Geology Question ...^

[mintokames]

Hi,

Can anyone shed any light on the following formation. Curious as to how it may have formed. South side of Soay ....



Thanks

Rob

[Ceegee]

The top and bottom are Torridonian sandstone. This is a pre-Cambrian megasequence of mostly fluvial (i.e. river derived) sands and grits. The actual strata are almost horizontal, but the strata are divided into packages, called foresets, of cross-bedding. This is where sand bars in the ancient river channels migrated along, like underwater sand dunes, leaving locally inclined bedding, at the angle of repose of the cascading sand (if that makes any sense).

In the middle is a much more recent dolerite sill. These are slightly more evolved magmas than basalt, and were the last stages of the ancient volcanic centres associated with the Atlantic opening in this area around 55my ago. Basically the Cuillins, Rum, Skye etc.) are ancient dissected magma chambers filled with the residual crystal mush after all the plateau basalts (Antrim, Staffa etc.) were erupted. The residual magma got squeezed out as a series of horizontal sheets (sills) and radial vertical dykes as the centres collapsed. If you looked carefully, you would see a glassy margin where the sill chilled as it was injected into the cold rock, and a "cooked" contact against the sandstones.

Steve

[Bards]

Err, yeah... what Steve said stacks up; as it were ;-)
Weathered cross-bedded sediment stratas on their own I generally find absorbing to the blinkered exclusion of any interesting birds flapping around to the wonder of ornithologically inclined mates. I'm from Jurassic country but the Proterozoic appeal up over there holds huge appeal.

Keep on rockin'.

Bards

[rockhopper]

The top and bottom are Torridonian sandstone. This is a pre-Cambrian megasequence of mostly fluvial (i.e. river derived) sands and grits. The actual strata are almost horizontal, but the strata are divided into packages, called foresets, of cross-bedding. This is where sand bars in the ancient river channels migrated along, like underwater sand dunes, leaving locally inclined bedding, at the angle of repose of the cascading sand (if that makes any sense).

In the middle is a much more recent dolerite sill. These are slightly more evolved magmas than basalt, and were the last stages of the ancient volcanic centres associated with the Atlantic opening in this area around 55my ago. Basically the Cuillins, Rum, Skye etc.) are ancient dissected magma chambers filled with the residual crystal mush after all the plateau basalts (Antrim, Staffa etc.) were erupted. The residual magma got squeezed out as a series of horizontal sheets (sills) and radial vertical dykes as the centres collapsed. If you looked carefully, you would see a glassy margin where the sill chilled as it was injected into the cold rock, and a "cooked" contact against the sandstones.

Crikey, that's technical (but very accurate).
Basically, top and bottom are rocks that have been laid down as a sediment and in between is a rock that has been subsequently injected as a molten intrusion.

Isn't geology great!!

Rog.

[Ceegee]

Crikey, that's technical

Well, I didn't want to insult anyone's intelligence with a "soundbite" explanation. I was working on the principal that if 95+% of the contributors here can grasp the nuances of meterology, vector navigation and hydrodynamics (which judging by past discussions most of us can in spades), then a little geology 101 is withing the capability of nearly all ;-)

Nothing irritates me more than the creeping tendency over the last 10-15 years for so-called documentaries to dumb-down and increasingly gloss-and simplify their subject matter. Disect any "Discovery" type docu and you'll note that 60% is repetition and restatement of some fairly simple premise with no detail or analysis.

Steve

[grumpyoldman]

I think you got the level just right.

An O'level in Geology obviously wasn't wasted!

I tend to agree with you on the 'dumbing down' of scientific programs. The BBC's Horizon program used to be Educational not Sensational. I gave up reading New Scientist for the same reason.

[mintokames]

Folks,

Many thanks for the replies. Not even having an 'O'' level in Geology, Steve's second paragraph was a bit over my head. But as I look as if I might be snowed in this weekend, I suspect Wikipedia will be my friend :-).

Seriously though, thanks for the replies, and I will be trying to understand it when I get home.

Cheers

Rob

P.S. Currently working through 'Death of an Ocean' to get some of the basics ....

