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Marion Becker
We found this interesting fossil on the beach between Mundesley and Trimingham (Norfolk) and like to know if it is a sponge. It seems hollow with an opening at the top. It has two layers like a smooth mantle surrounding the inner body. The entire length measures 8 Cm. Any information as to species, age and life would be much appreciated. Click image for larger version - Name: 90C5728F-C853-4348-978C-1C915B898884.jpeg, Views: 35, Size: 480.96 KB Click image for larger version - Name: E121C659-333B-4235-B5D6-6FFD6F22700E.jpeg, Views: 34, Size: 322.83 KB
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prep01
Hello Marion and welcome to the forum. Yes, it is a sponge fossilised in flint and is aroun 75 - 90 million years old. I can't see enough of it really, but may well be a Ventriculites.
Colin Huller
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Barrow Museum
Marion,
I am afraid that I disagree with the previous identification.  It is an ironstone nodule, created by what is now iron oxide/hydroxide of a constituent clay pebble concentrating around its outer margin, probably within a mainly sandstone deposit.  They have been called "Boxstones".  These occur in several geological horizons, any one of which could be the origin of your sample.  For example, the Jurassic or Lower Cretaceous would be quite possible for your Norfolk find, which will likely have been transported there by glacial ice.  Your specimen has, I suspect been broken exposing the softer core of the clay pebble in the middle which has been washed away, leaving the harder concentrically mineralised rim or crust.  The iron mineral is loosely termed Limonite, but may well have been deposited in another form and subsequently been transformed into its current mineral composition.  So, not a fossil, but an attractive piece of stone with a fascinating history nevertheless.  Compare it with the example I found online...


Limonite nodule.jpg 
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TqB
I agree with Barrow Museum. [smile]
Tarquin
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