In the Tagus drainage (Spain), drovers have been building up these pillars with stones at selected locations. They stand up in prominent hills and outcrops. Being curious about their meaning, I inquired a shepherd about their function and the criteria they use to choose their location. His answer could not be more enigmatic: the pillars have no meaning, they are not landmarks, and surprisingly they build them in places which seem to be affording for holding one. So, their value as true landmarks is highly questionable.