[rockhopper]

Well, I didn't want to insult anyone's intelligence with a "soundbite" explanation. I was working on the principal that if 95+% of the contributors here can grasp the nuances of meterology, vector navigation and hydrodynamics (which judging by past discussions most of us can in spades), then a little geology 101 is withing the capability of nearly all ;-)

Nothing irritates me more than the creeping tendency over the last 10-15 years for so-called documentaries to dumb-down and increasingly gloss-and simplify their subject matter. Disect any "Discovery" type docu and you'll note that 60% is repetition and restatement of some fairly simple premise with no detail or analysis.

Yup, agree with all your sentiments Steve. I just didn't want to leave the other 5% out !!

:-)

Rog.

[tg]

Steve,

Repose... I thought you meant get ready for another picture:-)

5% wasthatthen..!

Tim

[John N]

Fascinating. Level 2 Geology GCSE (a bit less than 'O' level). Loved the subject at school but all the teacher could offer was enthusiasm - he got no support from the school. I think they saw it as an 'easy' stat. and not much else - a sign of the way things were going to go, and this back in 1971.

I keep thinking I ought to check out the Open University - see what they have to offer.

In full agreement with the comments on current documentaries, HORIZON is but a shadow of it's former self - and whatever happened to that excellent C4 copy EQUINOX?

[rockhopper]

Plus... it was one of the subjects you used to get to go on field trips for. We went to Bristol !!!!....not that interesting geologically but great digs and Scarborough/Whitby/Robin Hoods Bay/Filey Brigg which was brilliant geologically - and we got to stay in a hotel.... 20 or so 15 year olds.... in a hotel (with a serving bar!)... for 4 days.... those teachers were very committed and brave!!!!.

Rog.

[Ceegee]

...and we got to stay in a hotel.... 20 or so 15 year olds.... in a hotel (with a serving bar!)... !!!

Field trips were always great fun. In the good old days before geology became a "desk-top" science 2-3 week+ trips a year were the norm on any degree course. Britain has some of the world's most diverse geology - it's no co-incidence that we were the cradle of the science, and the best place to see it is invariably along the coast line. Going on and/or leading field trips you get to see just about everything, that you hopefully can also come back and paddle along too. East Yorkshire, Wessex (Jurassic) Coast, Lizard/Cornwall, Pembroke and Anglesey, Mull to Skye and Applecross/Torridon-Sutherland have just got to be some of the best (especially when viewed with a teenage hangover ;~s)

Steve

[bastonjock]

I enjoyed your technical answer very much,I have never studied geology but I do find the subject very interesting.I often wonder as to how formations are formed.I once found some egg sized pebbles at the bottom of a scottish loch,they looked to all intent and purposes to me as stones in the making,they were still soft and crumbled in my hand,I remember thinking that given enough time they will be stones.

[John N]

Not that I (we) felt that hard done by at the time but our sole 'field' trip (our only trip in 5 years of secondary education) was a day trip to the North Downs to see the geologicaly exciting Box Hill. Unfortunatley it was all under the grass.
It was a hot day and I recall thinking a pint of best would go down well now, (not ever having had a pint before) but for a few short years lager got in the way. Sadly, the teachers enthusiasm was such that we never got see any of the fine hostelries in that area.

[Ceegee]

especially when viewed with a teenage hangover ;~s)

disclaimer required here I'll warrant...

I refer of course to 18 and 19 year-old (adult) undergraduates drinking as hotel guests in a licensed establishment after regular hours, and in the company of "responsible" academic staff, and in no manner or form do I intended to encourage or condone under-age or binge drinking, ahem.

Steve

@bastonjock - no need to have studied geology to enjoy it. The science was invented as a hobby by philosophical gentlemen amateurs and bored clergy in the 18th Cent. Anybody can and should feel entitled to have a go.

[PeterG]

Evidence of recent (ice age) effects is also fascinating. Those hanging shingle beaches on Loch Sheil (Freshwater Harry Potter habitat, not the sea one). An impossiblity unless they formed from stream deposits running alongside or over the ice.

I love the basalt transitions along the coasts of Mull and adjacent islands.

[Owen]

I keep thinking I ought to check out the Open University - see what they have to offer.

I did that, be warned it takes all your money and all your free time, at the end you get a nice certificate that employers laugh at. Still fascinating subject and very worth doing.

[Somerled]

at the end you get a nice certificate that employers laugh at.

