This geology and trail guide is for anyone who likes to walk, be outdoors, and is interested in the geology in and around Flagstaff. The geologic story is focused on Rio de Flag because it flows through Flagstaff, touches or passes by all of the geologic elements in the city, and is accessed easily. The origin and history of the Rio, and how geologic structures and deposits have controlled and changed its watercourse through time, are presented with extended text, maps, photographs, and trail guides. Words in bold font, or bold and underlined, are defined in the Glossary. Trails along Rio de Flag include segments of the Flagstaff Urban Trails System (FUTS), local unofficial social trails used for convenience by people and animals, and the Flagstaff Open Space Program trails at Picture Canyon Natural and Cultural Preserve.