A few days ago I asked you what this was….


…and I have had some very interesting and entertaining suggestions.   Thank you all very much!

I must admit that, if I didn’t know better and someone showed this object to me, I’d be imagining that something awful had happened to their pet tortoise.

But congratulations to Nicky Moxey, who guessed correctly that it is a clay nodule with a fossil inside!  Well done, Nicky!

This is what geologists call a septarian nodule.   I was delighted to see it (among several others) in a rock and fossil shop called Cornerstones in Ingleton, West Yorkshire, which I can completely recommend as a very absorbing place to spend an hour or more.

Septarian nodules are believed to have formed on the sea bed between 50 to 70 million years ago in the Cretaceous period, when sediment settled around the decomposing body of a small animal – a mollusc, perhaps.  There was a chemical attraction between the mollusc and the sediment, and the silt layer gradually formed into a ball around the body.

When the sea level dropped, the balls were exposed to the air, which caused them to shrink and dry out.   Cracks formed in the surface, and mineral-bearing liquids seeped in to fill them.  Calcite crystals formed in the crevices within the mud ball, and some of this was transformed into aragonite where it came into contact with the clay surface.

So, if you split the whole thing in two, you would probably find a very small fossil of a bivalve or something equally humble, imprisoned in a cave of cream calcite crystals and dark brown aragonite.  When the halves are sliced and polished, they are incredibly beautiful.

Section of a septarian nodule, part of the Pinch Collection at the Canadian Museum of Nature, Ottawa; courtesy Keith Pomakis via Wikimedia

Section of a septarian nodule, part of the Pinch Collection at the Canadian Museum of Nature, Ottawa; courtesy Keith Pomakis via Wikimedia

I know that the excellent Mr Wood’s Fossils in Edinburgh has some wonderful examples (or at least, he did when I was last there).

The word ‘septarian’ refers to the fact that the clay surface often cracks in seven directions, and it’s interesting to see that my own rock has seven radiating cracks – although this isn’t always the case.

Image credit neptunerover via Wikimedia

Septarian nodule sliced and polished. Image credit neptunerover via Wikimedia

The owner of Cornerstones in Ingleton told me that some septarian nodules have been found in streams around Yorkshire and Cumbria, but looking online I see that some of the best examples have been found in Utah.

Incidentally, I remember a friend showing me one of these nodules years ago, when we lived in North Wales.  He had been given it by one of his own friends, who I think had found it in a local river, and he wanted to know what it was.  We puzzled over it for a long time and I reluctantly gave it back, none the wiser.   But now I know – it was a septarian nodule!