May 16, 2011

Mystery Rock

Posted by Evelyn Mervine

Mystery Rock, Photo 1.

A few weeks ago a blog reader named Bob wrote me to ask if I could help identify a “mystery rock” in his backyard. Bob sent me several photos of the rock and wrote:

Hi Evelyn,

I’m reasonably sure I’m wasting your valuable time with these “silly”
rock pictures, but I’ve been searching the web for similar pictures or
descriptions with no real satisfaction.

I am a computer tech by trade and know nothing about rocks.

I found this one unique looking rock on my property in Bergen County NJ.
It was one of I’m sure tens of thousands in a large rock wall that is at
least more than 75 years old. (It could be much older.)
I have seen none else that look like this one so far.

To keep this email short, here is my question:
Should I be more curious about this rock or just place it in a garden
bed for decoration?

Thank you very much for humoring me on this.

Well, as a geologist I like nothing better than puzzling over an interesting rock– so, Bob, you’re not wasting my time at all!

My fiance Jackie (a geologist who currently works for AuruMar) and I took a close look at all of the photographs Bob sent of this mystery rock, and we’ve tried to identify it to the best of our ability. I thought I’d write up a post with some pictures of the mystery rock and our attempt to identify the rock.  If any other geologists want to weigh in on the identification of this rock, please do so in the comments.

Before I proceed with the pictures and our identification, I want to make two comments:

-First, identification of this rock is limited by two factors:
1. We are identifying this rock from pictures, which is not ideal. Geologists like to see rocks in person so that they can squint at them with hand lenses, scratch them with their fingernails, feel their density, break them open with hammers, and– on occasion– taste them.
2. This rock was in a stone wall, not in situ. Identifying rocks that are no longer in situ is always more challenging because of the lack of geological context.

-Second, Bob sent some good pictures in his first email, but when I wrote back to him I asked him to do two things: 1. If he was willing, break off a piece of the rock so that I could see the less-weathered interior, and 2. Try pouring some weak acid on the rock to see if it fizzed. You can see pictures with broken surfaces below. Bob informed me that when he poured muriatic (aka hydrochloric) acid on the mystery rock, “the muriatic acid from the hardware store caused my patio pavers to fizz up!… but not the ‘odd rock’.” I asked Bob to pour acid on the rock because carbonate rocks (such as limestone) fizz when acid is poured on them. Since the rock did not fizz, it does not have a significant carbonate component.
Here are some more pictures of Bob’s mystery rock (click to enlarge):

Mystery Rock, Photo 2.
Mystery Rock, Photo 3.
Rock wall where the mystery rock was found.
Mystery Rock, Photo 4.
Mystery Rock, Photo 5.
Mystery Rock, Photo 6.
Mystery Rock, Photo 7.
Mystery Rock, Photo 8.
Mystery Rock, Photo 9.
Mystery Rock, Photo 10.

Here is our attempt at identifying this mystery rock:

Identification: A poorly-sorted sandstone containing some angular rip-up clasts and with possible animal burrows (edited to add: probably not, but that would have been cool). We suspect this rock formed in a fluvial (rather than marine) environment. This rock likely formed in a high-energy stream or river and was also rounded in this fluvial environment. Edited to add: Another possibility is that this is a glacial erratic (a rock picked up by a glacier and dropped elsewhere). Perhaps the rounding occurred in the glacier? 

Now, let me explain our reasoning behind the identification:

The first question to answer when identifying a rock is if the rock is natural or man-made. Geologists must be careful not to misidentify anthropogenic materials such as brick, cement, and road tar as natural geological materials. We believe that this is a natural rock, not an anthropogenic material.

The second question to answer when identifying a rock is if the rock is igneous, sedimentary, or metamorphic. This rock is clearly sedimentary as you can see rock fragments and grains of different sizes which have been cemented together.

Okay- so we’ve identified the rock as natural and sedimentary. Now, let’s move on to some of the rock’s features. The rock has larger clasts (some of which are angular) of at least a couple of different rock types (maybe the one angular clast is a red mudstone?) set in a finer-grained (but still fairly coarse) matrix that looks (to us) to be mostly quartz grains, which would make the matrix of this rock a sandstone. Because of the rock’s sandstone matrix, we don’t think the angular clasts indicate that this rock is a breccia. Rather, we think these angular clasts are something called “rip-up clasts”– basically, fragments of rock that are ripped up in a high-energy sedimentary environment. This environment could be a high-energy stream or possibly a marine environment (for example, turbidites often have such rip-up clasts in them). Since the rock is well-rounded, we suspect this rock came from a river or streambed, meaning that it may have originally formed in a streambed as well.

Finally, we wanted to comment on the rounded holes located in the rock. These could be cavities which used to contain large, rounded clasts that have now weathered out of the rock. However, we think these large, deep, round holes could be some type of animal burrow.

That’s our attempt at identifying this rock– any thoughts from other geologists?

I hope this helps you, Bob! Thanks for sending so many great pictures of this mystery rock. Finally, to answer your original question: please do place the rock in your garden bed, but be sure to point it out to people and say, “did you know that’s a poorly-sorted sandstone?”