Petroglifo da Pedra da Cabra

so you turn off the road, end up in the mountains and you're following these.

until they end. leaving you at the high cross (a common feature of the Galician landscape)

but petrogylphs tend to be on bedrock outcrops like this one.

There it is! This is dated to the arte rupestre period in Galicia, c. 3000-1000BCE.