Lamb Of God & Mastodon at Red Rocks Amphitheatre

Lamb of God

Lamb of God is regarded as one of the "big four" bands of the New Wave of American Heavy Metal movement, along with Killswitch Engage, Shadows Fall, and Unearth. The band is primarily recognized as a groove metal band. Their music has also been categorized as metalcore, thrash metal, and death metal. Their 2000 album New American Gospel was characterized as "grindcore and death metal for the hardcore kids." The band's 2004 album Ashes of the Wake was characterized as "to showcase their thrash and melodic death metal influences and break out of the metalcore category critics tried to push the band into."

Wrath (2009) and Resolution (2012) demonstrated the band's direction towards incorporating more thrash metal elements in later albums. Burn the Priest has been described as a combination of death metal, punk, and thrash metal. Legion: XX, the Burn the Priest cover album, has been described as crossover thrash, hardcore punk, punk rock, and sludge metal.

Mastodon

Progressive metal, alternative metal, sludge metal, stoner metal, psychedelic metal, experimental metal, and groove metal are some of the genres that Mastodon has become known for. James Christopher Monger of AllMusic calls Mastodon "one of the more notable new wave of American heavy metal acts, a genre spawned in the mid-'90s" and adds that "Mastodon's innovative, lyrically astute blend of progressive metal, grindcore, and hardcore helped position the band as one of the preeminent metal acts of the early 21st century."

Their bassist Troy Sanders says, “We don't consider ourselves a metal band. We recognize there is a lot of metal in us, but we also want to believe we have a lot of rock 'n' roll in us, and we have a lot of progressive rock in us, we have bits and pieces of thrash and punk, and psychedelic-ness sprinkled throughout. A lot of times for me personally, just having the metal tag itself seems kind of limiting on us because we have such an appreciation for all styles of music and we like to incorporate bits and pieces of those into our songs. But we also recognize all the different subgenres and the millions of times people say, ‘Oh, what do they sound like?’ and you say something, I understand that. But, it doesn't affect how we create anything at all.”