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Thread: ASSYNT ROCKS

  1. #21
    ‹bermensch Ben Hedley's Avatar
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    have readable handwirting now tho!

  2. #22
    Mini Goon
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    All the information boards in Assynt were temporarily removed for maintenance when I was there...back in the first week of May. I wonder if they ever appeared over the summer or not?

  3. #23
    Ultra King MoS's Avatar
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    Thanks Ben, so the answer is probably to do with what happened during the time gap represented by the unconformity at the base of the Torridonian. As you say, unconformities are funny things. I was forgetting about Cambrian being directly on top of Lewisian in places, so likewise Torridon Group could rest directly on top of Lewisian at Slioch.

    I haven?t found a stratigraphic sequence for the Torridon area that would explain the situation at Slioch. Maybe the Stoer group never existed there. Or as you say it could have been deposited and then eroded away again.

    I just expected the younger Torridonian rocks in that area to be higher up the sequence and the contact with the gneiss to be well buried. So it was a surprise to find them sitting on the Lewisian Gneiss as in the Assynt area.

    Would be good to find out more, but don?t put yourself out, it was just one of those things that bugged me.

    Just to clarify, the Cambrian quartzite is a quartz sandstone (orthoquartzite) rather than a metamorphic rock.

  4. #24
    Ultra King MoS's Avatar
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    James, they hadn't been replaced the week before last when I was there, just notices saying removed for updating.

    I guess the new ones will refer to the geopark status.

    Or maybe they will be changing the dates

  5. #25
    ‹bermensch Jim Chalmers's Avatar
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    Hi Elaine

    The story on the Lewissian is getting extremely complicated in the literature and folk nowadays have problems even with the traditional nomenclature of Scourian and Laxfordian.

    However: some ages. The oldest rocks seem to be the metamorphosed remains of quatz-rich (tonalitic) intrusions from about 2700 to 3000 million years ago (shortened to Ma by geologists). They were all metamorphosed in what has traditionally been called the Scourian event at 2700 Ma.

    A suite of mafic to intermediate (basaltic to doleritic) dykes were injected at 2400 Ma and again at 2000 to 2100 Ma and more acid intrusives at about 1850 Ma. All of these were again metamorphosed in the Laxfordian event at about 1600-1700 Ma, that was accompanied by even more acid intrusions.

    All of this happened at great depth, probably more than 10 km down, but by about 1200 Ma had been lifted and their cover eroded so that they were near the surface at that time, the fossil landscape that had hills up to about 300 m high that you mentioned. Then the area started to pull apart, with the formation of rift basins into which sediments were deposited. The oldest sediments are from about 1200 Ma and are the Stoer group. They consist of fault breccias and shallow-water sandstones and mudstones.

    200 Ma later, between 900 and 1000 Ma, the area extended again with a major fault running along the present day east coast of the Outer Hebrides. There were mountains to its west and a rolling landscape with hills up to about 600 m high to its east that subsided as the fault moved Two big rivers carried coarse sediments from the mountains and deposited them in two huge fans that today comprise the Applecross Group of the Torridonian, the coarse pebbly sandstones that we are all familiar with the high hills of the NW Highlands. The sandstones lapped onto the hills, built up around them and eventually drowned them and you can see this process spectacularly by looking across Loch Maree from the Beinn Eighe nature reserve towards Slioch. The upper part of Slioch is Torridonian sandstone, but the lower part is a drowned hill of Lewissian gneiss.

    Hope this is of some help

    Jim

  6. #26
    ‹bermensch Jim Chalmers's Avatar
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    Re your comments about the extent of the Stoer Group, Elaine. It appears to have been very local, deposited in a little rift basin in Coigach. It certainly never extended farther west, but could have extended farther east and have been eroded before deposition of the Applecross Group.

    The latter is much more extensive, all the way from south Skye to well north of Cape Wrath and underlies the Minch.

    A present-day analogue to how the Torridonian Basin may have looked in Applecross Group times might be the area south of the Atlas mountains, where big transient rivers from thunderstorms and Spring snowmelt carry coarse stony sediments out into the NW Sahara Desert.

    Jim

  7. #27
    Ultra King MoS's Avatar
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    That?s brilliant Jim, many thanks. You have answered my question about Slioch and put it all into context.

    Slioch is another place where the geology is fairly obvious, even to the untrained eye. Looking across Loch Maree, the fossil landscape of the Lewisian with a valley and hills either side is spectacular. The surface of the Lewisian being the unconformity, cutting through Slioch, with the Torridonian above.

    I was unaware that the Stoer group was so localised. I had it in mind that the way the bedding planes dip in that area, it was quite possible that the younger Torridonian rocks to the South represented those higher up the stratigraphic sequence. But I guess geology is rarely that simple.

