Petit and Grand Som via the Grande Chartreuse monastery

Park in the Correrie car park.

(S/E) Set off in the opposite direction. After 200 m, take the path blocked by a barrier that branches off to the right. Continue until you reach the Grande Charteuse Monastery, remembering to respect the quiet zone. Once past the monastery, head up the hill to the right to enjoy a lovely view.

(1) Head back down the same path you came up, then continue to the right until you reach Notre-Dame de Casalibus.

(2) Leave the path on your right (which will be your return route) and, at the three-way junction, take the middle path leading to the Chapel of Saint Bruno.

(3) Retrace your steps and continue to the left. Keep going straight on until you leave the forest and reach some ruins.

(4) Continue along the edge of the forest on your right. Once at the Prairie du Col, turn right.

(5) Leave the return path on your right and continue towards the Col de la Ruchère. Before reaching it, turn right and begin a series of hairpin bends until you reach a crossroads with a sign pointing to the Pas du Loup.

(6) Turn right and follow the path towards the Pas du Loup. Continue until you reach the Habert de Bovinant.

(7) Head right, southwards, to the Col de Mauvernay, then continue until you reach the start of the Pas de Suiffière.

(8) Head due north, starting with a short, easy climb: the rest of the route will not be any more technically challenging, but there will be much more exposure. Continue along the ridges, following the signposting (which is not always clearly visible) to reach the summit of the Grand Som.

(9) Follow the path that veers slightly to the right, still heading north. After about 800 m, take the path branching off to the right, following the markings until you reach the Col de Bovinant: take care on the descent, which is usually slippery.

(10) Turn left towards the Petit Som.

(11) Pass via the Col de Léchaud and continue straight on to the Petit Som.

(12) Take the path heading south, then turn south-west to descend towards the Col de la Ruchère: be careful, this section can be very slippery.

(6) Continue down the same path you took on the way up to reach the junction at Prairie du Col.

(5) Take the path on the left towards the forest and follow the signs back to Notre-Dame de Casalibus.

(2) Head back down the path you came up to reach the car park (S/E)

Allow 7½ hours at a leisurely pace to reach the Petit Som (including a lunch break), then two hours for the descent.
Although impressive and exposed, I don’t think the Pas de la Suiffère is the most dangerous section. It is mainly on the descents towards the Col de Bovinant and the Col de la Ruchère that you need to be careful: you’re on steep slopes with a mix of slippery rocks and muddy paths.
So it’s best to wear good walking boots, carry walking poles and check that it hasn’t rained for at least three days.
However, the Pas de la Suiffère remains exposed with short sections of easy climbing: it is therefore not necessarily recommended for those prone to vertigo. Furthermore, the distance and elevation gain require a reasonable level of fitness to complete the route.