Geologist (born Brian Ross Weitz on March 26, 1979) is a founding member of Animal Collective. He provides electronic sound manipulations and samples for the band. Geologist grew up in Philadelphia and Baltimore and currently lives in Washington DC.

The "Geologist" nickname comes from a friend's misinterpretation of Brian's university major. Geologist attended the Park School of Baltimore where he met future Animal Collective bandmates David Portner (aka Avey Tare) and Josh Dibb (aka Deakin). Avey Tare, Geologist and Deakin first started an indie-rock band called "Automine" with two other schoolmates. Deakin introduced his childhood friend Noah Lennox (aka Panda Bear) to Avey Tare and Geologist. Throughout their high school years and winter/summer college breaks, the four of them played music in different variations and often solo, swapping homemade recordings and sharing ideas.

Geologist went on to study at Columbia University, while Avey Tare went to NYU, Panda Bear to Boston University, and Deakin to Brandeis University. Avey Tare recorded tracks with Panda Bear which eventually became Spirit They're Gone, Spirit They've Vanished, which was released under "Avey Tare and Panda Bear" in August 2000. After Panda Bear and Deakin both left school and moved to New York in 2000, the group's music became more collaborative, and Avey and Panda began playing clubs around New York. Geologist soon began performing with the group who eventually became "Animal Collective." He first appeared on their 2001 release Danse Manatee, and has contributed to every Animal Collective recording to date, except for their 2004 Sung Tongs LP. Before that time, Geologist had gone to Arizona for graduate school.

He has performed DJ sets both as Geologist and under the name Obrian System, released "Seeing Twinkles" with fellow Animal Collective member Deakin on the Music for Plants Compilation, and in 2011 released his first solo track on the Keep Company's Animal Collective Tape.