Geographers were once concerned largely with exploring areas [unknown to them] and from describing distinctive features of individual places.
I believe it's the unwanted "from." And personally, I would have said "... were once largely concerned with ...," but that was accepted in the original. Ignoring the position of "largely," it breaks down like this:
Geographers were once concerned with
1) exploring areas unknown to them
2) describing distinctive features of individual places.