This is a fabulous image -- Fedchenko Glacier in Tajikistan. The biggest glacier outside of the polar regions -- 70 km long.
This isn't too bad either -- Bering Glacier in Alaska:
This isn't too bad either -- Bering Glacier in Alaska:
There was a sad event last night -- the winding up dinner of the Preseli Tourist Association, after 43 years of promoting NE Pembrokeshire as a tourist destination and trying to sell the unique character of the landscape and its history. Over the years we have had tremendous support from the Welsh Govt and from the local authorities, and as a membership-based organization we have supported those involved in the tourist trade, held literally hundreds of talks, site visits and specialist meetings (and convivial social gatherings!), lobbied government, designed out own promotional campaigns, and produced our own leaflets, maps and web sites. We have never employed anybody -- everything has been funded through membership subs and small grants for specific projects.
But times have changed. When we started, Preseli District Council was our local government admin area, with its HQ in Haverfordwest. Then came Dyfed CC -- it came and it went away again. Patterns of holidaymaking have changed, people now get their information via the web, and Visit Wales and Visit Pembrokeshire use all the latest marketing techniques to compete with other tourism areas to attract future visitors. Small "regional" or "sub-regional" marketing exercises don't really have a role any more. So effectively the PTA has become redundant, having done its job over many years -- just like Eco Centre Wales, which I founded back in 1980 and which promoted energy conservation and renewable energy for 35 years until it was wound up in 2015.
Bluestone Country was our brand -- based of course on the idea that this where the Stonehenge bluestones came from and covering the area across which the Fishguard Volcanic Group rocks are exposed at the surface. Of course the landscape is unique too -- dominated by Mynydd Preseli and rougher and stonier than the landscapes to the south and west. Over the years PTA produced a number of leaflets and maps extolling the virtues of the area.
Now, of course the branding label has been stolen from us -- we have the huge Bluestone National Park holiday resort in mid-Pembrokeshire, Bluestone Brewery and assorted other bits of branding as well, not to mention Australia and the USA, which have their own "bluestone branded areas". Such is life......
Ironically, the Bluestone resort is not in the National Park, and neither is it in an area where bluestone rocks occur -- except as glacial erratics.....
I have been looking again at those red sandstone boulders and cobbles found in the roadworks near Penblewyn, near Narberth. There are quite a lot of them in the glacial deposits -- mostly quite small, less than 50 cm in diameter. They are sub-rounded, but they do have glacial facets on them, and some are clearly striated. HH Thomas (yes, he of the bluestones) recorded other red sandstone boulders in the area, and assumed they had come from the ORS red marl beds -- which are exposed around 3 km to the south of the find site.
This is what I said in another post:The biggest puzzle relating to the glacial deposits at Penblewyn is the relative abundance of boulders and cobbles of red marls -- coloured bright red and pink. What on earth are they doing here, 3 km to the north of the ORS outcrop? Are they derived from ancient river gravels that have been carried northwards from the outcrop and then incorporated into glacial deposits? That would be vanishingly unlikely, because there is no reason to assume that local drainage ever flowed northwards from the outrop, across the Afon Marlais valley (Lampeter Vale) and up the south-facing slope of the ridge. I checked the old Geological Survey Memoir for the Country around Haverfordwest (1914) and found that similar deposits from nearby are described on page 221: "The presence of red marl from the Old Red Sandstone in this drift.........shows that the transportive agency had a certain amount of northerly direction." The author? None other than our old friend HH Thomas. He was confused, and so am I........
At the time nobody knew very much about eustatic and isostatic interactions, but we did know that sea-level had been low during the big glacial episodes, and if there had been a sea-level drop of c 120m at a time of extensive and intensive glaciation, then the coastal platform would have been exposed, and available for ice cap growth. This was the map which we put into the scientific domain:
We postulated in a number of articles that as sea-level fell with the onset of a big glacial episode, more and more of the coastal shelf would have been exposed, leading to a northwards migration of the ice cap axis, in response to high precipitation rates to the north and west of the island ice caps, which must already have been in existence. We thought that there might have been an intervening ice-shelf phase, but we had no way of determining that from the evidence as we saw it.
