Fossils from the field

A and her daughter have been picking up stones from one of the fields today. Farm machinery could break if hit by a large stone while the field is being worked.  These appear to be be fossilised cockles, still tight shut.  Our valley used to be under water millions of years ago.  Shells and debris are often seen embedded in the rocks and stones but these are the first complete cockles I have ever seen.  Sections of ammonites were also found.