Originally posted by GunsForBucks:

The elder isn't in a swamp. If you are just seeking out the elder you might want to avoid swamps beyond scouting the edges for resources and marking any crypts you find.

He can be in a Black Forest surrounded by Swamp (or Plains, or Mountain).

I once had a Moder altar that was literally inaccessible without going through Plains first (yes, I circled the entire island).