The Elko Formation in northern Nevada’s Elko County is an oil shale-bearing geologic unit of lake and delta deposits that preserves changing environments across several spans of Cenozoic time. Its shales and limestones record intervals from the Middle Eocene into the Middle Miocene, with dated sections ranging from 46.1 million years ago to about 11.6 million years ago. It is important both for its oil shale and for fossils, including frogs, an ant, and freshwater invertebrates. Some species were named for the formation, underscoring its value as a well-dated record of ancient lakes, delta systems, and Nevada’s paleontological history.