The sediments forming the Citronelle Formation occur as a nearly continuous blanket along the southern margin of the Coastal Plain and can be traced from Texas across the Gulf Coast into peninsular Florida. The lack of fossil evidence, however, has made this formation the subject of a heated age controversy since it was first described in 1916. At present, its age is given by various investigators as late Miocene, Pliocene, Plio-Pleistocene or Pleistocene.
The recent discovery of abundant vertebrate fossils, at a site in northern Mobile County, Alabama, near the base of the formation, indicates that the maximum age of the Citronelle is mid-Pliocene (Hemphillian). Other evidence is present to indicate that the rocks of fossil zone represent a brackish estuary that was later filled by river encroachment, forming the overlying fluviatile sediments typical of the Citronelle Formation. Hence, the faunal evidence, coupled with existing pollen data from the upper sediments of the Citronelle in nearby Florida, now indicates that deposition of this formation began in the middle Pliocene and continued into the pre-Nebraskan Pleistocene.