I found this in skegness. I know it is not a fossil, but it is very interesting. Half of it is flint and the other half is chalk.. How could that have happened?
I think we need Mike to tell us how flint is formed in chalk, but just for interest I have added an image of a sea urchin made of flint with flint matrix attached to it that has come out of the chalk.
The white crust (called the "cortex") is not made of chalk, but fine-grained opaline silica.
This is a very basic definition, but flint is a silicon based rock which formed in the upper chalk. The silicon was sourced mainly from sponges which filled cavities left from previous marine life. Flint can be found as nodules and sheets and often contain fossils such as your echinoids Tabfish.
As far as I am aware, that is more or less the current theory of flint formation but it is still being discussed!
Thank's UK Fossil Guy for explaining some of the makeup of what Dan and I have found.
I do not collect in the chalk but as you know occasionally find the fossils of the chalk on the Holderness coast.
Yes, thanks UK Fossil Guy.
Tabfish, I can't do better than Luanne's explanation of flint and chert here