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....
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Friday 13 October 2023

How do ice sheets grow?


One of the small wasting ice caps of northern Baffin Island.  As it thins, the edges are retreating towards the core.....

As readers of this blog will know,  I have frequently bemoaned the fact that we know a lot about ice sheet collapse and disintegration, but not a lot about the manner in which ice sheets grow, through a process which we might call "blanketing the thickening".  See the following:

From this earlier post:

Following a warm (interglacial or interstadial) episode…. the first thing that happens is that temperatures drop, with the mixed deciduous forest replaced by a boreal forest and then by tundra. Then we see the growth of seasonal and then continuous permafrost. That creates conditions where seasonal snow-melt is reduced and eventually eliminated. So where snowfall is substantial (fed by moisture-bearing winds) snowfields develop in the highlands and then in the lowlands, with seasonal melting having less and less of an effect until the whole landscape is covered with snowfields and firn. In favourable locations — on the highest plateaux and in mountainous areas where there are substantial snow-collecting hollows facing north or north-east the firn might thicken substantially until we see the creation of glacier ice. Where there are convenient slopes or discharge routes, glaciers may start to flow and expand, pushing into areas which are otherwise covered with perennial snowfields.

But at what point does streaming ice from far afield start to affect an area like West Wales, where there may be lowlands, upstanding hill masses and plateaux which might support local ice caps? It's clear that this happens very late in the day, towards the back end of a full glacial cycle. The glacierization build-up phase (with extensive snow and ice cover on the landscape) may last for 70,000 years or more, interrupted by short-lived cooling and warming phases or by changes in precipitation and other types of "climatic forcing". But eventually, when the whole land surface has become deeply buried by locally-generated ice, maybe 500m or more in thickness, the big brute from the north begins to dominate -- in this case with the arrival of the Irish Sea Ice Stream - as cold-based ice is replaced by ice that is capable of sliding on its bed. Streaming ice then begins to do serious damage, eroding bedrock, picking up erratics and incorporating pre-existing periglacial and other deposits, and modifying a previously protected landscape in a multitude of different ways. The direction of ice flow is now determined not by the details of local topography but by the surface gradient of the ice stream; in the case of Pembrokeshire ice movement seems to have swung through an arc of maybe 60 degrees, but with a dominant flow from NW towards SE. It appears that the Welsh ice cap did not greatly affect Pembrokeshire, but ice from the Welsh uplands may well have filled Cardigan Bay, creating a constriction that affected Irish Sea ice flow directions. From the BRITICE-CHRONO modelling, Irish Sea ice affects the area for maybe 3,000 years.

This is an interesting short article:

It makes the point that the ice in these small ice caps and ice fields (which were probably initiated in the Neoglacial, after c 7,000 yrs BP) is very thin and cold-based, frozen to its bed, and effectively preserving without disturbance what was there before -- both landforms and sediments. 

In previous posts I have speculated on the lessons that might be learned from Drangajokull and Glamajokull in NW Iceland.  Use the search facility to look them up!

As I have suggested on earlier occasions, Salisbury Plain may well have been submerged beneath an extensive snowfield / icefield for many thousands of years, over and again during the Quaternary.   The enigmatic clay-with-flints may tell us something about what happened, and when........  But if the bluestones at Stonehenge really are glacial erratics, carried in by the ice from the west (and maybe the north as well), exactly how did this moving ice arrive, and what were the basal ice conditions at the time?  It may well be impossible to determine where the ice edge was located, because that would have been impossible if we had looked down at a white surface from a satellite at the time.   More to the point, where was the easternmost extent of MOVING ice capable of carrying extraneous or erratic material?  Work in progress.......


1 comment:

New Chris said...

Long time fan of the blog - thought of you as soon as I saw this!