Markings on Thurso Beach Rocks - is it fossilised plants?

Can anyone help identify the cause of the markings I noticed on the rocks at Thurso beach?

Photos posted in photography section at http://forum.caithness.org/showthread.php?t=44741

The white stuff is some sort of algae. The short dark markings are where snails or limpets have grazed on the algae leaving the dark rock surface. Interesting in itself but not as exciting as finding a fossil.

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Originally Posted by Stargazer

The white stuff is some sort of algae. The short dark markings are where snails or limpets have grazed on the algae leaving the dark rock surface. Interesting in itself but not as exciting as finding a fossil.

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Interesting indeed - I never even contemplated that it could be something like that. Thanks for the info. I must have another close look next time the tide is far enough out.