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Rosetta Stones


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Mysteries at the Beach!

The views expressed are those of the author and are not necessarily those of Scientific American.


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Beach rocks are often mysterious rocks. Ripped from context, bits battered away, portions dissolved out, the whole rounded and polished, they can be nearly impossible to identify. Not to mention the fact that the waves have often removed distinguishing features that help us tell man-made from natural rocks – betcha I’m not the only one who’s bounced home with a chunk of concrete that looks like an awesome piece of conglomerate.

Add in an area covered in glacial deposits, where bits of rock might have been transported hundreds of miles from home, and you have a situation where rock identification gets tough. But it can be fun to try.

Take Onamission5′s puzzle:

Was on the coast of Maine this weekend and saw something I did not expect to see at all— something which looked an awful lot like waterlogged, ocean shaped red cinder. Just little lumps of it here and there amongst the seaweed and piles of snail shells. It wasn’t porous enough to be red cinder I don’t think. Now I wonder if it could have been red sandstone?

Could be, possibly, sandstone – but that description set bells a-ringing. Seems I’ve seen something very much like this before.

The majority of Puget Sound’s beaches are loaded with cobbles. We have a ridiculous number of rocks lying about.

Beach at Whidbey Island, Washington State, showing all the lovely rocks many of our beaches are famous for.

Beach at Whidbey Island, Washington State, showing all the lovely rocks many of our beaches are famous for.

I find little rounded lumps of red rock all over. They do look an awful lot like cinder. Same firey red, lots of little vesicles, same tendency to absorb water. But many of them are nearly fist-sized, while cinders are generally pretty small. And, as Onamission5 pointed out, they’re not nearly porous enough. Maybe they’re the remnants of volcanic bombs? But somehow, that doesn’t seem right, either. Having grown up in an area lousy with cinder cones, where our roads were chip-sealed with cinders and we used cinders rather than salt on snowy roads, I’ve got a feel for cinders. These aren’t them.

I mean, I doubt many cinders would erode in this weird elongated shape, but I see it a lot around the Sound:

Rather strange cobble at Lincoln Park. You can find lots like these, if you spend much time on our beaches.

Rather strange cobble under the water at Lincoln Park. You can find lots like these, if you spend much time on our beaches.

Sandstone? They sure seem like it. There’s that gritty texture. The particles are fine-grained and well-cemented. So that’s plausible… except something seems off about that, too. Not that I’m the world’s sandstone expert or anything, but something’s always seemed artificial about these rocks. Odd.

But if you nose around the Sound a bit, you’ll soon stumble across the solution to the mystery.

Our red cobbles in the making! These were on South Beach at Discovery Park, where there's more than just outstanding bluffs to keep you occupied.

Our red cobbles in the making! These were on South Beach at Discovery Park, where there's more than just outstanding bluffs to keep you occupied.

Ah-ha! All those strange little cobbles turn out to be former bricks! You can see a fairly fresh fragment at the top, while the middle and bottom bricks have wave-rounded corners. Apparently, they’re being eroded out, discarded, or finding their way into the Sound any-old-how, and after being rolled and tossed and ground down in the surf, they live on as beach cobbles.

A nice example of a brick-become-cobble, from the same area at Discovery Park.

A nice example of a brick-become-cobble, from the same area at Discovery Park.

Now, I have no way of knowing if bricks are what Onamission5 saw. It’s very possible those actually were bits of sandstone. Goodness knows what might be eroding out of all the stuff the last ice sheet dumped all over Maine. But the description does sound awfully like the bricks-become-cobbles that add such a bold splash of color to our Puget Sound beaches.

Your mission, Onamission5, should you choose to accept it, is to see if you can find additional evidence to show just what these mystery rocks might be.

And you, my dear readers, are encouraged to share your beach rock curiosities and triumphs with us. I know it will be very hard to drag yourselves down to the shore, whether that be your local river, lake or sea, especially in lovely weather, but do try to make the sacrifice – it’s for geology. To the beaches!

Dana Hunter About the Author: Dana Hunter is a science blogger, SF writer, and geology addict whose home away from SciAm is En Tequila Es Verdad. Follow her on Twitter: @dhunterauthor. Follow on Twitter @dhunterauthor.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. kim lombard robson 11:24 am 08/1/2013

    I’ve collected several eroded sea-brick pieces from the beach off San Diego. I believe they come from lobster traps. A brick is placed in the trap to help weigh it down. Eventually they erode away to the point where they fall out of the trap or the fisherman tosses them away.

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  2. 2. RCWhitmyer 6:29 pm 08/1/2013

    If the stones are denser than average, what about an iron or other metal ore? In Central Indiana I’ve seen and picked up golf ball size reddish to brownish stones. They feel much heavier than they should for their size and are composed of fine granular material. Could they have been brought down from ore regions up north?

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  3. 3. StarMom 10:27 pm 08/1/2013

    I have beach rocks galore! How do I share an image, pray tell?

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  4. 4. Heisenbergensis 7:59 pm 08/7/2013

    Loved your article Dana! :-) I’ve got a nice rock to share too. Its a beautiful qtz breccia I found on the shores of the southern ocean, SI, New Zealand. Like StarMom i’d like to share.

    Link to this
  5. 5. Heisenbergensis 8:02 pm 08/7/2013

    RCWhitmyer, I wonder if you have found some meteorites???

    Link to this

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