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FATMAP difficulty grade
Follow the line and route description of Spencer Couloir initially.
From the mid-station of the Aiguille du Midi cable car (Plan d'Aiguille), follow signs for the Lac Bleu and start skinning eastward and under the Aiguille du Plan and the Blatière.
Cross over several small moraine walls (might require a couple of very short boot packs) and head towards the Nantillons Glacier.
Drop over one final short but steep moraine wall and onto the Nantillons Glacier.
Skin up a little further until under the first rock wall you come to.
Transition to boot packing and climb up a brief steep and rocky section often involving very faceted snow (tiring boot packing).
The angle decreases before steepening again up a 45 degree couloir.
After the couloir, it is normally possible to transition back to skinning before reaching another steeper (sometimes icy) slope.
It is possible to skin past the entrance to the Spencer until you reach a small col overlooking the Mer de Glace.
Ski back down the same way.
NB, as of summer 2017, a section of blank rock slab has opened up in the upper section of the Nantillons Glacier due to the recession of the permanent ice there.
If there is not a good build-up of snow covering it, this could be problematic.