Fourwinds matey,
the latitude for the Devils Arrows is approximately 54 degrees 20 minutes.
Geology may help with this one. the Devils arrows are Millstone grit. and from a site about five miles away from where they now stand. What are your stones?
these rocks weather along their bedding planes, so if you stand a rock upright with its bedding planes running vertically you will get this form of longitudinal erosion, not to say that this cannot be exagerated by pecking the lines out.
Another possiblity is the striations you get on glacial material (erotic erratics).
Happy hunting, I'm following your travels with great interest and some envy.
Fitz

It has been so long since I visited the Devils Arrows that I cannot remember whether the grooves appear on 'all four sides', as it were.

I have waited until I got my photies back to confirm that the Carlow stones do have grooves all the way around. To me, this does not seem entirely consistant with erosion on the bedding plane. Erosion on the bedding plane would imply erosion on just two opposite sides ... am I wrong?

I have scanned in the first one and added it now

http://homepage.ntlworld.ie/tom.fourwinds/williamstown/williamstown.htm

I have had to increase the compression on my images to save space, so I appologise for the quality. This will be resolved when the new site is up and functioning.