The area is full of cinder cone volcanoes and intrusive lava plugs, which occur in irregular patterns. Made up of red to gray cinders, some are loose and agglutinated, and some contain intrusive rock plugs, while others do not. Cinder cones south of Mount Jefferson erupted lava flows, such as Forked Butte and North Cinder Peak. About 1,000 years ago, the South Cinder Peak cinder cone erupted, generating a lava flow that reached Marion Lake. Other volcanic cones associated with Mount Jefferson include Forked Butte and Horseshoe Cone.