Park at Porte d'Herbouilly (D221), altitude 1,280 metres.
(S/E) At the direction sign for Favier, head towards the north-west, following the Yellow-Red waymarking to begin with. After a short climb and then a descent on a wide path, you will reach Favier at 1,211m.
Ignore the path on the left and continue to the right for a short distance up to the point marked: "le Chemin de la Pia" 1,200m.
(1) Take this path to the right in the direction of 'Pas de Saint-Martin', Yellow-Green waymarking.
The path climbs into the forest and quickly reaches some rocks which are easy to climb, although you need to be careful in wet weather. On the edge, you have a beautiful viewpoint over the "Vierge du Vercors".
On arriving at the foot of the cliffs, follow the path to the north - this is the 'Chemin de la Pia'.
Cross through a paragliding launch area, ignore the path on the right and continue ahead, still in a northerly direction, on (what we call in Chartreuse) a "sangle" ('strap'): a ledge.
Pass in front of the Saint-Martin cabin (a rough shelter on the ledge) and continue on until you reach the Pas de Saint-Martin.
Beautiful views to the north over the "Vierge" and to the west over Saint-Martin en Vercors and its meadow, and to the south over Grand-Veymont.
On the right-hand side: the cliffs, on the left-hand side: the void, but the path is wide enough, all the same. You need to be careful if you have children with you, but it doesn't present any difficulties because it is flat.
Enter into the forest. The path is still waymarked Yellow-Green, heading to the east.
(2) At the crossroads, it veers to the left and loops to the north-west. It quickly resumes its west-east axis at the place called "La parcelle 5", at an altitude of 1,350 metres. This is the highest point of the hike.
(3) Continue towards the east through the forest, ignoring a path on the left, until you arrive at the road climbing up to Chalimont Pass at the place called "Les Tenailles" at an altitude of 1,310 metres.
Cross the road and go back downhill opposite through a small section of the forest until you reach Herbouilly Meadow.
(4) At the crossroads on the Grande Traversée du Vercors path, turn right, heading due south and join the road to "Pot du Loup", at 1,300 metres altitude.
Turn left onto the road for about 500m until you reach a crossroads with a wide path.
(5) Follow the path to the left and in about 20 minutes you will reach Roybon.
(6) Turn right and follow the return path to the north-west, going in front of the 'Gîte d'étape' (large gite) called 'Auberge de Roybon'. After a short descent you will reach Porte d'Herbouilly (S/E).
