Interesting, my thoughts ,
the right hand side of the picture is lower than the left
Some of the markings are symetrical aligning on a crack, almost contours, it appears to me they represent different water levels and as a consequence evaperated salt deposits
Alternatively the rock is composed of very thin different coloured layers which have been eroded /dissolved by running water,
how hard is it
Any other suggestions like Mike I never seen anything like it
So we are in the Mylor Slate Formation (Devonian) - composed of slate and siltstone.
But those have been subject to hydrothermal metamorphism, which may be a factor in your particular rocks.
However, the structures appear at least partly sedimentary to me.
I have found nothing in the literature to explain it.
I think we need some local knowledge.
So may I suggest you contact the Royal Geological Society of Cornwal (200 years old this year)
And please let us know if they give you an explanation!
Jessica Winder has posted on her Nature Blog re post-storm newly-exposed rocks nearby at Mewslade Bay on The Gower
The connection is tenuous, but I thought she'd be interested anyway.
So I've given her a gentle nudge to look at this discussion to see if she has seen anything like this.
Seeing as the the thought is going through my mind, I might as well share it...
I could imagine these structures to have been formed by soft-sediment deformation. Some sand/silt volcanoes arise from sand/silt dykes (the effusion tending to change from being dyke-wide to discrete vents upwards). I have seen this in the Namurian on the west coast of County Clare, Ireland. The symmetry of the layering either side of what are now dark lines goes along with an effusive process (on a much smaller scale somewhat like the mirrored magnetic striping in the lavas either side of the mid-atlantic ridge).
The photos show some other hints that support that idea (rather difficult to explain without pointing an gesticulation).
There are aspects that I have not seen before: primarily, the mirroring and aspects of the geometry.
Just to be clear: the processes I am writing about took place before the sediment was lithified, and probably not long after its initial deposition.