The Willwood Formation is a sedimentary sequence deposited during the late Paleocene to early Eocene, or Clarkforkian, Wasatchian and Bridgerian in the NALMA classification.

[1][2] It consists of fine grained clastic rocks (mudstone and shale) interbedded with medium grained clastic rocks (sandstone) and sporadic conglomerates.

The formation underlies portions of the Bighorn Basin of Big Horn, Hot Springs, Park and Washakie counties of Wyoming.

[3] Radiometric dating of volcanic tuffs, combined with comparisons with other formations using magnetostratigraphy, using numerous samples from various levels of the formation suggest an age range of 55 – 52 million years ago, placing the Paleocene-Eocene boundary near the base of the formation.

[5] Fossil birds include Gastornis, Neocathartes and Paracathartes.