Penedos, Alien trails ?

I'm not sure how accurate this is but according to Mr Google, those white lines on rocks are called "veins" and are formed by the process of mineral deposition. Veins are formed when fluids, such as water or mineral-rich liquids, infiltrate minute cracks or fractures in rocks. As the fluids flow through the rock, they deposit minerals along the walls of the crack or fracture, forming the white lines you see.

Of course, the easy explanation is they're put on the rocks by aliens as you suggest :-))

Both are possible I suppose. Either way it's an interesting image!

Alien snails? ;-)

See my answer to Keith above!