No idea, but I'd be prepared to wager a couple of quid on the latter.
Fossilised worm burrows perhaps?
Definately look like trace fossils to me. (some sort of scale would be useful though - I usually use a lens cap for reference)
I would be fascinated to know the answer if you ever fathom this one out. Who ever did it had a talent for graffiti!
Fossils are always solid, so if this is a fossilised worm track then there should be a corresponding piece of shale in which the tracks are protruding proud of the surface.If you have a geologist's hammer, or a chisel, then try splitting some of the adjoining strata to see if you can find examples of both types of track.
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