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New Publication explores the origin and geology of the Rio de Flag of Flagstaff, Arizona

“This geology and trail guide is for anyone who likes to walk, be outdoors, and is interested in the geology in and around Flagstaff.”  Richard F. Holm

The newest Down-To-Earth booklet from the University of Arizona’s Arizona Geological Survey recounts the geology of Flagstaff, Arizona, with emphasis on the history of the Rio de Flag drainage that runs through the town.  Part 1 is about the region’s geology and Part 2 includes trail guides. (

Title: Geology of Flagstaff and Geologic History of the Rio de Flag, Northern Arizona: with Trail Guides to Geology along Rio de Flag

Headwaters of the Rio de Flag are at 9,200 feet AMSL near Snow Bowl on the west side of the San Francisco Peaks. The channel runs southeasterly till reaching the city of Flagstaff, where it turns to the northeast before flowing east to its confluence with Walnut Creek at 6,170 AMSL.

Written by Dr. Richard F. Holm, Professor Emeritus at Northern Arizona University, the illustrated 70-page text includes 16 maps and 42 photographs and illustrations. Part 1 provides a synoptic overview of the geologic setting of northern and central Arizona and includes a primer on some of the key volcanic features of the nearby, active San Francisco volcanic field. Holm describes the origin and geologic history of the ancestral Rio de Flag. Spoiler alert: volcanoes and lava flows encroaching on the course of the ancestral Rio de Flag shaped and reshaped the channel path, which was later altered by urban engineering.

The age of the Rio de Flag is uncertain, but the earliest channel formed more than 1 million years ago. Episodic volcanism at Dry Lake domes, Elden Mountain, Sheep Hill, A 1 Mountain, and Observatory Mesa disrupted the ancestral river, which responded with course changes and by forming small, temporary lakes.

bottomless pit
This sinkhole in the Permian Kaibab Formation is referred to as the Bottomless Pit.

Part 2 comprises four trail guides to the geology encountered along the course of Rio de Flag, from west to east:  Cheshire Park to Crescent Drive; Sawmill County Park to Little America; Old Walnut Canyon Road to Big Fill Lake;  and Picture Canyon loop. Each section is accompanied by a description of the length, relief and level of hiking difficulty, and topographic and geologic maps. Geologic features adjacent to the trail and features appearing on the horizon are described in some detail. The first two trail sections are part of the Flagstaff Urban Trail System (FUTS).

Trail highlights include: The Narrows – a small canyon carved into basalt at stop 3 on the Cheshire Park section;  Sawmill County Park/Willow Bend Environmental Center at the head of the Sawmill County Park section; the Bottomless Pit just 750 feet from the trailhead of the Old Walnut Canyon Road section; and the waterfall and rock-art pecked into basalt lava flows in Picture Canyon.  

Background. The UA Arizona Geological Survey’s Down-to-Earth series serves the Arizona public and interested visitors to jargon-free, illustrated booklets spotlighting the geology and geologic processes that shaped some of Arizona’s most iconic locations: including the San Francisco volcanic field, Oak Creek Canyon, the Chiricahua Mountains and the Santa Catalina Mountains – both Madrean Sky Islands, Sabino Canyon, Saguaro National Park, and the White Mountains.

The Down-to-Earth series includes 23 booklets, available in high-resolution PDF format and free to download from the UA Arizona Geological Survey Document Repository, no registration required: