These basins were likely part of a single large continuous basin that stretched across southern Wyoming during the early Paleocene. Coarse grain deposits are found at the contact between the Hanna and Ferris Formations. Biostratigraphy of mammals, freshwater mollusks, leafs and pollen indicate an early Torrejonian age (63 mya) for this event (Lillegraven et al. 2002). However, the Ferris Formation represents a rapid period of subsidence from ~65 to 63 mya, indicating that regional uplift likely occurred much earlier than 63 mya, with localized uplift and deformation occurring from 63 to ~59 mya (Kraatz, 2002; Eberle & Lillegraven, 1998; Secord, 1998).