Evidence for post-Laramide (Eocene and younger) tectonism and graben development is widely exposed in the Rocky Mountain region, but the overall temporal progression of events is poorly known. Young strata deposited in the eastern part of the Sweetwater Arch in central Wyoming preserve an illuminating record for the Miocene. The Sweetwater Arch represents remnants of a large Laramide mountain range that was once similar in magnitude to the Wind River and Big Horn Mountains of today. The Granite Mountains, which are contained within an east–west graben superimposed on the Sweetwater Arch, were down-dropped beginning in the late Tertiary. Presently, these mountains are erosionally reduced to granite knobs mostly buried by Miocene strata, such as the lower Miocene Split Rock Formation. Continued sagging and down-drop later in the Miocene allowed development of lakes along the axis of downwarp and deposition of lacustrine sediments in the Moonstone Formation. The age, geology, and paleontology of this formation have not been studied in detail; however, it contains datable tuff layers and mammalian fossils that can be used to study the timing of concurrent tectonic events. Constraining the timing of tectonism in this area is important because strata of similar age are sparsely distributed across Wyoming and, in general, are poorly dated. Vertebrate collections from outcrops of the Moonstone Formation have yielded the remains of a proboscidean, Aepycamelus?, and Plionictis ogygia. The collective presence of these mammals within a short stratigraphic interval suggests that the fauna is not older than the Barstovian North American Land Mammal Age (NALMA) and probably is older than the Hemphillian NALMA (i.e., Barstovian to Clarendonian). This biostratigraphic interval ranges from 15.9 to 9.0 Ma in the mid to late Miocene. Further constraints will be placed on the age of the Moonstone Formation with U-Pb dates of zircons from airfall tuffs within the study area. Magmatic zircons have been separated from one such tuff that is located stratigraphically between the localities bearing the proboscidean and P. ogygia. Knowledge of the faunas and age of this formation will allow correlation with similar units in the central Rocky Mountain region to constrain the timing of post-Laramide graben development and landscape evolution.