Geologically, the pass is located on a fault that runs along the Mosquito Range. Road cuts along the eastern approach to the pass reveal Pennsylvanian black shale and thin gray limestone layers that were laid down prior to the creation of the Ancestral Rockies. These Paleozoic layers were not uplifted during the creation of the Ancestral Rockies approximately 300 MYA and thus were not eroded to the same degree as other areas to the east and west.