This is a 16km walk, climbing around 1200m of altitude. It is a challenging walk as there’s a full selection of rock scrambling, steep gravel paths, and lots of zig zags.
The route starts starting at the southwestern corner (850m) in the carpark for the Musee de la Grand Chartreuse (La Correrie). I suggest doing the route anticlockwise, starting off with the big climb to the summit, and then taking the more gentle path down to visit the refuge at Col du Bouvinat. After that you turn towards Pas du Loup, then down to Col du Ruchere, before heading backwards towards the Grand Chartreuse Monastery at 950m
I’ve done a couple of walks in this area (all clockwise!), and had a hard time finding the path from the top of Grand Som back down to the Musee. After doing part of the route in the anticlockwise direction, I discovered that the path between the Tour de Chartreuse path and the high path at Col du Frenay isn’t signposted or marked in any way. It is on the map - but for some reason it isn’t marked on the ground. I can only presume that it might be considered a little tricky - but I didn’t think it was any worse than any of the other trails in the area.
If you do miss this trail - there is an acceptable (although longer) alternative marked below in blue.
Within the forested sections there are many logging trails, most of which are not marked on the map - so you’ll need to pay attention to the trail markings when navigating.
Note that there are a couple of dangerous sections close to the summit - and I’ve marked these in red areas on the map. According to a sign at the Musee, there have been 5 deaths in 10 years on these sections. I’ve walked the one down from Pas de la Suffiere to the south west, and while it is a bit of a scramble, with some steep drops, I wouldn’t say it was too scary.
On the other hand, the unmarked trail from Pas de La Suffiere directly to the top of the mountain is very scary. (I say unmarked - because it is so bad, it isn’t even shown on the IGN map! It is however marked on the ground with some arrows painted on the rocks, and the trail is well travelled enough to be able to see where people have walked.) This should only be attempted if your middle name is Mountain Goat, and you have no fear of heights. It is also a route that I would only attempt going up the mountain, not down. The more rational of us should take the slightly longer, and signposted path that leaves the summit to the east.
Grab the kmz for my route plan for Grand Som from here, or take a look at the route plan below.
You can also see my first attempt at everytrail.com.