intaglio

In the a tradition parallel to trap streets (see previously here and here) but with more delightful consequences, we discover that Swiss map-markers faced with the potentially tedious task of reproducing the country’s varied topology are not above, like the marginalia of a medieval scribe, of seeking a bit of relief by the occasional hidden doodle in a mountain face—those isometric lines indicating relief, the angle and the orientation of the slope (and in no way snuck into a guide for climbers mind you), being called hachures (Schraffen oder Bergstriche). See more examples at the link above from Amusing Planet.