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Geology/crab legs fossil?



Hi Keith,
Check out this rock my boyfriend found in a stream at Peekamoose in the Catskill Mountains. It's about 5" wide. Any ideas about it? Looks like crab legs to me! Thanks for any help.

What you have looks to be clay rip up clasts embedded in siltstone.

Not being able to really discern the granularity differences in the inclusions and the surrounding matrix stone, I am taking a educated guess.

Remember that the inclusions, are in the stone in three dimensions, and what looks like a "slender" leg actually extends into the stone as a flat plane.  The shapes will vary depending on how the surface of the rock "cuts" or intersects the inclusion.  For example if I cut you hand in half perpendicular to your palm it would look narrow and curved, if I cut it in half flatwise, parallel to your palm, it would look wide and flat.  So it is with inclusions and how they look on the outside of the rock they are encased in.

You may have see this at work in the ladies room, if you have ever been in one where they have marble containing "ghost" sea shells.  Some are intersected by the blade that cut the stone and look like commas , others look like donuts if cut flat wise as I demonstrated by the palm example.

What you might do is try to map any of the inclusions through the stone from one side to the other.

How rip up clasts occur is this:

You have a layer of mud it begins to dry and crack.  You know you've seen low areas in a stream where mud has settled, and wide cracks open.

Then comes rain, a torrent, and a kind of like a flash mudslide fills the stream bed, and it peels up layers of the clay and they get mixed in with the silty sediment that formed the mudslide.

The clay clasts get preserved in their tangled orientation since they cannot settle out in the encasing sediment.

This all gets buried under other sediment and eventually lithifies into rock.

Then you piece weathers out makes its way to a stream and gets bounced around and rounded to its present shape.

Now, if these were truly fossils, there would be planes of weakness along the different material types, and the rough handling in the stream that caused the rounding would have broken the rock apart before it got to its well rounded state.

Now, if on closer examination of the rock you see that there is absolutiely no difference in the granularity, or crystalline structure, then I could be wrong.

Clay deposits on winter lakes, create varves, alternating light colored and dark colored clay layers.

It oould be that some how some of the light colored layers got intermixed in with the darker clay by turbulence or stream or flood action and created your rock.

Have you tried to put any kind of acid on the different parts to see if there is any calcite present?  


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Keith Patton


I can answer questions concerning physical and historical geology, environmental geology/hydrology, environmental consulting, remote sensing/aerial photo interpretation, G&G computer applications, petroleum exploration, drilling, geochemistry, geochemical and microbiological prospecting, 3D reservoir modeling, computer mapping and drilling.I am not a geophysicist.


I have 24 years experience split between the petroleum and environmental industries. I have served as an expert witness in remote sensing, developmental geologist, exploration geologist, enviromental project manager, and subject matter expert in geology and geophysical software development.

American Association of Petroleum Geologists
American Association of Photogrammetrists and Remote Sensing

Bachelor and Master of Science
Registered Geologist in State of Texas

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