There has to be a good reason to mark this spot on the map. It must have been important, at least to him. This location is very difficult to get to. It would probably take a day or more to hike in. I cannot see a cabin in the photos but cabins usually don't show up, especially in the trees.
Main camp is in a clearing.
One of the letters was extremely interesting also. It seems that he corresponded regularly with his niece. Of course you would never get to read what he wrote. There were other letters, but none with any references of this nature. I intend to follow up on this bit of information and possibly make contact with his niece.