Relative to global geological chronology (the geologic time scale), it is usually considered to fall within the earliest part (early Rupelian stage) of the Oligocene epoch, beginning around the same time as the Eocene-Oligocene boundary.
[1] The Orellan is named after the Orella Member of the Brule Formation, a widespread subunit of the White River Group.
In the White River badlands of southwest South Dakota, an equally extensive Orellan component of the Brule Formation is known as the Scenic Member.
Orellan strata of the Brule Formation can be found in more restricted exposures across northeast Colorado (Cedar Creek Member) and North Dakota.
Other mammals which first appear at the start of the stage include Leptomeryx evansi (a leptomerycid ruminant) and Palaeolagus intermedius (an early lagomorph).