Although stoner bands like Bongzilla, Electric Wizard, & Unearthly Trance, and the like are often misclassified as sludge, the similarities are primarily a matter of historic concept & some musical attitudes, but they sound distinct -- stoner metal tends to be more melodic, while one of the prime aesthetics of sludge is discordance -- in a Godflesh or Scorn, uncompromising sense.
It grew out of Lousiana in the early nineties, although it is remarkably similar to the Melvins' work prior to that, and there is a significant amount of influence. Though Flipper, Swans, Neurosis can be credited as the first proto-sludge bands, I would opine that EyeHateGod, Crowbar, & Buzzov*en were the earliest distinct bands (88-90). Acid Bath ('91) could also be credited as a sludge band, but I believe they are distinct, although having influenced & been influenced by the genre.
Soilent Green (formed '88) and Dystopia ('91) are the next bands on the family tree, and practiced a grind-influenced form of sludge that's best described as a cacophony of hatred & pop culture references, overladen with discordant bass lines and almost nulled-with-distortion guitar work, staples of sludge metal.
Cavity ('92) formed in Florida, and has broke up since ('03), but are notable for their relatively prolific number of works.
Iron Monkey emerged around '94, taking more influen...