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Steve Tguiri April 25, 2012 04:42PM

While doing some construction in my back yard (in NW San Antonio, TX) I found this rock that had a hole through it. It is a little bit bigger than an american quarter. It also appears to have a old fashioned "6" or "9" depending which way you look at it. The hole doesn't seem to be made from dripping water because the hole is not smooth or straight and the entries of the hole are cone shaped. I think it is calcite.

I claim to be no expert and I do think the "6" or "9" could be a natural occurring coincidence but this rock just looks unusual to me and I wanted to see if I may get some expert opinions.

(I did wash off the mud and dirt)


open | download - photo.JPG (796.1 KB)
open | download - photo (1).JPG (695.7 KB)
open | download - photo (2).JPG (640.6 KB)
Don Saathoff April 25, 2012 06:12PM
Hello Steve, welcome to Mindat!!

North of SA, especially north of the Balcones Fault, there is, in the Edwards Limestone, the remains of an ancient aquifer. This limestone, locally called "Honeycomb Limestone" often contains fossils in addition to the honeycombed appearance. I think you have a small piece of this limestone - we have several pieces in the yard in excess of a meter long completely riddled with both large & small holes. Put a drop of white vinegar on the piece and watch for small bubbles to confirm limestone. You will see honeycomb in many aquariums around SA.....the fish love it!!

George Eric Stanley Curtis April 25, 2012 06:32PM
Hi Steve,
It looks like a small flint nodule to me.

Flint often forms around sponges which leave a characteristic hole, and the '6' swirl mark is also quite often seen on flints, though I am not sure what causes them.
The way to tell is with its hardness.
Flint is a variety of quartz, and is quite hard. It will make a sharp 'click' sound if you tap it with a metal rod.
Limestone will sound dull.

All the best,

Eric :-)

United Kingdom, Cornwall
Paul Brandes April 25, 2012 07:11PM
I'm with Don on this one. I have seen many of the limestones coming off the Edwards with similar holes and fossil impressions on them, and your piece looks dead on Steve.
Roberto Bosi April 25, 2012 08:11PM
Hi all, I also agree with Don and George. Somewhere in Puglia region (east-southern Italy) you can find many similar flint nodules with an hole (sometimes perfect) in the middle. These nodules are used as ballast for the fishing nets by local fishermen. The whitish colour of the specimen looks like the typical coating (a sort of oxidation) of the flint old surfaces. Especially in the first image, more over, I seems to notice that this coating is detached in some places revealing a "fresher" surface of a yellowish-beige colour.

Better to be just than good (Kempis)

Edited 4 time(s). Last edit at 04/25/2012 08:49PM by Roberto Bosi.
George Creighton April 25, 2012 08:23PM
George Eric Stanley Curtis Wrote:
> Hi Steve,
> It looks like a small flint nodule to me.
> The way to tell is with its hardness.
> Flint is a variety of quartz, and is quite hard.
> It will make a sharp 'click' sound if you tap it
> with a metal rod.
> Limestone will sound dull

My first impression was also a flint nodule.

Thanks Eric that you explained how to detect flint covered in limestone from limestone itself,
never knew this of test method.

Live and learn.

Regards george
Norman King April 25, 2012 08:23PM
I agree with the others. The apparent numeral looks to me like an "involute" cephalopod--that's one where the later whorls (coils) of the shell cover the earlier whorls. It is too worn to make a guess as to whether it is an nautiloid or ammonoid cephalopod.
Knut Edvard Larsen April 26, 2012 12:15PM
Well, for me it looks more like fossilized gastropod. The "number" above your right thumb on the first photo is remnant of the dextral coiling typical of gastropods. The hole in the piece can be interpreted as the opening of a shell of an gastropode that has been eroded through.
Norman King April 26, 2012 07:08PM
Knut, with all due respect, you are not very close on this. I have not been able to confirm the existence of a gastropod having such form during the Cretaceous. Moreover, there have never been any gastropods having a circular aperture. That hole is somewhat irregular, and you seem to be trying for a “freebie” to claim it was something that it does not resemble. The hole in the rock is not part of the outline (the “numeral” 6 or 9), so I am mystified as to how you connected those, anyway. I dispute that such an approach is good science. That would be tantamount to saying that a smoothly rounded crystalline material someone asked about is orthoclase, and the fact that it lacks cleavage is a result of abrasion or solution, so lack of visible cleavage doesn’t matter in its identification.

The figure on that rock is an involute cephalopod. I include illustrations of them below. Note that there is a plan view and a cross-sectional view for each one (nautiloid and ammonoid). I also show an involute planispiral gastropod for comparison. It looks OK, except that is an archaic type from the Paleozoic to Triassic (most lived during the earlier part of that time span).

This information is not secret. You can find it in any good geology library. I referred to these sources:

1. Arkell, W. J., et al., 1957, Treatise on Invertebrate Paleontology, Part L, Mollusca 4 (Ammonoidea): Geological Soc. America and Univ. of Kansas Press, 490 p.

2. Shimer, H. W., and Shrock, R. R., 1944, Index Fossils of North America: Cambridge, MA, M.I.T. Press, 837 p. (for the gastropod illustration; a dated reference but readily available and comprehensive enough for amateurs).

3. Teichert, C., et al., 1964, Treatise on Invertebrate Paleontology, Part K, Mollusca 3 (Nautiloidea, etc.): Geological Soc. America and Univ. of Kansas Press, 579 p.

When using those, you have to be sure that the form you choose for the correct identity lived during the proper time (e.g., Early Cretaceous as opposed to Tertiary, etc.) and that it lived in the correct region (e. g., North America rather than Europe or Asia (today’s lands, of course, may have not been so far apart in the past–that needs to be taken into consideration).

So, again with all due respect, I don’t think people (especially those on the Mindat management team) should pretend to be expert commentators on subjects in which they are not particularly knowledgeable. We should give other sciences all the respect they deserve. While it may be true that mistakes in paleontology may not come back to “bite” you (like they might in chemistry or thermodynamics when discussion mineralogical issues), such mistakes may mislead the person who posed the paleontological question. They also suggest we think it is not important to be correct in what we say in other fields. As for me, I think we should provide state-of-the-art commentary in all scientific fields. If we cannot, then we should not risk confusing things with poorly-advised opinions. Any commentary should be left to those who have appropriate expertise.

Edited 2 time(s). Last edit at 04/26/2012 07:57PM by Norman King.
open | download - Eutrophoceras (nautiloid).jpg (170 KB)
open | download - Bucanopsis (gastropod).jpg (48.8 KB)
open | download - Phylloceras (ammonoid).jpg (59.1 KB)
Steve Tguiri April 30, 2012 12:55AM
To all that responded,

Thank you. Your input helped me identify this rock and open up more areas of study for me to explore. It was also very interesting to read the small debate that this raised in concern of properly identifying objects and backing up your observations with references.


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