ASSYNT ROCKS

[MoS]

Just back from a week in Torridon/Applecross followed by a week in Assynt. I guess many of you will know the mountains but unless you are into geology, do the rocks say much to you?

A brief overview .

Lewisian gneiss basement, some of the oldest surface rocks in Europe, metamorphic and highly altered rocks which show beautiful patterns of different coloured folded bands. The Lewisian gives the classic knock and lochan landscape found West of Loch Assynt. Sitting on top of the Lewisian is the Torridonian sandstone. Much of this has been removed by erosion and the remnants can be seen in the form of mountains like Cul Mor, Canisp, Suilven and Quinag. Many of these mountains have a quartzite cap - the first of the Cambrian rocks deposited. These rocks are undisturbed and often referred to as the unmoved foreland.

To the East the situation is much more complex and major thrusts - the main one being the Moine Thrust - have caused huge sheets of rock to be moved 10s of kilometres. These earth movements were the final stages following the collision of continental plates which gave rise to the Caledonian Mountain Belt. The result is that older rocks often appear on top of younger ones - not the normal succession you get where rocks are undisturbed. One example is where the Moine Thrust cuts through the Stack of Glencoul, the older Moine rocks sit on top of the younger Cambrian ones. The view towards Stack of Glencoul near Kylesku bridge is one of many where it is possible to pick out the different rock types and see how features like thrusts, faults, folds and unconformities have shaped the landscape.

Some of the car parks have pictures and descriptions on boards - temporarily removed for updating at the moment. I wondered if people appreciate these or are they an intrusion? Should people be left to do their own research or is this sort of information and places like the Knockan Visitor Centre welcome in such a wild and beautiful landscape?

[AlisonS]

There are also the caves near Assynt where bones thousands of years old were found; including bear, reindeer, wolf and humans. This proved important to our understanding of environmental history in the area. It was mentioned in a documentary recently. There's a board near the car park.

[Chris Townsend]

I think information boards in car parks are useful and worth having. They're certainly much less of an intrusion than the car parks themselves! The siting and disguising of car parks is another matter.

I remember visiting the Knockan Visitor Centre for the first time many years ago and being fascinated by the information provided.

The geology of the area is impressive but not really explicable even in a basic sense without some background information.

It's a wonderful area, one of my favourites. Any information boards should also strongly promote the value of the area and the need to protect it.

[Richard Kermode]

It has just been declared a geopark I think as well, which is a great step to preserving it all and I think that is why the boards have been removed to be upgraded and hopefully improved, but time will tell!!

[Chris Townsend]

The Geopark was opened on the 12th of this month.

See:

http://www.assynt.info/page17.html
http://www.greentourism.org.uk/Defau...03.Lang-EN.htm

[BigDug]

You are quite right Richard K.

The following link has all the info about Geopark:

http://www.assynt.info/page17.html

Also, here's a link to a nifty wee site that tells you about the various rock formations:

http://earth.leeds.ac.uk/assynt/

And the following links have some superb panoramic views of the region. If you don't have Broadband or better you might want to forget these two:

http://www.camvista.com/scotland/pan...u/kylesku.php3

http://www.camvista.com/scotland/panoramas/suilven.php3

[BigDug]

Ah! Chris beat me to it

[Chris Townsend]

Not for the panoramic views. I hadn't come across those. They're wonderful.

[MoS]

Thanks for the camvista links Oot. Not seen those before - specially like the Suilven one.

I didn?t get to the Knockan visitor centre this time, I think it has been updated recently. I looked at the Knockan Crag leaflet and was a bit put off to see that they have Peach and Horne portrayed as rocket men in a comic strip. I guess there are many ways to bring geology alive but I think there are enough ways to inspire the imagination without resorting to this sort of distortion. I can?t comment first hand, but I do have a problem with developing such a scientifically sensitive site as Knockan cliff with trails and art work. Maybe it will encourage a lot more people to physically go and get some hands on experience of the rocks. I have mixed feelings.

The rock route is outlined on the reverse of the Knockan Crag leaflet and ties up with the information boards. I have an old leaflet called the Assynt geological motor trail which is fairly basic and an excellent over view. It includes photos of views like Knockan cliff, looking towards Spidean Coinich (Quinag) from Stronchrubie and the double unconformity across Loch Assynt from Ardvreck Castle. Some of the photos have the rock types and thrust planes superimposed. Some are reproduced with the rock types shown in different colours. The leaflet was all the visitor needed to get a feel for the geology and there is plenty more information available in the form of books and guides. I?m not keen on the boards. I?m all for inspiring interest in the rocks, but they seem a bit ?in your face?. The motor trail leaflet had the advantage that you could read it at leisure and when driving off to walk, stop to check out the area from a distance and know what to watch out for. You then had it with you to refer back to. Information boards are all very well to explain to somebody what they are looking at there and then. I hope some people go on to wanting to learn more.