    As for the Lewisian, many episodes of deformation and intrusions as you say. On the small scale it?s quite interesting to look at the cross cutting nature of folds and dykes to try and work out which came first. I've looked at the Canisp Shear Zone near Clachtol, the fabric within the gneiss is almost vertical, in contrast to the surrounding gneiss. To be honest, I just enjoy looking at the beautiful patterns produced by the banding and folding. I get a kick out of spotting something like a refolded fold and knowing that it's evidence of more than one phase of folding.

    Thanks again for the bigger picture Jim.

  8. #28
    ‹bermensch Jim Chalmers's Avatar
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    Just for fun, I looked up some info on what the climate was like back then. Some clever folk have been able to use the chemistry of both the Stoer and Applecross Groups to deduce that the Stoer Group was deposited when/where there was a long hot dry summer and a cool winter. Rain was confined to the winter and amounted to a total of 500-1000 mm per year. They were probably at a latitude of 10-30 degrees at that time.

    The Torridonian rivers flowed in a more temperate climate, similar to present-day Pennsylvania and New Jersey. The mountains above today's Outer Hebrides were probably high enough to be snow covered, at least in the winter. One major difference would, of course, be the total lack of vegetation back then. Nowadays erosion and sediment transport in a temperate climate is limited by the almost continuous vegetation cover. Back then, the bare ground would have reacted much more like one of today's deserts, hence the coarse nature of the sediments, quite unlike those deposited today in temperate climates.

    Jim

  9. #29
    Mini Goon
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    >>> All the information boards in Assynt were temporarily removed for maintenance when I was there...back in the first week of May.

    Shame: I was up there in the summer of 04 and I thought they were superb. Knowing very little about geology, a little understanding of how the landscape had formed added enormously to my appreciation of it.

    Having stumbled across one of the boards almost by accident, I made a point of stopping to look at all of them as I made my way north. I was so impressed by them that I decided to write to the people who had put them there when I got home to say thankyou. Unfortunately, like so many good resolutions, I never did anything about it, I'm sorry to admit.

    I particularly remember one a few miles south of Kylesku, I think, where the road has been cut into the bedrock, the scar revealing a cross section of bands & folds hidden below the surface. The board nearby gave a brilliant, grand scale anatomy lesson.

    It's a great area, too, for "erratics", boulders dropped by retreating glaciers at the end of the last ice age (-?- someone correct me if I'm wrong about this). A bit further north, there is an incredible one the size of a house perched on a pinnacle above the sea a couple of miles east of Durness. So carefully positioned, it looks as if it must have been put there on purpose. Wish I had a photo of it. I wonder what on earth people made of those before the proper explanation was identified?!

  10. #30
    Ultra King MoS's Avatar
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    Amazing, how a detailed picture of an ancient environment can be built up from careful observation and analysis of its rocks. I think it really helps to have the comparison with what goes on in parts of the world at the present. Interesting to note that the lack of vegetation had such an effect on the type of deposition. I suppose the same thing happens when forest disappears and there is no protection for the soil and underlying rock, allowing it to erode and wash away relatively quickly.

    I?d love to get hold of a book which illustrated the environments in which some of the major rocks were deposited. A sort of artists impression of the landscape of the time

    The mountain range that gave rise to all that eroded material must have been quite something. There are 2000m of sediments in the Stoer group and another 6000m of Torridon/Applecross sediments above it. As Jim said, the second group cover an extensive area.

    Thanks for the additional info. Jim.

  11. #31
    Ultra King MoS's Avatar
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    Ian, I?m interested in hearing your take on the information boards. It?s always good to hear people getting enjoyment from understanding the rocks. As you say, it adds a whole new dimension to appreciating the landscape. The road cuttings at Kylesku and Northwards are quite something, they produce lovely cross sections through the gneiss. I first visited the area before the bridge was built and there was a little car ferry which took you across. The road was all single track then. I was disappointed when they ?improved? it but I guess the transport link is good for the fishing at Kinlochbervie. It?s also a great road to drive along and although the rock now exposed is unnatural it?s stunning all the same.


    I don?t know the erratic at Durness you refer to. Yes erratics were left behind when the ice melted, just as a river would deposit its load if it dried up. Glacial deposits are important in working out the direction that the ice moved and erratics often travel long distances. If anyone is heading up Quinag, there are Moinian and Lewisian erratics (small ones) up there resting on the Cambrian quartzite.

  12. #32
    ‹bermensch ~Batman~'s Avatar
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    Of course, not just the landscape would have been different during pre-cambrian times.....the atmosphere was oxygen depleted and CO2 rich. This affects weathering rates, as a warm atmosphere rich in CO2 precipitates more than a cool atmosphere - the CO2 can form carbonic acid - and leads to increased weathering - and increased CO2 drawdown.

