Can anyone suggest the origin of these rocks?

Reply Wed 18 May, 2016 07:06 pm
I came across an area of the West Coast of Ireland where erosion had exposed soil and rocks as you'd expect.

It was an interesting place - there appeared to be a mix of Sandstone, Limestone, Shales and Basalt all within an area of a few hundred feet.

Within the cliffs I found many of these types of "Rocks".

As I hope is clear from the few photos they had a hard outer surface and onion skin type layering - they weren't particularly brittle - but with a good whack of a large limestone rock they were easy to break, revealing the layering inside and then once broken the layers were kinda brittle.

What I found particularly unusual was the sometimes very soft grey core, particularly in the smaller one - i.e. 2-3 inches diameter.

I'm no geologist and have very limited knowledge related to geology but if i had to guess, which I did at the time - I'd say the grey core might be very very fine ash - it certainly rubs on the hand like ash.

Further as I hope is somewhat apparent from the last 2 photos there was sometimes areas of little black crystals just outside (moving away for the core) the grey core but still well within the rock itself.

I wonder if anyone could maybe offer some useful insight into the origins of such rocks as I've never seen anything like them.

My own uneducated guess was that they might be pieces of magma/ash that was extruded from long extinct volcanos and blown out of the crater into the sky and dropping into the ocean therefore cooling very quickly from the outside in, and perhaps therefore giving some basis for the hard layered exterior, the tiny crystals of what might be obsidian and the soft ashy core - of course I could be way way off.

I mentioned basalt earlier as being at the location - at least I think it was basalt - It doesn't look like the giants causeway - with the hexagonal shaped pillars, but instead is a dense black rock (interior, outside was grey or sometimes redish) surface with groves and channels like you'd often see in images/videos of cooling magma on TV in Iceland or Hawaii etc.

Gonna get to posting the images now.
















Reply Wed 18 May, 2016 07:30 pm
Hard to tell. Coprolite maybe? Petrified Poop.
Reply Wed 18 May, 2016 08:26 pm
They are iron concretions. Theyre common in the Jurassic Limestones and are solution/precipitate deposits of iron oxyhydroxides. There are several ways these things form an each one is a key to the mineral abundance so prospectors use em as indicators of whats nearby.
These are either Goethite, (if non magnetic) or Maghemite (if they are magnetic).

One of the kind of concretions common in the various iron rich environments are called "Moqui Marbles" and are reminiscent of certain concretions qeve seen otherplaces recently. (hint: think space program)

Id say these are goethite and are probably no deeper than the subsoil ("C horizons')
Reply Thu 19 May, 2016 04:13 am
Cool. Thanks.

What causes them to form like that?
Reply Thu 19 May, 2016 05:19 am
since these nodules are deposited through microorganism's help(ulothrix)< gases are probably involved, (mostly H2S as the stuff is reduced and then oxidized into iron oxyhydroxides).
Colonists used to make cast iron railing and cannon balls and then stove plate and pots and pans put of bog ore (It was cheap and renewable ore).
Theres a national historixal site over in Mass called the SAUGUS IRON WORKS, that used bog ore in the late 1600;s. Its by the Saugus River and shouldnt be a bad day hop to the site.

Mot all deposits were associated with the bottoms of ground water drainage areas(near headwaters of creeks and spring areas).

Ill see whether I can find anything interpretive ,I got nothin else (Im just a glorified prospector)

Reply Thu 19 May, 2016 06:55 am
That's fine. I was just curious what would form a structure like that. Thanks.
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Reply Thu 19 May, 2016 09:41 am
Thanks for the answers guys.

I'd have been very surprised to hear it was petrified poop - I don't recall ever having heard of land fossils discovered having being discovered in Ireland, just plenty of marine fossils in the limestone, plus the rocks were too plentiful and spread out among a few metres of subsoil.

I'd say from recollection that the rocks were found about 6-8 feet below the current soild level - though its hard to be certain given the erosion - i.e. the actual cliff face was staggered down toward the sea.

Great info Farmerman - I've always wondered what those blueberry formations on Mars were. Now I guess I know. I had even wondered if they might be stromatelites as I'd seen some recently in an Australian museum that had a similar bobbled surface.

Any ideas on what the black crystals might be?

Incidentally I put the rocks near a magnet and I don't think there was any perceivable attraction.
Reply Thu 19 May, 2016 01:40 pm
Miki I'm having no luck at all loading the images
Reply Thu 19 May, 2016 05:54 pm

Any ideas on what the black crystals might be
thats the actual goethite, the bog ore concretion is mostly limonite .
Stromatolites in Australia are in entirely different environments. We usually find the goethite/limonite/maghemite stuff on the top of where the water table sits. Most of the big ore deposits are less than 100 kybp
Reply Thu 19 May, 2016 06:20 pm
That's odd - apart from the first image (Which may be a dodgy link) all the rest load fine for me.

You could try this thread on an Irish Forum where I had posted them originally and where I've added the img links here from:


You'll see the image attachment links further down in that thread if the images don't load in the html img tags.
Reply Thu 19 May, 2016 06:30 pm
Oh I see thanks.

Interestingly I came across a different area in the centre of Ireland a few months back where a meter or so below the surface (at the bank of a path cut into a boggy mountain area) there was a large soft flakyish bright orange feature, which was forming out of the layer of clay immediately below the bog layer. I now suspect that was one of those type of concretions forming. It was clearly oxidised iron and again very spectacular to see for someone not used to seeing such things. Thanks again.
Reply Thu 19 May, 2016 06:57 pm
the iron in sloution probably comes from some basic igneous rocks. Are there ab=ny gabbros or basalts nearby? also, limestones, when they dissolve, their contained iron an manganese goes into forming iron concretions. Mostly though, it comes from layers of iron sulfides (pyrite family) that reduce to iron sulphates nd then to oxy hydroxides (limonite/goethite)

You can find some really showy samples if there are bigger crystals of the goethite. It takes a "bubbly look on the surface" (this is called a botroydal habit) and the crystal (in cross section) look like radiating arms of shiny blackness.

The American Indians would colr themselves with the red iron (called it red ochre) and they would have lumps of actual iron silicate in the bottoms of their cooking fires. They had no metallurgy so the iron silicate (We call it fayalite), that was left, became just an artifact till some archaeological wag decided to interpret the age of the fire pit by remnant magnetism left on the fayalite as it cooled and recorded the magnetic north for that time.
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Reply Fri 20 May, 2016 11:03 am
the rest load fine for me
Thanks Mik but exactly how do you do it
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