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View Full Version : Artifact, Junk, or Natural Formation?



Fazor
2013-Feb-23, 02:13 PM
This (image below) has sat in a box containing a random collection of rocks gathered by my brother and I throughout our childhood. I was kind of surprised to find out my mom had kept them.

We found this stone in our yard when our house was built (25 years ago) and kept it because we thought it looked like it might be a Native American artifact. It looks like a mortar or grinding stone. My dad, who has a masters in Geology, thinks it may be a naturally-formed shape created when a pebble gets stuck in a swirling current and slowly grinds a smooth depression. I forget the term he used for that.

I could accept that, but it seems odd to me that there would be two depressions, one on each side. The chances of that happening natural seem to make him question that theory as well. The depression on the bottom has a much sharper edge to it, which also makes me wonder if it was carved by hand rather than by nature.

But it appears to be made of hardened (fired?) clay, which makes me question it as a Native American artifact. Also, I've read that the Native Americans tended to use much larger mortars as they practiced communal cooking, and as such made larger meals. This makes me thing it's more recent.

The land it was found on is in central Ohio, and when we built our house, it was farm fields. The area had been farmed for quite some time, but there had never been a home at that site. That makes me wonder if it could be "trash" from an old (1800's?) farm house in the area, that got dumped or cast off in the field.

When I saw it again last night, I asked to take it so I could head to the best place I know for this type of discussion. (And then I came here instead, ha!)

Okay, CosmoQuest; Thoughts?

18177

Solfe
2013-Feb-26, 01:01 PM
I have seen rock carved depressions and holes while hiking and camping around NY, but usually on a larger scale. I have been told they form from water pushing a rock or boulder around a natural depression until it carves right through.

You samples are much smaller, and I think they look man made. I wonder what the verdict will be?

Very cool in either case.

Swift
2013-Feb-26, 02:10 PM
We found this stone in our yard when our house was built (25 years ago) and kept it because we thought it looked like it might be a Native American artifact. It looks like a mortar or grinding stone. My dad, who has a masters in Geology, thinks it may be a naturally-formed shape created when a pebble gets stuck in a swirling current and slowly grinds a smooth depression. I forget the term he used for that.
They are commonly called "potholes" (yes, like the ones in a road). The gorge where I lead a lot of hikes has lots of them.


I could accept that, but it seems odd to me that there would be two depressions, one on each side. The chances of that happening natural seem to make him question that theory as well. The depression on the bottom has a much sharper edge to it, which also makes me wonder if it was carved by hand rather than by nature.
Actually, side-by-side is not unusual at all, especially for smaller ones. One image (http://www.americanwhitewater.org/photos/archive/medium/12192.jpg)
But the shape doesn't look exactly correct to me for a natural pothole. I have seen some shallow ones, but the steeper sided ones (as in the linked picture) seem more common.

But it appears to be made of hardened (fired?) clay, which makes me question it as a Native American artifact. Also, I've read that the Native Americans tended to use much larger mortars as they practiced communal cooking, and as such made larger meals. This makes me thing it's more recent.
From the picture it doesn't look like fired clay to me, it looks like natural stone, but that might be the photo.

I agree with you, it looks like a grinding stone. You might bring it to a local natural history museum and see what they say.

jokergirl
2013-Feb-26, 03:26 PM
IANAGeologist, but I would go for that it's a frost-created erosion (those are also called potholes, just to add to the confusion). It looks like there are multiple small ones on top of the big one, which reinforces the theory.

Swift
2013-Feb-26, 03:53 PM
IANAGeologist
It took me several seconds to translate that to "I Am Not A Geologist". :D

I like the new avatar jokergirl; one of your drawings?

Fazor
2013-Feb-26, 04:31 PM
Actually, side-by-side is not unusual at all, especially for smaller ones.

Side by side, sure. But this is on the opposite side of the same rock. Certainly possible that after one side had started eroding, something flipped the rock over, and the same thing continued erosion on the other, though.



From the picture it doesn't look like fired clay to me, it looks like natural stone, but that might be the photo.

To me it has an odd resonance for stone. But I'm no expert.



I agree with you, it looks like a grinding stone. You might bring it to a local natural history museum and see what they say.
I'll have to see when the Ohio Historical Society would have someone on hand to take a look at it. Probably the best close authority.

And to both Jokergirl and Swift (or anyone else); do you have any examples of water/frost "pot hole" erosion on small rocks? A quick search seems to only be turning up examples of these formations in large, flat rock surfaces.

jokergirl
2013-Feb-26, 05:07 PM
Look at this, this should be close to you:
http://ohiogeologyandbiodiversity.blogspot.se/2013/01/hemlocks-shading-black-hand-sandstone.html

I had to do a bit of Google fu until I figured out that ice erosion is called spalling in English, upon which I found this site. Does that look like it could be the source of your stones?

Solfe
2013-Feb-26, 05:59 PM
And to both Jokergirl and Swift (or anyone else); do you have any examples of water/frost "pot hole" erosion on small rocks? A quick search seems to only be turning up examples of these formations in large, flat rock surfaces.

I don't have pot holes but I have a pair of images of a groove cut into a rock on one of my blogs. I have no idea if this is erosion or just happenstance that a rock fell into the groove. It is pretty cool looking. The images were taken a Chestnut Ridge Park in Orchard Park, NY.

http://wondersofny.com/wp-content/uploads/2012/06/2012-05-23-11.47.33.jpg
http://wondersofny.com/wp-content/uploads/2012/06/2012-05-23-11.47.58.jpg

Swift
2013-Feb-26, 06:38 PM
And to both Jokergirl and Swift (or anyone else); do you have any examples of water/frost "pot hole" erosion on small rocks? A quick search seems to only be turning up examples of these formations in large, flat rock surfaces.
I don't have a convenient picture, but I have held such rocks. In the gorge I hike in, the exposed bedrock is shale, a rather soft one, and we get a lot of little potholes. Pretty frequently pieces of shale break-off, and once in a while I've seen pieces with little potholes in them. I'm leading a gorge hike Sunday, if I think of it (don't hold out much hope of that with my brain), I'll try to find one.

I'm pretty positive that the mechanism would be pothole forms in big formation, then little rock breaks off of formation. I don't think they'd form in little rocks, since the amount of water flow you'd need to make a pothole would just move or tumble a small rock.

Swift
2013-Feb-26, 06:41 PM
I don't have pot holes but I have a pair of images of a groove cut into a rock on one of my blogs. I have no idea if this is erosion or just happenstance that a rock fell into the groove. It is pretty cool looking. The images were taken a Chestnut Ridge Park in Orchard Park, NY.

http://wondersofny.com/wp-content/uploads/2012/06/2012-05-23-11.47.33.jpg
http://wondersofny.com/wp-content/uploads/2012/06/2012-05-23-11.47.58.jpg
Looks similar to "my" gorge. I'd guess the grove was a seam or fault that eroded out faster than the surrounding rocks.

Someone get geonuc to get over to this thread.

publiusr
2013-Mar-01, 11:43 PM
I wonder how many times pumice and slag get called meteorites.