Hagerman Fossil Beds National Monument (HAFO) is home to the world-famous Hagerman Horse Quarry. This site gives important insight into Pliocene age paleontology. Absolute ages of this site have been elusive, although researchers have assumed the site to be around 3.2 Ma. This study sought to determine more precise ages for the previously documented Hagerman Horse Quarry and Peters Gulch ash layers at HAFO thereby defining a more specific date for the Hagerman fossil assemblage, and to describe and interpret their sedimentary horizons. Detailed stratigraphic columns were created to describe sedimentary layers and establish marker beds. The sedimentology and stratigraphy are consistent with that of fluvial/flood plain depositional environments. Two ash samples were collected and analyzed using U/Pb zircon geochronology via laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Similar to previous studies, the majority of zircon crystals in these volcanic ashes are detrital in origin, ranging in age from 4 Ma to Precambrian. While the depositional age of these ash beds remains elusive, the detrital ages were grouped into three major volcanic events and described in order to identify the source of the detrital zircons and hosting sediment. These source areas include the Atlanta Lobe of the Idaho Batholith (80 – 100 Ma), the Challis Volcanic Group (45 – 52 Ma), and rhyolitic Yellowstone-Snake River Plain volcanics (4 – 17 Ma).