The Word Formation is a geologic formation in the Glass Mountains of West Texas, probably named for the Old Word Ranch. It preserves fossils from the Permian period and is especially significant because the Wordian age of the Permian takes its name from it. The formation has three members, ordered from youngest to oldest: the Appel Ranch member, which is Wordian, and the China Tank and Willis Ranch members, which are Roadian. Its limestone beds record a marine environment and have produced fossils such as gastropods, brachiopods, ammonites, and the vertebrate Helicoprion davisii.