We've covered a fair bit of ground with our Starter Kit series, where we select a handful of key records that highlight a niche musical style or penetrate the prolific status of a staple genre. Unfortunately, this format doesn't lend itself to covering proto-genres—microcosms of musical history comprised of a specific set of albums released in a fixed period of time. But these movements are crucial to the evolution of our favorite genres, particularly when it comes to the trajectory of sludge metal. What's become a multifaceted and often refined style was once a disparate lineage of bands from different genres who all applied the "sludge factor" in different measures. While you won't find a dedicated section for proto-sludge at your preferred music store, the following albums an artists laid the framework for the modern sludge landscape. So whether your sludge purveyors of choice come from the atmospheric, blackened or progressive sects of he genre, they're all indebted to the groundbreaking statements these albums made.