What are these cut marks? glaciers/man made

[nwark]

Let me start off by saying that I love all rocks and have a big imagination. Ninety percent of the things I find are probably really nothing significant. Please be patient with me and I promise I will try not to overdue it on the questions. Thank for all your help and comments in advance, I really appreciate all of you.
My pictures are not very good because the sun was only out part of the time. I guess I should have waited but I was a little anxious. I am from Northwest Arkansas in Washington County close to the Oklahoma border. I find a few arrow heads but they are usually broken or are not as nearly as grand as the things I see on this forum. Not really worth sharing. I do have quite a few rocks that have cut marks and I have found many curious things that may just be mother natures handy work. The first rock has lots of cut marks that aren't all perfectly straight. The second one has a few straight cut marks but when I turned it over it looks like it has some sort of red clay on it. I tried to scratch it but it is just as hard as the rock. I hope you can see the red color, I may try to take a better picture later. Thanks so much, I hope the pictures upload, if not I will resize and try again later,

[Hoss]

I think they are natural nwark thanks for the way you presented your questions. Geology in your area is actually pretty cool have a look at this link and see. https://www.geology.arkansas.gov/geology/fossils.html

Scroll down and click on Psuedo fossils.

[nwark]

Thanks, I do find a lots of things like this. Any information is always appreciated,.

[painshill]

In pics 1, 2 and 4 if I zoom in I can see vesicles (bubbles) and that almost always means the rocks are volcanic. In geological terms that doesn't necessarily mean the big pointy things people generally think of at the mention of volcanoes... only that the rock has come from a magma chamber below the surface of the earth and subsequently solidified. In those kinds of rocks you will frequently get striations formed by shrinkage during cooling and also from the intrusion of veins of minerals with a different melting point or a different viscosity to the host rock. Cracks can also fill with secondary minerals a long time after the original solidification.

I don't believe those are glacial drag or scouring marks, for which the striations are almost always linear (straight) and often in groups where the lines are parallel to one another. The other thing that points away from glacial is that in pic 2, it's clear that the striations extend into the interior of the rock on a more recently broken and uneven surface that has obviously different weathering to the exterior (and its striations).

[nwark]

I know I should have gotten better pictures, the first rock is really bubbly. I see what you mean and I do find a lot of these types of rocks but they don't always have the marks on them. This is very helpful, thanks so much.

[painshill]

I suspect it's vesicular andesite or vesicular basalt. When the volume of vesicles gets to beyond 50% of the total rock volume we call it 'scoria' and when there are sufficient vesicles that the rock is very light (and may even float in water) we call it 'pumice' (or 'pumicite' if very fine-grained).

[nwark]

I see what you mean, I Googled it and it does have that look on the underneath especially but it is actually pretty heavy and I think some sort of flint because I know someone personally who makes sparks with it to start fires just for fun. They ask me to bring it to them if I find small nice pocket size pieces that are sharp. This one is gray in the middle and hasn't been broken yet. I hope this makes a little sense like I said I am just an armature, Thanks, I love all the input, I am learning a lot about rocks, and on my first post too.

[painshill]

Basalts are just about the most dense non-ore rocks such that even vesicular basalt is pretty heavy... until it approaches being a scoria. You won't usually get vesicles as such in sedimentary rocks such as flint or chert because vesicles almost always form by the expansion of air/gas/steam in molten rocks derived from magma. Flints, cherts and other sedimentary rocks may contain small numbers of randomly-distributed cavities known as 'vugs', or sometimes one large cavity of that type. They're formed by a different process, aren't usually spherical like vesicles, and may be filled or lined with crystalline minerals.

[nwark]

I think I see what you mean, I do get a lot of these and they are very porous and have perfectly round concretions in them. I often thought this looked like some kind of volcanic rock because there are so many little pebbles throughout too. I hope the pictures work. Sorry to take up so much of your time, thanks so much.

[painshill]

Hi again

This is just a short answer (I'll add more later) since I'm just about to head out. It's election day here and I need to vote, among other things.

The rocks you're showing are not all of the same type. You may be finding them all in the same place, but that doesn't mean they were all formed by the same geologic processes. For example, the second rock you're posting here with that classic concretion is sedimentary. Concretions like that don't occur in volcanic or igneous rocks, but they're very common in sedimentary rocks... notably sandstone, and that's what you have there

I wouldn't definitively identify the first rock based only on one picture, but it doesn't look volcanic. What typifies vesicles in volcanic rocks is that they're usually distinctly spherical. Think about the way bubbles form in a sponge cake when you bake it. Sometimes they will be elongated or ovoid if the magma from which they form cools slowly and is still flowing for a while, or if the unsolidified magma gets more material piled on top of it and is squashed, but you should still be able to recognise they started life as spherical.

More later.

[nwark]

Thanks so much for clarifying that, I have always wondered if the concretion rock was volcanic and now I know. Also, they were found a few hundred yards away from the first two rocks. The first and second rock with the cuts in them were found in a creek while the concretion rock were found on a hillside. Thanks again and I really appreciate all your time, this has explained a lot for me. Have a nice day.

[BabaORiley]

Mother Nature makes some of the most interesting things through her amazing geologic process. Those are cool rocks that would have come home with me also.

[nwark]

Thank you.

[Tam]

Painshill not to bomb this thread but what about iron ore . We go up in an area where it has not been collected from WW 2 . ( SWGeorgia ) use it for stepping stones and fire pits . Some of them look like a big bubbles 10 in across .
Then some have a crystal like substance that sparkles in purple colored ones .
I did look it up and how old they are but is it basically the same process , cooling , bubbling ?

[painshill]

Not intending this to become a ‘Geology 101’ thread but, just to clarify.

I said originally “I can see vesicles (bubbles) and that almost always means the rocks are volcanic”. Vesicles are hollow structures formed by gaseous expansion in molten rocks. What I could see, with some zooming in were exactly that, and with the typical spherical shape that arises from such expansion. Here’s a chunk of vesicular volcanic rock which shows it more clearly:



Iron ‘ores’, includes a wide array of ‘ironstones’ which are sedimentary rocks. They’re typically rich in iron oxides such as limonite, hematite, and magnetite; carbonates such as siderite; silicates such as chamosite; or various combination of the above.

Many of those minerals can occur in what look like ‘bubbles’ from the outside, but they aren’t formed by gaseous expansion and they aren’t hollow. Most usually they’re what are known as ‘botryoidal’ (from the Greek ‘botrys’ which refers to a bunch of grapes). Here’s a chunk of botryoidal hematite, but it’s completely solid:



These are not formed by gaseous expansion. They’re solid nodular (sometimes concretionary) structures formed by chemical reaction & precipitation or crystallisation. When huge they are sometimes called ‘mammillary’ (in reference to their resemblance to breasts) or ‘reniform’ (in reference to their resemblance to kidneys) and when smaller they are sometimes called ‘ooidal’ (in reference to their resemblance to tiny eggs as might be laid by lower order animals).

[nwark]

This is interesting because at a higher elevation I do have some of the bigger basketball size holes in the bluffs where I guess concretions used to be. Thanks so much all very helpful, I have wondered about these for years. Maybe sometime I will get a picture and post it, thanks.