Rock identification ?

[ChrisJB]

[quote=Dorts;871617]I frequently visit Black Rock Sands near Morfa Bychan on the Llŷn Peninsula. At the Western end of this fine beach is an out-crop of Cambrian rock which forms cliffs to 80ft, (25m) or so, in which are some fascinating 'sea-caves'.
There is a 'fault' on one part of these cliffs which are of great curiosity to me, trying to imagine the forces that formed it.



It appears that geologist's are not entirely certain how such formations among these Cambrian rocks are caused:-
"The often intensive metamorphism which these originally volcanic and sedimentary rocks have been subjected to and their generally faulted relationship to neighbouring rocks has meant that geologists’ understanding of them has been limited."

We can only guess!
Dorts.[/QUOTE

The linear feature evident in you photos look like a a small sill.

Chris

[Dorts]

Originally Posted by ChrisJB

The linear feature evident in you photos look like a a small sill. Chris

Chris, can you elaborate? Am I right in thinking that the rock in the center has forced its way between the rock below and above. And would this have happened while the rock was in its present state, or in an igneous state?

Wiki has a quite thorough piece here: http://en.wikipedia.org/wiki/Sill_(geology) , which may require simplification.
Thanks.
Dorts.

[ChrisJB]

Originally Posted by Dorts

Chris, can you elaborate? Am I right in thinking that the rock in the center has forced its way between the rock below and above. And would this have happened while the rock was in its present state, or in an igneous state?

Wiki has a quite thorough piece here: http://en.wikipedia.org/wiki/Sill_(geology) , which may require simplification.
Thanks.
Dorts.

Yes, you are right. A sill is an igneous sheet of rock (molten when implaced) that is forced along the bedding planes (parallel to the bedding) of the country rock (a dyke is similar, but cuts across the country rock). The Whin Sill of northern England is a good example. Looking at this one I'd say this sill was emplaced after the country rock has been metamorphosed. I say this as the sill has joints that do not match the foliation of the surrounding country rock. By this I mean, if you look at the sill, it has marked joints that run at 90 degrees to the alignment of the country rock. I suppose a bit like the joints in the basalt at the Giants Causeway.
Having re-read this, it might sound a bit confusing! Sorry.

Chris

[Dorts]

Originally Posted by ChrisJB

Yes, you are right. A sill is an igneous sheet of rock (molten when implaced) that is forced along the bedding planes (parallel to the bedding) of the country rock (a dyke is similar, but cuts across the country rock). The Whin Sill of northern England is a good example. Looking at this one I'd say this sill was emplaced after the country rock has been metamorphosed. I say this as the sill has joints that do not match the foliation of the surrounding country rock. By this I mean, if you look at the sill, it has marked joints that run at 90 degrees to the alignment of the country rock. I suppose a bit like the joints in the basalt at the Giants Causeway.
Having re-read this, it might sound a bit confusing! Sorry.
Chris

Not at all confusing Chris. A clear explanation put into words I understand.
Thanks very much.
Dorts.

[Paul mabbott]

Most of the exhibits are still there if you walk high enough although the actual rock samples to pick and examine have gone! Some have been replaced by more 'hands-on'(ironically ) exhibits about vulcanology, astronomy &c but still very worth a visit - now called the Earth Sciences Division of the Natural History Museum.

Originally Posted by Dorts

,,,,,,,,,As a young lad living in London, I would spend much of my time in those great museums we have. The most appealing to me was the Geological Museum in South Kensington, by the Nat. Hist. Museum. It was always virtually empty, and in those days you had free access to the thousands of specimens in the multitude of specimen drawers beneath the display cabinets. Yes, you could actully just pick them up and examine them! I have been hooked ever since, and in my botanical pursuits have been gratefull that I took such an interest in those early years. But how I wish I'd learned more. I cannot go anywhere without looking closely at rocks and their formations. Geology to me really is the most fascinating of all the earth sciences. And I would love to see more in that area on this particular Forum.
Dorts.

Another great source of urban mineralogy are various books and leaflets which have been printed about the stones of London buildings. Spent many a happy hour in my son's youth searching out interesting rocks and fossils: the South Bank buildings are a treasure trove!
Many listed here:
http://www.londongeopartnership.org....%20summary.pdf
Anything similar produced in other cities?

[Tringa]

I'd forgotten I had this one.

Gneiss



Dave

[Acutipuerilis]

Very gneiss.

Just spotted this thread, but thought I'd say hello. I normally lurk on the insect forums, but I'm a professional palaeontologist with general geology background; I'm happy to (try to) id or comment on a few rock photos if anyone has any burning questions.
Most of my fieldwork is in Wales, despite living in China - I'm coming back over for most of May to work near Bala... bliss.
Ta-ra,
Joe

[Dorts]

Hi Joe. Thanks for you offer of help with ID's etc.
As you probably know, I spend most of my time on the Wild-flower Forums, but like keep an eye here for anything of interest, as I still have much to learn in this area.
Dorts.