I am both an OU graduate and an 'employer' and have never come across this view. I don't mean to be defensive, but just surprised, as the quality of learning and materials are excellent.

[bastonjock]

[/quote]

@bastonjock - no need to have studied geology to enjoy it. The science was invented as a hobby by philosophical gentlemen amateurs and bored clergy in the 18th Cent. Anybody can and should feel entitled to have a go.[/quote]

Ive sent off today for 3 books on geology,I have in the past hired a helicopter to take a look at mount Kilua (SP) on Hawaii bubbling and smoking away,an other place of great interest was the Culdera on Santorini.im looking forward to learning more.

[John N]

I keep thinking I ought to check out the Open University - see what they have to offer.

I did that, be warned it takes all your money and all your free time, at the end you get a nice certificate that employers laugh at. Still fascinating subject and very worth doing.

In a few years, if not earlier I anticipate having lots of free time. And I don't expect to be to concerned about the employment viewpoint.
I'd just like to know more about something I have always had a latent interest in but done little about.

[Ceegee]

I can really recommend: The Map that Changed the Worldby Simon Winchester. Not geology per se, but the biography of William Smith, Oxfordshire farm boy and self-taught navigation engineer, who first mapped the geology of the UK. Looked down on by the "establishment" and after periods of penury and debtor's prison, he was finally recognized for his insight and achievements. Real Dickensian stuff. In many respects he was Britain's "other Charles Darwin".

Steve

[Mark R]

Quite good...
http://www.amazon.co.uk/Hidden-Landscap ... iversguide

[janet brown]

John N

I keep thinking I ought to check out the Open University - see what they have to offer

OU Geology courses are excellent: in the 1990's went straight into a second level Geology course, then Science Foundation, then another second level How the Earth Works. They prefer you to start with foundation course, but I had no problems coping having taken geography A level in the late 70's. Quite a few courses have a compulsory summer school, I did my Geology one at Durham, with good field trips to Shap (granite quarry) and Staithes on N.Yorkshire coast (sedimentary with fossils).

Main problem I had was that the OU year runs from Feb-end of Oct, and we'd moved into a new house with garden that needed a lot of work, plus son became more time consuming!

Janet

[Owen]

Main problem I had was that the OU year runs from Feb-end of Oct, and we'd moved into a new house with garden that needed a lot of work, plus son became more time consuming!

Janet

I think they now run two start dates per year, the Feb start and one following a more normal school year. This was just coming in as I finished, so I didn't take in all the details. Not sure whether its all courses or just a selected few.

[Helen M]

Ahhh - Geology

I love Geology. My dad used to take me fossil hunting at a young age - mum and sister sat in the car while dad and I scoured quarries etc for fossils.

I got an A level in it. Would have loved to be a Geologist.

My uncle's one Dr Jeff Wilson.

He discovered Macaulayite. Used to work for the Macaulay Institute.

Have a house full of random rocks and fossils. You can't beat natures art.

As for field trips - I did chesil beach and Moffat. Boy did those teachers have their work cut out 28 years ago!

Amazing subject.

H - x

[Jim]

I will always remember our geography field trip to Dorset (which was mostly geology), but perhaps for all the wrong reasons.
Several pupils were suspended, but only briefly because the suspension was illegally imposed.
Lulworth Cove (limestone outer, green sandstone inner?) and Durdle Door (sedimetary limestones), Kimmeridge shales, Studland and Swanage chalk, More limestone at Portland Bill and the grading of the stones on Chesil beach - perhaps I do remember it for some of the right reasons too?

[Ceegee]

Jim, you might be interested to know (since you're involved in the offshore industry yourself) that the "Wessex Basin" (as you just described it above) is a classic example of a hydrocarbon province, and much a studied example in petroleum geology courses in the UK (seeing as the alternative, the North Sea is inconveniently under water).

