Oligocene, Epoch. A division of geologic time that began about 37,000,000 years ago and lasted 14,000,000 years. The Oligocene is the third and latest division of the Paleogene Period. Most of the rock formed during the Oligocene is sedimentary-During this time some mountain building occurred in the northwestern United States. Petroleum and natural gas are found in commercial quantities in rocks of the Oligocene Epoch.