FMF - Friends of Minerals Forum, discussion and message board

[DaveP79]

Hello,

Looking for help in identifying this rock due to its unique appearance. Apologies in advance, as I am very much a beginner, so cannot identify luster, hardness, etc. However, attached are a few photos. Additionally, this rock is light reddish-brown in color, seems to be very hard (can scratch it with a metal screwdriver but it doesn’t flake), and has a dark gray vein running through it (as can be seen in the bottom right of the pic). I found it on an ocean beach in Massachusetts. The underside of the rock is also shown and has some interesting textures.

Thanks,

Dave

[Peter Megaw]

Tough call....not much information to work from other than it looks fairly homogeneous with minor fractures.The beaches of Massachusetts are mostly glacial dumps composed of rocks ripped up and transported by the continental glaciers that melted leaving Cape Cod, Long Island etc as their "terminal moraines"...think of the pile left in front of a dozer blade when the operator knocks off for lunch. And the scrapings can come from anywhere upglacier between that beach and the North Pole. Even if you identify what it's made of you won't know where it's from.

Call it a "glacial erratic" and enjoy it as a cool looking rock in your garden.

If you want to be more technical, we geologists have a fall back category for this sort of thing FRDK.. stands for Funny Rock Don't Know.

Keep looking...there's good stuff out there

[Peter Megaw]

Forgot to mention that those parallel grooves on the smooth surface were probably made by the glacier as it was dragged against other rocks

In short...what has happened to this rock is more interesting than what it is!!

[DaveP79]

Hi Peter,

Thanks for the help and the insight. Maybe not the most exciting find, but it’s definitely piqued my interest in searching for some more “FRDKs” in this area. Looking forward to learning more about geology in the Cape and in general!

Best,

Dave

[Peter Megaw]

Welcome to the Monkey House...

[Peter Megaw]

There's a really good beginners book on the geology of Cape Cod...I know where my copy is...at my sister's in Falmouth

I'll see if I can get you a title

[Peter Megaw]

Just Google "Cape Cod geology USGS" it'll take you right to the paper

[DaveP79]

Sounds like a must have for the Cape. I’ll look it up. Thanks for the recommendation.

[Pete Richards]

It's quite a bit of a reach, just from those photographs, but the texture of the object suggests bone. I suggest you get in touch with a vertebrate paleontologist who knows the paleofauna of the Cape Cod area to see if it might be a fossil of some sort. The hardness is appropriate for fossilized bone. There is also a suggestion of bilateral symmetry (suggestive, not diagnostic).

[Pete Modreski]

That is all very good advice about your rock that you received from Pete Megaw.

Likewise, Pete Richards' suggestion is good--I also thought that its shape "might" suggest some kind of fossil, though, the little (quartz probably) veinlets running through it would argue, probably not really a fossil.

As you will learn if you try to study and learn about rocks some more, a very fine-grained, fairly hard rock can be quite hard to identify, even for "any good geologist"! There are many, though very different in origin and nature, kinds of such rocks that can appear very similar, and lacking any real obvious distinguishing characteristics: quartzite, sandstone, meta-siltstone, even a very fine-grained granite. The fact that you say it's pretty hard seems to rule out one other possibility, limestone. "In theory", if your rock is quartzite, you should NOT be able to scratch it at all with a knife or other sharp tool. But there is a continuous gradation between quartzite, sandstone, siltstone, argillite, and others (degree of purity of quartz of which it is composed vs. content of clay and other soft minerals, plus degree of metamorphism, all resulting in a range of possibly hardness). This all makes geology so interesting, and complex! You may never get a completely satisfactory, positive answer as to "exactly what this rock is"!

Best of luck with this & your future "interesting rock" observations,

Another Pete M. (Modreski)

[DaveP79]

Thank you for your help, gentlemen, and for providing a bit of informal education to this very novice rock hunter. I will certainly try to find a paleontologist and get his/her take. Otherwise, happy to relegate it to the confines of my garden and find more interesting rocks to keep it company!