The question goes a little deeper than simply depress over the Rockies or Alps. Nowhere has such expansive or high terrain as the Himalayas and Tien Shan mountains to the north. Quite fascinating to fly over there at 35,000' and see terrain so close! The problems involved there are that the terrain vastly exceeds 10,000' for such a large area you can get easily get caught exceeding the emergency oxygen supply for the passengers in a depress by having to remain above MSA which will greatly exceed 10,000'. This sort of terrain even a 747 cannot fly directly over, but it is acceptable to skirt the edges where you are still affected by high MSAs. The requirement on those particular legs where MSA is so high is you need to be able to reach a point and descend within your passenger oxygen limit, and if need be exit your route onto a lower MSA and descend. Those legs with the problem have escape routes published off the main route, and one must be aware which is nearest to get to and exit and descend within the time available. I recall this is a problem near Urumchi, north western China. It goes without saying that the exit routes should lead you to a suitable diversion route and airfield itself unaffected by oxygen requirements en route, as you will then be at a maximum of 10,000'.
It is advisable to warn the cabin crew when you are approaching such areas so they can be advised that any emergency descent will not get rid of the requirement to remain on oxygen.