Mastodon isn't sludge in any way, mate... sludge is hardcore punk + (proto) doom metal. It's what the Melvins, the B-side of Black Flags's My War and early Flippers spawned, so mostly the NOLA scene (Eyehategod, Crowbar, Down, Acid Bath, Buzzoven, etc...) and "others" (Grief, Floor, 16).

Perhaps we need a word to disambiguate "atmosludge" from actual sludge, for the same reason "skramz" was invented.

If there’s no bug infested dreadlocks then it ain’t sludge!

Except Dixie Dave. Eyes crossed from Jim beam and cough syrup works too.