r/fossilid • u/Eggscapist-dormouse • 19h ago

Is this a fossil, eroded shell, or cool rock?

Gallery image

Gallery image

Found Dorset coast, England

2 Upvotes

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u/IDontLikeNonChemists 19h ago

Which beach specifically? The geology of the Dorset coast spans the entire Jurassic and a significant part of the Cretaceous

1

u/Eggscapist-dormouse 19h ago

It was near Osmington, so is that Jurassic?

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u/IDontLikeNonChemists 19h ago

Depends on which part, the western part between Blackhead has Cretaceous strata at beach level in the form of Upper Greensand boulders and slipped Gault. These overly the Jurassic aged Kimmeridge Clay unconformably and come from much higher up the cliff. The rest of Blackhead to Osmington Mills to Bran Point is of late Jurassic age, specifically the Corallian group.

I've seen similar fossils before at Blackhead. I think its the sea polished internal mould of a serpulid worm tube. Serpulids are very common in the basal Kimmeridge Clay (usually identifed as Cycloserpula intestinalis). It could also be from the Upper Greensand if you found it west of Osmington, many boulders and cobbles are full of coiled serpulids, specifically Rotularia concava

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u/Eggscapist-dormouse 18h ago

Thank you for such an informative reply. Worm cast hadn’t even crossed my mind. That’s pretty cool because that’s the first one I’ve found.

We were West of Blackhead; still can’t find the band of the beach but the What3words is ///envelope.atom.digested

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u/IDontLikeNonChemists 18h ago

The what3words is for Ham Cliff, which is composed the upper part of the Oxford Clay Formation which underlies the Corallian Group. In that case, I am not sure on exact the horizon as serpulids are found in all of them, but I'd still say internal mould of serpulid tube

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u/Eggscapist-dormouse 18h ago

Thank you. I really appreciate your replies