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Wednesday, January 7, 2009

ROCK

Lucy Corrander - Rock - 24th November 2008

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11 comments:

The Weaver of Grass said...

That rock picture is amazing - I just cannot make out what it is. I think it is a Gestalt situation. It I knew what it was I am sure it would help.

Lucy Corrander said...

Hello Weaver of Grass

It is, simply a rock.

A rock on the coast of Dorset.

The reason this coastline is good for fossils is that there is a mixture of different kinds of rock from different periods and many of them are soft so the sea washes the fossils out of them.

In this case, the water has washed away the soft sandstone and left the harder shapes behind.

I'm just about to say things I think are the case but may be wrong - so don't quote me . . . !

The mid-area of the Dorset coast is where Kimmeridge Clay (common to the east of here) begins to give way to what I think of as Lyme mud which gets more common as you go west, towards Lyme Regis.

This Lyme mud is a soft, cloy-y kind of stuff, which is hardly rock at all, and it seems to release fossils better than about anything else.

It's a slatey-grey. If you click on the picture, you'll see a smudge of it.

There's also Oxford clay . . . and I'm afraid I don't know the difference between Oxford and Kimmeridge!

(This is ignoring Portland stone - from which St Pauls Cathedral and a large part of London was built and which comes from the island just to the south of here . . . well, it's more complicated than an island, so I'll leave that!)

What I find especially interesting in this rock is that a piece of harder material seems to have been broken in half. Then the two pieces seem to have been moved away from each other, then re-set in the mud which enveloped them and which then turned to stone.

Right . . . having displayed what may turn out to be ignorant twaddle, I'll pop over to a palaeontologist with a blog and see if she'd be prepared to comment. (Can't think of any geologists to ask!)

Glad you like the picture, anyway.)

Lucy

JJ said...

Hi Lucy, one palaeontologist reporting for duty!

Not knowing the precise location I can't really say anything about the type of rock, but you're almost certainly correct that this is something harder that has been left behind after the softer rock was eroded away.

If these lumps are about the width of a fat thumb, then you're probably looking at a fossil burrow from a snail or bivalve mollusc (like a mussel), or even a worm - there are some pretty big juicy worms that hang out in the sand on a beach, and the Jurassic Period was no exception.

If they're larger than that, getting on for fist size or larger, then my guess is that there was just a bit of sandstone that happened to solidify a bit more than the sediment around it. It happens sometimes - if there was a little particle in the middle of the lump it would have provided a focus for harder sediment to bind to. This is how flint nodules form further along the coast, but it doesn't have to be flint.

As for it being broken in half, if it is a burrow then it's very likely that at some point the burrow caved in before harder sediment filled in the mould.

Does that help at all? I can share the post with some better sedimentary geologists and see if they have anything to add.

Gordon Mason said...

Hey, Weaver of Grass ..... bet you're glad you asked!

No, seriously that really helps to explain the background to a really challenging (to the mind) photo. There are so many images that come through.

Wonder if in a future 'age' someone will find a rock with a mobile phone/camera bulging out and question what it could be??!!

Again, Lucy, great use of angle of light.

Philip Bewley said...

I love the image and the commentary!
clay, whatnot...there is a universe in"just a rock"
Your post, lucy, delighted me.
Philip

Lucy Corrander said...

Thanks JJ. I really appreciate that you came to have a look and left such an interesting and informative comment.

As to where this rock is, it is on the Portland Harbour side of the Weymouth peninsular, ten or fifteen minute's walk from the mouth of The Fleet Lagoon.

However, I'm not sure this is of much use to know because all sorts of rocks have been brought here from different parts of the nearby coast to help with coastal defence and the kinds of stone vary enormously. This includes flint and some rather repellent pink stuff and a rock that is pitted with lots of little holes which makes it look a bit like grey and weathered concrete . . .

I keep on tripping myself up with this blog because I forget important details. I think the lumpy shapes are largish, which is disappointing because I think it'd be more exciting to think they're parts of prehistoric burrows than bits of harder sandstone - but I'll go back soon and check. The nice thing about rocks is that they tend to stay put so, as long as I can find the right one . . .

(I probably can.)

Again thanks.

. . . and if anyone has anything to add, that would be really interesting too. It's wonderful how much of the history of the world can be touched upon in a photo of one, more or less, randomly chosen rock!

Lucy

Lucy Corrander said...

Gordon . . . as long as they don't crack the rock open and find a fossilsed me attached to the other end of the phone!

Lucy

Lucy Corrander said...

Philip - glad you like the image.

I too am enjoying the conversation it has given rise to!

Lucy

Silver Fox said...

"Wonder if in a future 'age' someone will find a rock with a mobile phone/camera bulging out and question what it could be??!!"

Definitely! And it'd be great to be around for that. ;)

Lucy Corrander said...

Hello Silver Fox

I've been taking a look at the dramatic rocks on your blog.

. . . Mobile phone sticking out of rock . . . I have the impression that whenever archaeologists find something they don't understand, whether an aritfact or building, they say it must have had a religious use and significance.

Maybe, when someone comes across the phone in the rock, they will use it to show Startrek was documentary and not fiction.

Lucy

Jan (Thanks For 2 Day) said...

Hi Lucy,
I know nothing about geology but I will say, that one 'lump' looks strangely like a sock over a foot:)
Jan

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