A sharp eyed walk along the beach at Cocklawburn or on Lindisfarne and with a bit of luck you may find a small disc, shaped like a tiny petrified polo mint.
Before I answer that question, here is a little bit of background information from a manuscript fragment recently discovered in an obscure roman outpost at Ecclesia Novum-Oppidum near to the Fluvium Collegium (surprisingly close to where I live!) and published in the journal Falsus Nuntium.
This project, which was completed during 2015 organised by the Howick Heritage Group and supported with HLF funding, delivered a book, some educational work with a local first and middle school and a series of walks and talks. In this section of geo-diversity resources you can have a look at the presentation given for the walks and talks as well as much of the material which was used in the educational work including lesson plans, activities and exercises. There is also a handy guide to fossils to be found on the beach at Seahouses (and elsewhere on the coast).