Rock identification help

[Fletc]

I found this rock when on the beach the other day, not sure what it is- it was whole and rounded in shape before I split it in half. It’s slightly magnetic and fairly heavy. Any ideas what it is? Cheers

 

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[Back-of-the-boat]

It is what would be classed as a concretion rock.

 

[Fletc]

Cool thank you mate didn’t think I would find anything interesting at all on my local beach but you never know till you have a go ��������

 

[Red-Coat]

Welcome to Tnet.

I have to disagree. This is not really a concretion in a geological sense. Concretions form by mineral precipitation around a nucleus or in the spaces between multiple nuclei.

It’s a ‘breccia’, which is a rock composed of broken fragments of other rocks which have become cemented together by a fine-grained matrix. You can see that those fragments are angular, which is what distinguishes it from a conglomerate, where the fragments have become rounded by erosion before they get cemented together.

When the fragments are all of the same rock type, we call them ‘monomict’ and when they are from two or more different rock types, we call them ‘polymict’. So that’s a ‘polymict breccia’.

Assuming your ‘Brighton’ location means Brighton on the south coast of England and that’s the beach on which you found it, then it will likely have come from elsewhere. Most of the ‘native’ cobbles and pebbles on the beach are flints and quartzites that have come out of the chalk and sandstone cliffs, but material continually comes in from elsewhere by ‘longshore drift’. The Council has also imported supplementary material from further west at various times to bolster the sea defences.

 

[Fletc]

Yes mate it’s the south coast Brighton and really interesting I am a newbie to rock hunting and really appreciate your knowledge on this subject great stuff

 

[Back-of-the-boat]

Thank you Red-Coat I just knew it was a form of concretion by being cemented together I just didn't know the other term being "breccia"

 

[tamrock]

I wish I cold find some high-grade gold bearing brecciated quartz

 

[TheCannonballGuy]

I applaud Red-Coat for his very educational reply about Fletc's mystery-rock.

Let me mention one other possible ID for it... which I think is the correct one. I used T-Net's "super-enlarge the photo" option to closely examine the matrix which surrounds the polymict stones. To me, the matrix looks very much like the color and consistency of ordinary concrete, and it contains the usual ordinary gravel as aggregate. In my opinion, Fletc's chunk of broken concrete has gotten rounded-off smoothly into a potato shape by decades of being rolled around in the ocean surf.

That ID could also explain why, as Red-Coat astutely noted, it is "geologically out-of-place" for the location where it was found.

 

[Fletc]

That was my first thought the canonballguy but was much harder and less brittle than I expected some forms of concrete to be and more shaped like a egg than round but I have only just started and joined this site so expect a few more post like this one I thought it would be fairly easy to identify what I find myself with the technology and apps and google but my god how wrong was I really appreciate it guys

 

[dozer dan]

Hardpan?

 

[Red-Coat]

There may be some over-thinking going on here. I’ve been hunting our South Coast beaches for years and there are plenty of places where you can find brecciated cobbles that have eroded out of the crumbling cliffs and been ocean rolled. Generally they’re found to the west of Brighton (which is in East Sussex) and on the beaches of towns more towards Devon.

Cliff and beach erosion is a common problem in many areas of the south coast and material generally get carried eastwards, so finding brecciated cobbles on Brighton beach (where they wouldn’t be typical for the local geology) would not be especially unusual. As I said, the Council has also imported material from elsewhere from time to time to protect the beach, but they don’t always say exactly where it has come from.

Much of the South Coast brecciated material is silica-cemented and contains hematite (some of it in sufficiently high quantity for the matrix to be bright russet-coloured) and also secondary magnetite in amounts that cause it to be weakly attracted to a magnet. There are also areas where it’s gold-bearing such as around Torquay in Devon, but not in commercial quantities.

I don’t think this is concrete but couldn’t completely rule it out without in-hand examination since they can of course look very similar. But I don’t see anything in the pictures which would cause me to think concrete rather than breccia.