The Little Butte Volcanic Series dates from the Oligocene to the Early Miocene epochs. "This series of volcanic rocks, which is hundreds of feet thick and wide spread, consists of lava flows and vast amounts of tuff and ash that erupted from local volcanoes over a period of several millions of years." - The Ore Bin, Vol. 30, No. 4, April 1968, State of Oregon, Department of Geology and Mineral Industries.