DETERMINING THE LITHOLOGY AND GEOLOGIC MAP UNITS OF MOUNT EVERTS, YELLOWSTONE NATIONAL PARK, WY

Mt. Everts is an iconic landmark near the northern border of Yellowstone National Park that spans the divide between Montana and Wyoming. It is currently represented by two geologic maps, generally aligned with state lines, that disagree on their interpretations of Mount Everts lithology and designated formation names. The northern Montana map, the Gardiner 1:100,000, interprets Mount Everts as Archean schist and hornfels (Ash) while the southern map, an unpublished mylar of the Mammoth 1:62,500, as a series of upper Cretaceous rocks (Kl, Kev, Ke). Initial investigation of the northern terminus of Mount Everts quickly determined it is composed of sedimentary rock, not metamorphic rock. Samples were taken from the northeast side of Mount Everts to petrologically compare units across maps. Hand samples and thin sections made from the samples closely matched descriptions of the upper Cretaceous units mapped on the Mammoth 1:62,500. The thin sections were also compared to samples taken from mapped upper Cretaceous units (Klf) on the Montana map to determine if there are striking petrologic differences that might have led to the contrasting unit designations. No obvious differences were detected in hand samples or thin sections between rocks found on Mount Everts and rocks found in the Klf unit. To further confirm unit relations between maps, as well as remap the northern portion of Everts, eight more thin sections are being analyzed and field work will continue through 2022. Future work in the area will concentrate on the southern portion of the mountain to verify contact locations between units. Accurate geologic mapping is especially important for public land, such as Yellowstone National Park, so land managers may mitigate safety risks, mindfully place infrastructure, and communicate scientific knowledge to the public. This work highlights the importance of reassessing older geologic maps and making necessary updates when the need is recognized, particularly along disagreeing shared boundaries.