Erratics of many different lithologies on washed rock surfaces in the Stockholm Archipelago.
It's intriguing that ice-rafted erratic boulders are still being talked about in the context of the large erratics found on the shorelines of southern Britain. Ice rafting seems to be the "process of preference" among those who -- for whatever reason -- do not wish to accept that full glacial processes have substantially affected the Bristol Channel coasts of Cornwall, Devon and Somerset.
About a year ago I posted these items on the blog:
https://brian-mountainman.blogspot.com/2023/12/coastal-geomorphology-in-devensian.html
https://brian-mountainman.blogspot.com/2023/12/the-big-erratic-boulders-of-devon-and.html
........... and I have discussed the matter in many other posts as well. Use the search box to find them.
The latest spat is with James Scourse, who argued in a very strange paper that the big coastal erratics were emplaced by floating ice at a time of high relative sea level (RSL) and cold climate.
https://brian-mountainman.blogspot.com/2024/03/new-scourse-paper-on-giant-erratics-is.html
That of course flies in the face of our understanding of eustatic / isostatic relationships during the Devensian and earlier glacial episodes. In the latest rather crude attack on me by Pearce, Ixer, Bevins and Scourse they argue again that the big coastal erratics (like the Limeslade erratic) on the Bristol Channel coasts are "ice rafted" -- and that there was "asynchrony between Early and Middle Devensian regional ice sheet development and global sea level". I don't have a clue what that means, and no evidence in support of the contention is provided. I see no sign at all of Early and Middle Devensian glacial activity in the Quaternary stratigraphy of West Wales, and I see no evidence of substantial isostatic depression at the time, or tectonic tilting. Bennett et al, in the new paper on the Quaternary in Devon, say with respect to the coastal erratics: "...........another view is that the coastal platform erratics were delivered by icebergs calved from glacier ice farther north, an interpretation that is consistent with the widespread distribution of erratics on shore platforms on the Bristol Channel and English Channel coasts."
Even if there was a substantial ice mass in Wales during the Early and Mid Devensian, global sea level at that time was around -70m, and I can see no mechanism for a sinking of the Devon and Cornwall coast by an equivalent amount, thereby creating conditions for ice rafted debris to be dumped on the coastal rock platform. On the contrary, when one looks at known isostatic adjustment mechanisms, there would have been a hinge line along the Bristol Channel and a rise in the relative altitude of the South-West Peninsula, making giant erratic emplacement from floating ice even less likely.
Icebergs do not, in general, deliver erratics to distant coastal platforms. Dirty icebergs moving away from floating ice fronts almost always operate in deep water, dropping their debris load as a consequence of bottom melting -- ending up as glacio-marine sediments with added dropstones. There are vast thicknesses of these materials in the Celtic Sea and in the outer Bristol Channel. (They have received impressive attention during the BRITICE studies.) Icebergs do get stranded in embayments in the intertidal zone, and they do break up into bergy bits and release big boulders onto mud flats. But most of the ice with "rafting" characteristics is sea ice which develops as flat sheets and then gets broken up by lateral pressure, and broken ice floes are almost always clean. Where there is an ice foot on a polar coastline, it is almost always a mixture of sea ice, frozen spray and detached chunks of winter snowbanks. It sometimes contains erratic boulders, but these are generally taken from pre-existing coastal materials including beach accumulations, till and scree. They are seldom carried in from the sea; they are more likely to be carried OUT from the coastline to be dropped in deeper water offshore.
As I see it, the big erratic boulders scattered along the coasts of the Bristol Channel are NOT all concentrated in the current intertidal zone. They occur at altitudes up to at least 80m. They have to be residuals or remnants of ancient glacial deposits scattered along the Bristol Channel coasts. As I have said before, the "concentration" of these erratics on the intertidal rock platform is more apparent than real, since these are washed surfaces similar to those associated with "kalottberg" hills around the marine limit in Sweden. Nobody ever suggests that the boulders found at the marine limit are unique in any way; they almost always also occur above and below the limit. So the position of the marine limit -- like RSL on the coast of the Bristol Channel -- is interesting but not geomorphologically significant.
https://brian-mountainman.blogspot.com/2018/07/kalottberg-features-in-northern-sweden.html
It seems to me highly likely that the erratic boulders on the Bristol Channel coasts are simply the remnants of ancient glacial deposits that have been almost entirely removed on the coast by marine, periglacial and other processes, possibly over several glacial / interglacial cycles. Further inland, away from the coast, there may well be in situ glacial deposits (with giant erratics!) still awaiting discovery.
The boulders themselves may date from a number of different glacial phases, including the Anglian, Late Wolstonian and Late Devensian, in which Irish Sea ice from the N and NW crossed the coastline and pressed inland via convenient depressions and wide valleys. Just a few of the boulders (such as the famous pink erratic at Saunton) can be tied into the stratigraphic sequence, sealed by sandrock or slope breccia. At Whitesands in North Pembrokeshire the boulder bed (which I think is related to some of the erratics on coastal rock platforms) appears to consist of boulders emplaced prior to the last interglacial and then modified by wave action. It is not visibly associated with the Ipswichian raised beach.
Looking again at the ice-rafting hypothesis, I cannot see any tectonic or other evidence which suggests that floating ice was involved in the emplacement of any of the big erratics on the Bristol Channel coast. On those occasions when there were dirty icebergs in the Celtic Sea, sea level was probably at least 100m lower than today, and the coastline was at least 10 km further to the west than it is now.
4 comments:
Should we expect to see icebergs carrying rocks today, as we sadly witness global warming?
Yes, icebergs always have, and always will, carry debris including big erratics. Or SOME will be dirty, while the majority are rather clean. Big outlet glaciers with floating snouts erode vast amounts of rock, and it has to go somewhere. Most of the debris is just dropped from the icebergs in deep water, as they waste away.
In present-day glacier melt, I can see that dirty icebergs being produced, but they are likely to come from localised areas and drop the material relatively close by. In an end of Ice Age scenario, the zone of release of icebergs might be larger, along a wider front. Assuming the same ratio of clean to dirty icebergs, the number of dirty icebergs might be absolutely higher. The major influence is that sea level is lower, so they could raft out and lose their load in what is now much deeper water. Where icebergs are not forming, the ice is simply draining out onto what was then above sea level, but is now shallow, tidal or dry ground. Any material would just be dropped there.
It all depends on the local geography and the nature of the calving ice. If a big ice stream is discharging thousands of icebergs into a fjord system, with very deep water, debris may be carried hundreds or even thousands of miles from the source before being dropped. At a time of lower sea level you would not necessarily have a longer or more active calving ice front -- in the Celtic Sea, for example, you may have a pretty messy situation, with a grounded ice front simply disintegrating or breaking up, with vast discharges of meltwater.
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