Identifying Rocks - Geology ?

Globalti

Legendary Member
Yes it's igneous - lots of crystals. Some of those stones are quite rounded, maybe from river action, which will make the wall harder to build than if it was nice flat even slabs of sedimentary stone, like this:

Dry-stone-wall-Sand-stone.jpg


Fascinating things, are dry stone walls. I love looking at the different styles.
 

Firestorm

Veteran
Location
Southend on Sea
O level geology c 1974 here...
crystals = igneous
smooth generally means metamorphic
Grains - sedimentary
glassy or aerated - volcanic

Granite is igneous
slate , marble is metamorphic (heated but not melted)
Sand , limestone, shales are sedimentary
Obsidian / pumice - volcanic

how to build a wall with any of them...gawd knows
 

Globalti

Legendary Member
Presumably a good wall is one where the waller has been able to utilise the natural shapes of the stones without resorting to chipping or breaking.

In the Lakes I've seen walls just built in the middles of fields to use up stones and give shelter to animals.
 
[QUOTE 3428317, member: 9609"]In a recent thread there was a discussion about geology and different types of stone which I found particularily interesting. And as I am about to do some work with local stone (a dry stone wall in the garden) I am quite curious as to know more about it. It would generally be known around these parts as Whinstone, but can anyone help me be more specific.. It is basically field stone that the farmer has dumped in one corner.
What I know so far; according to this web site Link the geology of the area is


this is it in the corner of the field


This is a piece cracked apart with a hammer (sometimes a slight smell of flint when broken)


And this is a piece cut with a diamond tip blade


and I will give @McWobble a shout as he seems to know about geology things[/QUOTE]

Whinstone is basalt, or a volcanic magma in this case that didn't make it to the surface. See http://en.wikipedia.org/wiki/Whin_Sill

It's in a flat-lying 'seam' (aka a sill) under much of N England and the Borders, and it's also the crag that Hadrian's Wall is built on.

Basalt is the stuff that is erupted in Iceland right now, and also forms much of the Hebrides - think Staffa/Fingal's Cave?

Anything else you'd like to know?
 

Firestorm

Veteran
Location
Southend on Sea
Whinstone is basalt, or a volcanic magma in this case that didn't make it to the surface. See http://en.wikipedia.org/wiki/Whin_Sill

It's in a flat-lying 'seam' (aka a sill) under much of N England and the Borders, and it's also the crag that Hadrian's Wall is built on.

Basalt is the stuff that is erupted in Iceland right now, and also forms much of the Hebrides - think Staffa/Fingal's Cave?

Anything else you'd like to know?
Giants causeway Iirc
 
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Haitch

Flim Flormally
Location
Netherlands
 
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McWobble

Euthermic
Location
Minkowski Space
[QUOTE 3428317, member: 9609"]In a recent thread there was a discussion about geology and different types of stone which I found particularily interesting. And as I am about to do some work with local stone (a dry stone wall in the garden) I am quite curious as to know more about it. It would generally be known around these parts as Whinstone, but can anyone help me be more specific.. It is basically field stone that the farmer has dumped in one corner.
What I know so far; according to this web site Link the geology of the area is


this is it in the corner of the field


This is a piece cracked apart with a hammer (sometimes a slight smell of flint when broken)


And this is a piece cut with a diamond tip blade


and I will give @McWobble a shout as he seems to know about geology things[/QUOTE]

I'd really need to see the rock up close and personal, with my hand lens (and ideally have a thin section of it and a petrological microscope - but you probably don't have those!).

However... The rock in question is fine-medium grained and dark grey with small interlocking crystals visible. That strongly suggests that it is igneous and intermediate in composition (more silica than basalt, but less than granite). The size of the crystals implies that the rock solidified from its parent melt quickly and close to the surface. The colour of the weathered rock is rust, most likely from oxidised iron - again, this suggests an intermediate or mafic (basaltic) composition.

The dark flecks look like biotite mica, the white ones are probably feldspar. I can't see any olivine crystals, which would unambiguously identify it as a basalt, nor can I see any glassy silica crystals which would mean it's an andesite. My first instinct was it's an andesite, but the weathered rusty appearance suggests it's a basalt. On the other hand, the cut section is far too pale - that hints that it's andesite! Any chance of a close up photo of the cracked section - that might help. Have a look and see if you can spot any glassy and translucent silica crystals, or olivine - it'll appear as a dark green crystal.

What size are the crystals? That also has a bearing: basalt and andesite are both fine grained with crystals smaller than 0.25 mm. If they're bigger then it'll either be the medium grained basaltic equivalent dolerite or micro-diorite.
 
Location
Lothian
Geology was one of those subjects that completely passed me by at school, perhaps it was mentioned in geography but I stopped that at 13. But over the last 30 years its gradually seeped into my consciousness what a fascinating subject it is, and especially the historical significance around here, James Hutton, Siccar Point, Agassiz Rock etc, and how that fed into Darwin's Origin of Species. Plus it's great to look at when out for a walk.
 

Globalti

Legendary Member
Yes geology is fascinating. If you climb mountains you can't fail to begin to wonder what shaped them. Geology explains so much about our lives and why we live the way we do. If I had my time again I would definitely study geology. Might even do it when I retire.

The Whin Sill was a term I heard frequently when I lived in Newcastle and used to go climbing at Crag Lough with my Dad. Hadrian and his pals utilised the outcrop and the rock to good effect:

Hadrian's_Wall_and_Crag_Lough.jpg
 
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