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