Northern Ash is the heavy brother to the swamp ash. They must be sneaking in some northern pieces.
Getting a bit off the original topic but there is no "Swamp Ash" species. It is simply ash that is grown in wetter and perhaps a bit warmer areas. The trees grow faster and thus the wood is a bit softer and lighter. Most of it has been harvested. With the emerald ash borer problems there is going to be less and less ash available.