Rock Wedges, Human Made Or Biological?

[tlatoanitzin]

I found this rock in a field near a stream. I observed these wedges.

When I saw them was unavoidable to make an analogy with cuneiform script printed over baken clay tablets. Could be these wedges be human made or are they biological in origin? http://www.flickr.com/photos/51986587@N04/sets/72157632717451327/

[Scylla]

They are the imprints left behind by mineral crystals that have since weathered away. Pyrite, calcite and many other minerals can be selectively dissolved or oxidize away, leaving a very interesting defect in the host rock.

[PA Fossil Finder]

I'm sorry, but I don't see any fossil here. This rock looks like a worn pebble of granite. Granite is an intrusive igneous rock, and most have a lot of feldspar minerals in them. Feldspars, when broken, fracture in a way that is almost always a perfect right angle. This produces something that looks like a bunch of miniature steps. If there was a large chunk of feldspar that formed in your rock, a collision with another river boulder could smash it, fracturing to make these "letters".

Another mineral like this is galena. See here.

Edited February 8, 2013 by PA Fossil Finder

[painshill]

I would agree – neither man-made nor organic. Unfortunately that danged infernal slideshow doesn’t readily allow a single pic to be isolated and enlarged, but as far as I can see those are cubic holes and v-shapes from the corners of holes made by cubes. This sort of thing:

[image by Roel Tijskens]

[image by “Wrench” on Treasurenet forum]

Those ones are the result of minerals with a cubic crystal habit being eroded or tumbled out of the rock matrix. The most likely minerals for this would be fluorite, pyrite, magnetite, calcite (plus pseudomorphs), feldspar and – if you were in the right area (in Mexico you probably aren’t) – pseudomorphs after glendonite.

As PA Fossil Finder says, the cobble looks granitic (or at least igneous), so feldspar would be likely.

[tlatoanitzin]

Sorry for that flickr slideshow, I don't know how it showed up like so. Here are direct links:

http://www.flickr.com/photos/51986587@N04/8457097890/sizes/k/in/set-72157632717451327/

http://www.flickr.com/photos/51986587@N04/8455999021/sizes/k/in/set-72157632717451327/

http://www.flickr.com/photos/51986587@N04/8457096756/sizes/k/in/set-72157632717451327/

http://www.flickr.com/photos/51986587@N04/8457095924/sizes/k/in/set-72157632717451327/

http://www.flickr.com/photos/51986587@N04/8455997797/sizes/k/in/set-72157632717451327/

http://www.flickr.com/photos/51986587@N04/8457094468/sizes/k/in/set-72157632717451327/

http://www.flickr.com/photos/51986587@N04/8457094376/sizes/k/in/set-72157632717451327/

http://www.flickr.com/photos/51986587@N04/8455994931/sizes/k/in/set-72157632717451327/

Thank you guys, for your input, the pictures you posted speak for themselves, they are very informative.

[middletown]

Very instructional posting for those like me who are still learning about rock business, thanks Scilla, PA Fossil Finder, Painshill and

tlatoanitzin, great!!