I think that's right. The carvings you mention look very modern. ie V-shaped (carved by metal perhaps) not U-shaped.
My most recent (frustrating) experiences with the more obscure (but catalogued) rock art is that if you can't see it in broad daylight, then it must be genuinely prehistoric! You get to the rock with coordinates and yet you see nothing but a couple of very vague and eroded carvings. Cupmarks are the exception, being easier to spot and, for some reason, harder to erode(?).
Mind you, the most amazing designs do sometimes unfold under a torchlight.
http://www.murosarqueoloxico.org/?page_id=219&album=2&gallery=10
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