Identifying Rocks - Geology ?

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John the Monkey

Frivolous Cyclist
Location
Crewe
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.
I loved the A-level - I went on to do it at degree level, on a course that seemed pretty much designed to suck all the fun out of it. I dearly wish, in my darker moments, that I'd had the bloody mindedness I developed in later life, and stuck with it.
 
I did A Level Geology and even now as I'm going about I look at any rocks and try to work them out.
Sometimes I'll hit a rock on the track - "Ouch! Bugger! Hmm... That looked metamorphic..." :rolleyes:
 
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Berk on a Bike

Veteran
Location
Yorkshire
You can usually tell the origins of rock from the wording that runs all the way through it.

This is from Blackpool...
Blackpool-Rock.jpg


...while this (though similar in appearance) is, if I'm not mistaken, from Scarborough...
2a05e438-6259-4b04-b596-ee22030327e7-620x372.jpe


You're welcome.
 

Ganymede

Veteran
Location
Rural Kent
This is bringing back all those Open University Geology field trips to me... Can't add anything that the more current experts haven't said already though!
 

Fnaar

Smutmaster General
Location
Thumberland
[QUOTE 3429708, member: 9609"]Yeh but - I have a passion for stuff that free.[/QUOTE]
Yeah, I was just messing about. One of my possible plans if I ever get to retire is to try out some dry stone walling... I'm always amazed at the miles and miles and miles of walls, and in awe of the work that's gone into making them.
 

Mr Celine

Discordian
[QUOTE 3429701, member: 9609"]

As for my attempt today, spent about 3 hours, lost my temper with it and flattened it - very frustrating, I reckon there is something wrong with the rocks I've got. Might cheat tomorrow and use some mortar to hold the dam things in the correct position, as long as it's deep inside and not visible then I will be happy with it.[/QUOTE]

I rebuilt the dry stane dyke at the end of my garden but that took about 3 years!! It's slow work and better suited to an hour or two at a time after work on a summer afternoon before the midgies come out. As for your rocks, my wall is built with cobbles which must have been part of the bed of the River Tweed at some point and they're a pig to get to grip each other.

The dry stone walling association publish a very useful guidebook which covers the basic principles. Have you sorted your stones out ? - foundation stones, through stones, coping stones, building stones, hearting, and you'll find loads of stones that are no use at all, particularly if round.
 

vernon

Harder than Ronnie Pickering
Location
Meanwood, Leeds
Get some bricks from Homebase, and be done with aal that faffing about with stones.

[QUOTE 3429708, member: 9609"]Yeh but - I have a passion for stuff that's free.[/QUOTE]

Then go to Homebase at night, cut a hole in the fence and steal the bricks.
 

Wobblers

Euthermic
Location
Minkowski Space
[QUOTE 3429690, member: 9609"]Thanks for all that, here are close up in better light, (the first two were under florescent tubes) the scale on the ruler is ½mm

cracked open
rock5_0343_zps8a1410a8.jpg~original


cut open
rock4_0348_zps61ea402a.jpg~original
[/QUOTE]

Hey, what's this, you expect me to work? How very dare you! :smile:

Let's start with the easy bit: the crystals are about 0.5 mm in size (thanks for putting the ruler for scale, that really helped!). So this is a medium grained rock - which rules out basalt and andesite, which are finer grained. After looking at your new photos, I can see a few pale green crystals. That's olivine (a little paler in colour than I remember, but the only other common mineral that's green is chlorite - and it certainly isn't that). Its abundance is around 5%, which is typical for a rock of mafic composition. I can't see any quartz, but there is a fair number of translucent grey-white plagioclase feldspar crystals. You see plagioclase in that quantity in both mafic and intermediate rocks. With its dark colour, plagioclase abundance, ochre appearance of the weathered surface and olivine I'm reasonably sure that it's mafic. With this medium grain that suggests that it's dolerite. There are a number of volcanic dykes that formed about 60 million years ago in your corner of the country as part of the enormous pulse of volcanic activity that heralded the opening of the North Atlantic. There's a good possibility that your rocks come from one such dyke where the basaltic magma didn't quite reach the surface so cooled slowly enough to form those larger crystals.
 

Mr Celine

Discordian
[QUOTE 3431554, member: 9609"]sat out in the drizzle faffing about for nearly 3 hours today and I'm still not impressed, but it is better than yesterdays attempt, I will have a look at it in the morning and decide whether or not to start again. It isn't a proper dry stone wall as it is just to hide a retaining wall I put up earlier and i am cheating with a bit of mortar. It is in quite a damp shady spot so should turn quite green and mossy. [/QUOTE]

This is what I built round my pond about six or seven years ago.
pond.jpg


The pond as built by the previous owners had a 'wall' made partly of cast stone blocks and partly of vertically set paving slabs which looked rubbish. The liner was polythene and had started to leak so I relined the pond, recut and relaid the paving slabs round it and built the stone retaining wall to match the drystane dyke at the end of the garden, behind the camera.
The wall was built dry for two or three courses then backfilled with weak concrete, followed by another two courses and backfilled again with concrete. Because the wall is only half the height of a full sized wall it is built with smaller stones than normal. The copes are too small and light to hold themselves in place so are set in mortar. There are also a couple of slabs laid flat to form a seat.
There is one larger stone with a hole bored through it which is connected by an underground pipe to the rhone on the house, so the pond now fills with rainwater.
 

Wobblers

Euthermic
Location
Minkowski Space
[QUOTE 3431793, member: 9609"]Thanks for that, fascinating stuff even though I am struggling with it. Any descent websites you can suggest that might help me identify my own ?

One thing that puzzles me - Since Dolerite does not seem to form any of the bed rock in the area they were sourced (should have been Andesite) and since the stones are lifted from fields rather than quarried, is it likely they have been dumped here from glaciers during the last ice age and have just broken up in the fields over the millennia?

Even though the wall does look a multitude of colours, I reckon it is all pretty much the Dolerite that you have identified, although there are occasional (1-2%) pieces of a very red sandstone.

edit; (one final thing about this bloody rock) it must contain some iron, or at least one of those very powerful magnets found in hard drives, sticks to it.[/QUOTE]

I did the Open University geology courses - there's nothing like learning from experts who do this sort of thing every day! If you don't want to go to all that expense, time and hard work (understandable!) try this web page - it seems well thought out with a whole set of questions to lead you through to an identification.

The dolerite will have come from a fairly narrow dyke. You've probably seen videos of those Icelandic eruptions which have lava erupting from long fissures in the ground. That lava will cool rapidly to form basalt. The rapid cooling means that large crystals don't have a chance to form so you end up with a fine grained rock. The lava's being fed to the surface through a feeder dyke - once the eruption ends, the magma in that dyke will cool slower slower and form dolerite.

Your rocks are rounded. That suggests to me that they've been reworked by water. Possibly they were then transported to their final position by glaciation and dumped in a moraine - if they're amongst a whole range of unsorted and mostly angular material with a wide range in sizes, that would suggest glacial transportation. Otherwise, it's quite possible that they were moved by some ancient stream or river that is long gone.
 
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