The stratigraphic distribution of recently collected Uintan fossil mammal localities in the Wagonhound Member and Myton Member of the Uinta Formation, show evidence for a higher Uinta B2-Uinta C faunal transition and higher lithological boundary in the Uinta Formation. Continuing collecting efforts and stratigraphic work in the upper Uinta B and Uinta C intervals of the Uinta formation have resulted in the identification and in some cases the extension of the local ranges of numerous Uinta B2 mammalian taxa such as Epihippus gracilis, Mesomeryx grangeri, Pareumys milleri, Pareumys grangeri, and Protoptychus hatcheri. These and other mammalian taxa show extended local ranges above the Amynodon sandstone, a geological structure thought to be the boundary between the Uinta B and Uinta C intervals of the Uinta Formation and is often used to approximate the interval where the Uinta B2-Uinta C faunal transition occurs. Based upon these extended ranges of classic Uinta B2 mammalian taxa we suggest the faunal transition zone from Uinta B2 faunas to Uinta C faunas occurred 59-120 meters above the Amynodon sandstone. The lithological transition from Uinta B to Uinta C rocks in the Uinta Formation appears to occur higher than the Amynodon sandstone. A summary of descriptive microstratigraphy suggests that based upon a distinct change in lithological structure and color, the boundary should be placed 73-76 meters higher than the Amynodon sandstone boundary.