    About 700Ma ago, the Earth was plunged into an almost total ice-age, and only volcanic action and positive feedback of the greenhouse effect pulled us out of the 10Ma long deep freeze.


    The rifting formed extensional basins which would account for the torridonian lying unconformably on differing strata.
    As these basins filled, there was more tectonic rifting and passive basin subsidence - this is one way the 300-400Ma long deposition occurred.

  13. #33
    ‹bermensch ~Batman~'s Avatar
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    Ignore the bit about "Snowball Earth", it has nothing to do with this discussion - just interesting thats all

  14. #34
    Mini Goon
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    >>> It?s also a great road to drive along and although the rock now exposed is unnatural it?s stunning all the same.

    Agreed - not many good reasons for cutting roads into rock, but that's got to be one!

    >>> If anyone is heading up Quinag, there are Moinian and Lewisian erratics (small ones) up there resting on the Cambrian quartzite.

    I must have walked over/through them without realising. I've recently begun to think that this is a whole aspect of my hillwalking that I'm just not benefitting from. I'm always aware of my "distant" surroundings, but never really pay that much attention to what's under my feet, unless it's threatening to tip me over.

    Time I invested a bit of effort learning some geology, and some natural history, I think!

  15. #35
    Mini Goon Helen Hedley's Avatar
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    The BGS do a special geological map of assynt which is likely to contain the stratigraphical sequence and may cross sections of the area. Also worth a read is the sheet memoirs that go with it.

    Ben

  16. #36
    Mini Goon Helen Hedley's Avatar
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    oh poo! seem to be logged on as mum!

  17. #37
    Ultra King MoS's Avatar
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    Where does the evidence for ?snowball earth? come from then Batman. That?s too interesting a concept to ignore. And ?positive feedback of the greenhouse effect?? Not heard that before.

    Ian, I am lucky in that my partner is great on the Natural History, I?m afraid that he doesn?t share my enthusiasm for geology though He usually uses the time I spend looking at rocks for taking photographs so it works out OK.
    If you are looking for some books to start you off, there were some recommended on the ?Why Scotland and England are joined at the hip? thread some time back. One was Geology and landscapes of Scotland by Con Gillen, I?ve just got a copy and it does look good and very readable at first glance. The Leeds Uni. web site which Oot referred to in his post -
    http://earth.leeds.ac.uk/assyntgeology/site_map/
    is excellent. Enjoy!

    Ben, I?d love a copy of the Assynt map. I had planned to use the maps in ?Assynt District of Sutherland - geological excursion guide? to mark some of the main features and rock types on my ordnance survey map. Can?t have enough maps though.

  18. #38
    ‹bermensch ~Batman~'s Avatar
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    I'll have more time tonight to go into it, as my work computer blocks this forum for some reason, but there is lots of evidence.

    As for feedbacks, there are several cycles in nature - carbon, water, rock, sulfur etc. Eg. for part of the short term carbon cycle, increased CO2 in the atmosphere leads to increased atmospheric temperature which increases precipitation which increases weathering which actually REDUCES atmospheric CO2. This is an example of a NEGATIVE feedback loop (because one part of it has a negative forcing effect).
    The positive feedback I alluded to in an earlier post, concerns albedo (reflection of solar radiation) a snowball Earth would reflect a lot of radiation and could (theoretically) never warm up enough to melt!

  19. #39
    Ultra King MoS's Avatar
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    If anyone would like more information on the Snowball Earth theory, there is a summary from a BBC docmentary broadcast in 2001 here

    A recent article in New Scientist asks "Was 'snowball Earth' more of a slushball?" and reads -

    THE idea of a snowball Earth - a period when the planet was completely enveloped in thick sheets of ice - has received a knock. Evidence of prehistoric photosynthetic algae indicates that tropical oceans did not freeze over completely at the time.

    Geologists accept that Earth was in a deep freeze twice during a period from about 750 to 580 million years ago. But they disagree over whether it was a snowball or a "slushball" with areas of thin ice or open ocean.

    Now, Alison Olcott of the University of Southern California in Los Angeles and her team have found evidence for a slushball Earth in rocks in Brazil. These rocks were deposited during the deep freeze and contain biomarkers of a community of photosynthetic algae and other microbes. These could have lived in open water or under thin ice, but not under thick ice, which would block the light needed for photosynthesis.
    From issue 2513 of New Scientist magazine, 20 August 2005, page 18.

    I guess the debate goes on. To bring it back to Scotland, the Port Askaig tillite of Islay can be correlated with similar glacial deposits in Norway, suggesting widespread ice sheets. We need to look to the Dalradian rocks for evidence of glaciations which would tie in with the Snowball Earth theory.

  20. #40
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