Which band member and co-founder left R.E.M. in 1997 after 17 active years as drummer?

Explanation

R.E.M. was an American rock band formed by Michael Stipe (vocals), Bill Berry (drums), Peter Buck (guitar) and Mike Mills (bass) in 1980. The band's discography includes their debut album "Murmur" (1983), "Out of Time" (1991), "Automatic for the People" (1992), "Monster" (1994), "Accelerate" (2008) and "Collapse into Now" (2011). Over the years, the members of R.E.M. have been politically active, focusing especially on social issues.