Aha...

...I thought. (Sitting beside a drystone wall). Fossils! What fun. I shall go inside and identify them immediately.

It turns out that the identification was not only difficult but, it also made me question whether these little indentations are fossils (or just the marks of the nails of someone’s boots on a stone... ? Are there even nails in boots? How the plot (and my ignorance) thickens).
Maybe it’s coral??

I don’t know. :-)