Sludge is in development, last decade, sludge was a shit, whit country/blues influence, but now it is backing to its roots, as doom, hardcore punk and extreme metal (grind/death/black/thrash). Sludge is turning more musical, when it was only "music".
Look at this time line.
- Melvins (grungy sludge)
- Black Label Society (country idiot sludge)
- Crowbar (psycho/thrash sludge)
- Neurosis (math/grind sludge)
- Cult of Luna (atmospheric sludge)
- The Ocean (artistic/prog sludge)
Sludge are backing to his roots: Grunge, Hardcore Punk, Doom Metal, Extreme Metal, Atmospheric Metal. Sludge is Black Flag's My War, but the guys thing that sludge and stoner was the same thing and start putting blues/country in it. But later, turn more and more gothic (not "gothic"), backing to the real roots. With metal happen the same thing (first, cool proto-music: classic/doom metal; second, idiots mixing things: glam/pop metal; third, backing to the roots: thrash/speed metal; forth, alternative cool sound: death/black/grind/gothic metal).
I like to think that sludge is a new genre. Sludge isn't Metal neither Punk, is Sludge.