We suggested that the big glaciation, with ice streams flowing southwards and south-eastwards into the Bransfield Strait via the sounds between the islands, was not the Devensian / Weichselian / Wisconsin glaciation but the preceding one -- or two, or three........
Much ice has flowed and melted since 1966, and although our ideas have stood the test of time there has been a huge amount of research in the South Shetlands and on the Antarctic Peninsula, and it's now apparent that the "offset South Shetlands Ice Cap" which we described has existed on multiple occasions, including -- most recently -- the Weichselian. So the outlet glacier troughs have been occupied by streaming ice over and again, maybe getting deeper with each successive glacial episode. Bethan Davies has written about the most recent deglacial phases around the Bransfield Strait. Some of her maps seem to show an LGM ice cap axis over the island chain, with a grounding line on the shelf, and some show cold-based ice grounded between the South Shetlands and South Orkneys. I must check up on what the latest thinking might be.......
There is now a big literature.
What interests me about all of this is the extent to which we can extrapolate for our studies of "the South Pembrokeshire problem" -- was the area glaciated during the Devensian, or was it not?
If an ice cap could thicken and grow, and if its axis could migrate windwards across a low-lying coastal platform in the South Shetland Islands within the time-frame of a glacial episode, why not in Pembrokeshire too? I shall ponder further.....
This is a close-up of a photo I have looked at before -- showing the crossing striations on the surface of a red sandstone cobble found in the excavations at Penblewyn, near Narberth.
Here is another paper by Adrian Hall and colleagues -- this time applying their ideas about glacial ripping to the Loch Eriboll area of NW Scotland. I'm a bit more convinced now -- especially with reference to areas where rocks are flat-lying or where subsurface fractures or bedding planes might be parallel with the ground surface.
Citation:Hall,A.M., Mathers, H. Krabbendam, M. Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland. Geosciences 2021, 11, 232.
I'm particularly interested in the work done on brecciation beneath a glacier and the creating of what the authors call "rubble till". There is a great deal of breccia, sometimes with pseudo-bedding and sometimes not, beneath the Devensian tills of West Wales, and I have always interpreted this as rockfall debris or as an indication of a long period of periglaciation dominated by freeze-thaw processes and downslope movement associated with a permafrost active layer. Perhaps I have been wrong over all these years? Perhaps some, at least, of these deposits are indeed associated with bedrock disruption and ripping, as described by Adrian Hall and his colleagues?
And the material described in the past as "rubble drift" in Pembrokeshire -- could that be connected to the processes now described?
Some thought is needed, when I get a chance......
Here is the famous map of glacial erratics and drift in England and Wales, first published by the Yorkshire Geological Society in 1928. Thje map was made by FW Harmer. For its time, it was very detailed and comprehensive, but like most maps the density of symbols has more to do with the density of research than with the actual distribution of the things being mapped! Click to enlarge........
The routes shown for Shap granite erratic transport are themselves somewhat erratic, and will have been much more erratic than shown -- but the main point of interest is the final resting place. One Shap granite erratic (at least) is shown as having been dumped in the Severn estuary. So here is an interesting question: could the "granite debris" found at West Kennet be related in some way?
In their analysis of the granite "grus" at West Kennet Ixer, Bevins and Pirrie decide that it has probably come from Cheviot -- maybe from Cunyan Crags. This is well to the east of Shap.The West Kennet grus: how far?
Many thanks to Hugh for publishing this wonderful photo on his Preseli 360 Facebook page. It's one of the best photos I have seen which portrays the landscape on the north face of Preseli -- with the craggy tor of Carn Goedog in the centre of the photo.
