The boundary between natural and artificial is very diffuse with rock carving. What the ancients made of fossils is anyone's guess, really. These elongated 'slots' appear in grit stones where fossilised mussels have been bedded. Mussels are strange creatures - I've kept freshwater specimens as pets - and, perhaps, line up for some strange reason. The fossilised slots are discoloured and tight when the rock is split - the boat-shaped cupmarks, if indeed that is what they are, have been worked slightly, losing the dark colouration. I have a fossilised mussel - if anyone fancies trying it for fit - Cinderella style - with these cupmarks.