“I can remember I used to walk home from school across a hill with gravel deposits. I didn’t know they were gravel deposits then, but I would pick up the pebbles and take them home,” said Richard Holm, geologist and Northern Arizona University professor emeritus.

A former National Park Service “Roving Ranger,” Holm provided tours of the Rio de Flag and its surrounding geology for four years. Now, he has recorded his field notes for amateur geologists to take Flagstaff’s ancient rocks home with them.

His 72-page e-book is available for free download through the Arizona Geological Survey’s Down-to-Earth program. In it, Holm describes the geologic history of Flagstaff and especially the Rio de Flag. He also provides guides to four trails along the Rio de Flag, pointing out the abundant geologic features that can be seen along the way.

More technical than a basic trail guide, but using less jargon than scientific journal articles, readers will need to don their thinking caps – and flip to the glossary often – to uncover the geologic history beneath their feet.

“I was a teacher for 31 years. Knowledge, to me, is important. The idea of these trail guides is to give people a better understanding and appreciation for what’s out there,” Holm said.


Flagstaff sits on the southern part of the Colorado Plateau, which spans into parts of each of the four corner states: Arizona, Utah, Colorado and New Mexico.

Holm said the region was once located below sea level, but movement of tectonic plates, pieces of the Earth’s outer layer, 75 million years ago caused the formation of what is now the plateau.

He estimates the Rio is at least 1.29 million years old and said it was shaped by the interactions between lava and water, substances both similar and yet vastly different.

“Lava is just like water: it flows downhill. But lavas are a lot more informative than water. Water runs off, or it soaks in or it evaporates and you don’t have it anymore. You have the valley. The lava flow occupies a valley because it is there, but when the lava solidifies, it stays there. It’s still there,” Holm said.

He said this persistence of lava is what makes a geologist’s job possible: they can study the composition and map its location to see how it traveled and shaped the region over time.

“Since its origin, the Rio changed its watercourse several times in response to changes in Flagstaff’s landscape. Episodic eruptions of volcanoes and lava flows in different places are the agents of change that occurred independently of the river,” Holm wrote.

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These lava flows came from eruptions of San Francisco Mountain, Elden Mountain, Sheep Hill and A-1 Mountain, among numerous others. Holm said the most recent volcanic eruption occurred at Sunset Crater about a thousand years ago.


Holm’s research originally focused on deep rocks beneath the earth’s surface; however, when he first arrived in Flagstaff in 1970 to teach at NAU, his interests changed when he saw the potential of local volcanoes. His specialty quickly became igneous petrology, the study of rocks formed by magma. This field is closely related to volcanology, which emphasizes the processes behind volcanic formation and eruptions.

“They’re just out your door. You don’t have to go very far to study them, plus they’re young, so their features are preserved. The climate here is pretty dry, so the rate of weathering and decomposition of the rocks is slow,” Holm said.

Nearly 20 years after retiring, Holm continues his geological research through organizations like the Arizona Geological Survey.

Compiling years of field notes into a single e-book for the public became a hobby that lasted more than two years. He said he took his time because there were no deadlines: contributing to the Down-to-Earth series is voluntary and does not pay authors anything, except perhaps prestige.

F. Michael Conway, senior research scientist at the Arizona Geological Survey, said Holm’s e-book marks the 23rd entry in the series, which began in the early 1990s.

“They’re very popular online and they were popular in print. A number of them were sold in state parks or national monuments. I believe they enrich the experiences of people who are interested in the structures, landscapes and geology they see,” he said.

Holm is already on his way to setting records, with more than 500 downloads of his entry in just over a week.

Conway was one of Holm’s students while he attended NAU. He said Holm’s work, especially his geologic map of San Francisco Mountain, remains unprecedented.

“It is a gorgeous map. To my knowledge, nobody has gone back in and mapped to that detail,” Conway said of the map published in the late 1980s through the U.S. Geological Survey.

Holm will give an in-person geologic tour with the Friends of the Rio de Flag Thursday, July 11.

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Kaitlin Olson can be reached at the office at kolson@azdailysun.com or by phone at (928) 556-2253. 


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