This north flank was perfectly placed for the accumulation substantial thicknesses of snow and firn -- and maybe glacier ice -- during the Early and Middle Devensian -- prior to the arrive of the Irish Sea Ice at the time of the LGM. This might have been a snowy tundra wilderness for the best part of 70,0-00 years.
To the left, on the rolling plateau ridge, I am now more than ever convinced that a Preseli ice cap waxed and waned several times as the climate oscillated between interstadial and continuous permafrost conditions. There is local till on these slopes, and across the moor to the right of the photo there are abundant spotted dolerite boulders that must have been carried NORTHWARDS by glacier ice. There is a lot of till on Brynberian Moor as well. On the slopes there are also traces of meltwater channels and morainic accumulations, and there are many examples of ice moulded forms on the tors -- I am still trying to work out how old these features are, and how they were formed.........
Patton, H., Hubbard, A., Glasser, N. F., Bradwell, T. & Golledge, N. R. 2013 (July): The last Welsh Ice Cap: Part 1 – Modelling its evolution, sensitivity and associated climate. Boreas, Vol. 42, pp. 471–490.
The last Welsh Ice Cap: Part 1 – Modelling its evolution, sensitivity and associated climate.
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.
Then, after maybe less than 1,000 years of "peak glaciation" the ice sheet starts to collapse, and the ice stream power is dramatically reduced. Forward flow is stopped, and catastrophic ice wastage sets in -- so that a landscape deeply inundated beneath glacier ice is gradually revealed, with an intermittent cover of glacial and fluvioglacial deposits which are remobilised and redistributed as the last remnants of buried ice melt away.
On a glacial cycle timeline this is all very asymmetrical, with (1) a long period of "increasing glacierisation" with minimal landscape effects followed by (2) a period of intense activity involving streaming ice and then (3) by a short period of catastrophic ice wastage and sediment redistribution. That's the simplest situation imaginable. Of course, in reality it is always more complex. Climatic oscillations occur on a variety of different scales, and long-term cooling is sometimes interrupted by intermittent warmer phases, just as long-term warming is interrupted by "cold snaps" such as the Younger Dryas phase of c 12,000 years ago and the Little Ice Age that set in during the Middle Ages. Changes in oceanic and atmospheric circulation can also have the effect of changing wind direction and strength, and changing patterns of precipitation. These are all things that are built into the increasingly sophisticated modelling of ice sheet growth and decay by the BRITICE-CHRONO group and others.
So just as the idea of a spectacular ice front marching across the landscape is an image that has no basis in reality, the idea of a later ice front retreating across a landscape during deglaciation is equally misleading. Ice stops moving forward -- it does not RETREAT. It simply wastes away where it lies, mostly from the top down. So just as the local ice caps of Aubrac in France and Glamajökull in Iceland melted away completely, leaving but a few traces of their former existence, the same thing is happening at the moment to Drangajökull, leaving very few traces up on the high plateau where ice movement was minimal, but complex associations of glacial and fluvioglacial deposits and landforms in the valleys affected by streaming ice.
And we should be careful about using the term "ice limits" as well. I have to admit to using the term far too frequently myself, as a shorthand expression which is easily understood. On Quaternary maps we see "ice limits" portrayed all too often as straight lines, even across mountainous terrain -- but in reality ice edges are generally "fingered" or crenellated in sympathy with the landscape, and beyond an ice sheet edge there are generally scores if not hundreds of smaller icefields, ice caps and snowfields -- again in sympathy with the lie of the land and the patterns of snowfall. So the real ice edge is not just crenellated but also fragmented.........
So how does all this relate to the evidence on the ground in West Wales and elsewhere? Watch this space.......
One of the biggest problems with all of this is the claim by Rolfe et al that Lundy Island was affected by glacier ice during the Early Devensian:https://brian-mountainman.blogspot.com/2015/03/a-lundy-island-spat.html