Steve

[Bards]

I believe the 'nodding donkey' at Kimmeridge is the oldest productive oil well in the world, and is a (tiny!) part of the largest onshore oilfield in W Europe. Some of the oil-shales (although not believed to be a direct source for the productive oil beds and quite sulphurous) are so hydrocarbon-rich they are inflammable, though I can't remember any of the infamous spontaneous flaming cliff incidents for the past 10 years or so.
The importance of local Dorset strata in the early days of geological study as a modern science have led to the terms 'Kimmeridgian' and 'Portlandian' still being recognised worldwide today as names for discreet geological Ages. The contentious Broad Bench surf feature at K-Bay is a ledge formed by a relatively resistant limestone bed in this complex. Maybe I should bin the spare paddle, and get an Estwing hammer fixed to the rear deck instead?!

On the subject of field trip jolly japes, I remember up in the Pennines a foray was mounted to the nearest (but still a good walk) boozer for some under-age supplies. The idea that getting Merrydown and Special Brew from the boozer to fashion some concentrated Snakebite contraband for ease of portage seemed logical at the time... though the fall-out was not what the school had in mind when me and Bill were let in! 25 years later he's one of my paddling mates; our fates were sealed early!!!

Fair play to Rob the OP who's let the genie out of the bottle on this one; maybe sea-paddlers beards aren't water-activity specific, but have been acquired during some of our geological histories?!

Bards

BTW, I'm not always this boring... ;-)

[Jim]

I believe the 'nodding donkey' at Kimmeridge is the oldest productive oil well in the world, and is a (tiny!) part of the largest onshore oilfield in W Europe. Some of the oil-shales (although not believed to be a direct source for the productive oil beds and quite sulphurous) are so hydrocarbon-rich they are inflammable, though I can't remember any of the infamous spontaneous flaming cliff incidents for the past 10 years or so.

I forget details, but do recall being told that in times past the oil shales from Kimmeridge were used for fuel. Would they have been used 'as is' (tossed in the fire?) or processed in some way to extract a fluid to put in lamps/stoves?

The importance of local Dorset strata in the early days of geological study as a modern science have led to the terms 'Kimmeridgian' and 'Portlandian' still being recognised worldwide today as names for discreet geological Ages.

Is Devonian also a term for a geological age? There are cross sections of river beds visible in the Grand Canyon (the spot I am thinking of is a side canyon but I can't decide which one off the top of my head, maybe stone?) which are called Devonian rivers. I find it fascinating when you get a section in which you can see some some of the past which was buried, then exposed and weathered for a period, and then buried again. A less impressive but equally important sight is the great non-conformity, where several layers of sedimentary rocks are missing from the section, the layers skip a few hundred thousand years (because the region was pushed up in the past and those layers eroded away before the newer layers were deposited).

By the time you get into the granite gorges you can see about 60 million years of the past (most of it pre-history) in the section. Presumably the granite and schist we have in Scotland is of a similar age, we just don't have all the later periods piled on top of it?

[Bards]

Technically an 'Age' such as Kimmeridgian is a much smaller subdivision of timescale than an 'Epoch', of which Devonian, Jurassic, Carboniferous etc are examples. Eons, epochs, ages, era, periods... that's where all the nomenclature gets more than a tad confusing and is best neatly sidestepped ;-)

My dim memory suggests that the Lewissian gneiss which underpins the Pre-Cambrian shield up there in Scotland is one of the oldest surface rocks on earth. The mountain range which sat on it was huge and is now just a residual stump, so all the overlying strata have been weathered away, doubtless having been recycled for several generations of rock formation since; I don't know if there's a valid guess at it's maximimum height/altitude achieved, but it's a nice idea to imagine it as having been the Himalayas of it's day (or indeed it's epoch, era, period, age...well, yes... whatever!!!). Where the granite intrusion fits into all this I don't know, but would assume it to be relatively far younger than the Lewissian complex...

The oil in the shale down here could be extracted to either fuel street lights, or be used as an ingredient for various products such as naptha. Even when the oil was extracted before burning it still made by all accounts a ridiculously unclean whiff, and it really doesn't bear thinking about the hideous fumes given off when burnt raw for various industrial purposes - no emission laws back then! I don't think it's use survived into the 20th Century; I suppose it was really Dorset's closest equivalent of peat back in the day... not sure there was any distilling associated with it, though!!!

That canyon must be an amazing geological cross-section; apart from sea cliffs all we have round here not too weathered are from new road cuttings af a 'fun size'. 'Non-conformities' are like sacred relics amongst hammer-swingers; you have seen the promised land with that one Jim!

Bards