Jackson, L.E., Jr., Little, E.C, Leboe, E.R., and Holme, P.J., 1996: A re-evaluation of the paleoglaciology of the maximum continental and montane advances, southwestern Alberta; in Current Research 1996-A; Geological Survey ofCanada, p. 165-173.

Controversy over the age of the maximum known advances of continental and montane glaciers in southwestern Alberta rests largely on the paleoglaciology of the maximum montane piedmont glacier. Reconstruction of it, based upon features that mark the former glacier surface in the Foothills, indicate that the glacier had a maximum surface gradient of 0.4-0.6° - less than half the value assumed by previous workers. The continental ice sheet that was roughly coeval with the piedmont glacier and coalesced with it in the Foothills. Paleoglaciological evidence favours a Late Wisconsinan age for maximum extent of continental and montane ice.

No comments: