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Geology/kind of rock?


Dear Robert:

I was picking up rocks for my husband, he farms and rocks breaks the equipment.  I found a rock that reminds me of a turtle's shell but it looks like some kind of cartilige joints the pieces together.  It is hard as the rock.  A piece was broken off and the inside is black and dark green.  I haven't ever seen a rock like this one.  Could you possilby have an idea?  I would really like to know.  Thank you,  Teresa Davis

Hi Teresa,
Without a picture it really is impossible to reliably identify your rock.  Also, you don't mention where you found it or how large it is, though I suspect it's at least fist sized since you mention your husband removes rocks that can damage his farm equipment.  However, your description of a "turtle's shell" instantly suggests a rock called a septarian nodule or concretion.

A concretion is a hard, massive sedimentary rock.  The original rocks are usually limestone or clay that, because of mineral replacement, have become much harder than the rock that surrounds them.  They are usually spherical-ish in shape but can be very irregular.  It's derived from Latin "con" meaning together and "crescere" meaning "to grow".  They appear to grow in place in sedimentary layers, and while the process is not well understood, there is usually a nucleus of some kind within the rock.  A "nodule" is much the same as a concretion but it usually doesn't have a nucleus at it's center but it also grows in place.  Most times, as the rock dries out and lithifies many cracks form inside.  Mineral laden water flows through these cracks and slowly fills the cracks up.  The result is a kind of tree pattern inside the rock and set of polygonal cracks on the surface.

They have long been misinterpreted as organic and some have even been described as fossilized turtles.  Here's a picture of a septarian from Arkansas;

This is only the outside of the rock.  This site;

gives you a little more information about septarians, including an interior image.

The problem is that you describe the interior as "black and dark green".  It isn't impossible that the rock is a septarian but they are not normally that color.  It probably still is some kind of sedimentary rock but to really identify it there is no getting around an image.  If you can't send me a picture, try taking it to the Geology department of your local college, or the Geological Survey #or equivalent# of your State government.

If you still prefer my help please send me another question with a clear picture with something for size in it #a coin works well#.  Also, tell me what State you are in.  Sometimes you can identify a rock just by knowing where it comes from.

I am still going to go with your rock being a septarian nodule, barring any additional information.

Hope this helps.


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C. Robert Reszka, Jr.


I can answer any general geology question (rocks, minerals, stratigraphy, geomorphology etc.). My expertise is in the geology of the Michigan Basin, PreCambrian, Paleozoic and Recent. I can answer questions concerning mining and petroleum exploration and production and the laws concerning those activities. I can also answer questions concerning stratigraphy of the Michigan Basin. I will also answer questions about mineral and rock collecting in the Basin. I won`t be able to answer many specific questions on hydrology, geophysics or geochemistry. I may be able to answer very general questions in those venues.


I have been working for the State of Michigan for 36 years as a Geologist and a Resource Analyst. I have experience with Subsurface Geology and Petroleum Geology, mining in Michigan, and Sand Dune Mining and Protection issues.

Michigan Basin Geological Society

Decade of North American Geology.
Bedrock Geology of Michigan

BS Wayne State University

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