[Chris Townsend]

I haven't seen the current boards as it's a few years since I was over that way. I have that motor trail leaflet from an early visit. I'll have to go over there soon and see what the current situation is. It's clear the Geopark is going to mean much more promotion for the area.

[MoS]

Information boards aside and leaving the complex thrust zone for a moment.

Rocks tell a story, wandering near the coast, I take a trip back in time and imagine the following.

The loch and knockan landscape of the Lewisian (minus the mountains) must look quite similar to the rock surface some 1000 million years ago. The sandstone rests on the gneiss unconformably - this means that there was a long time gap and then conditions changed to an environment where deposition could take place. At that time these old rocks (which were also connected to North America and Greenland) were located much closer to the equator and the climate would have been semi arid. There was probably a large mountainous land mass to the North West, beyond where the Western Isles exist today. The mountains were being eroded, perhaps by flash floods, and the sediments were carried quickly to be deposited in huge alluvial fans. We know this because of the high proportion of feldspars in the rock, these tend to get destroyed unless transport is rapid. If you look at the layers of sandstone - Clachtoll and Stoer are good sites - you can see signs of mud cracks on some of the bedding planes. So there would have been times when the water dried up. There are also ripple marks on bedding planes and cross bedding, so at those times the water was shallow and there were currents. There are places where the base of the sandstone is full of large angular blocks of metamorphic rock, these deposits look like they could have been a scree slope. At this time there was virtually no life on earth. But there is evidence of algae deposits, the first signs of life in the fossil record. There is even a layer thought to represent a volcanic mud flow, but no sign of the volcano which caused it.

Hope this view is OK by any geologists. Simplified I know but it helps me appreciate it. If anyone wants to offer a simplified view of the Moine Thrust zone it would certainly help me get my head round it.

[~Batman~]

Sounds about right to me. As a geology student (rock-head) this is somewhere I'm going next July for a week. Already can't wait!


Geology rocks my socks!

[~Batman~]

Ripples can also form by wave action.

Presence of feldspar in sedimentary rocks (known as arkose) indicates structurally immature sediment - as you say deposited rapidly (close to source of sediment).

Life has existed on Earth for around 3850 million years.

Complex (multicellular life) begun around 550 million years ago (cambrian explosion)

The lewisian gneiss is much older than 1000 million years.

[~Batman~]

Oh, I forgot - the Earth is 4550 million years old!

[MoS]

Batman, I think the Lewisian is about 2500 million years old. I was thinking of the time when the first Torridonian sediments were being deposited as 1000 m.y., but it may have been earlier.

Thanks for putting the numbers into context, they are pretty staggering without some sort of scale to hang them on.

[Chris Townsend]

Lewisian rocks are 3 billion years old according to Con Gillen's Geology and Landscapes of Scotland, 2900 my according to John L. Roberts The Highland Geology Trail. I don't think the odd 100my matters :-)

[MoS]

I'm happy to give or take a few hundred m.y. As dating techniques advance, and info. is updated, they do seem to be getting older.

I have a problem with dates and the Torridonian sandstone - maybe I?m missing something obvious.

I know that the Torridonian was deposited over a long period of time, 300/400 m.y. and within it are subdivisions. The Stoer group being the oldest, then the younger Torridon group and within this, the Applecross formation. Now all comonly known as Torridonian. I?ve looked at the unconformity in the Stoer area and on the lower slopes of Slioch. If there?s a time gap of 200 m.y. between the times when these rocks were deposited, how come they both rest on the same Lewisian surface. Could the topography of the Lewisian landscape account for the first Torridonian sediments to be deposited in the Stoer area 200 m.y. before the first ones in the Torridon area.

Am I being dim here? The old Lewisian surface rose up to 600 m. but would that be sufficient relief to account for the time gap?

[Ben Hedley]

The Cap of quartzite on top of the torridonian sandstones would prob have formed in a similar kind of way as quartzite is just a metamorphosed quartz rich sandstone, abundant throughout scotland.

Unconformities are a tricky subject. for example, in Bardon quarry (leicester) Triassic marls and mudstones sit directly on top of the pre-cambrian (Charnian) volcanics. Originally some of the Scottish higlands were of almost himalayen proportions. Now is missed the strat module this year cos im an industrial geol student and am trying desperately to remeber what happened in assynt but i think the basement has undergone massive amounts of tilting and folding. this would certainly allow erosion and weathering to remove preceding sequences over a period of Xmy. Events such as flash floods or marine transgressions leading deposition in a deltaic environment or foreland basin would give rise to the torridonian sandstone.

I will try and borrow t'other halfs nots from strat and let you know!

[Ben Hedley]

oh and being a geologist...i can't spell!

[Dave O aka Jungle Dave ;)]

You're a scientist Ben its allowed,