The Coconino National Forest contains Arizona’s second largest landform: the Mogollon Rim (you likely know what holds first place in this category). Also called the Tonto Rim, or simply “the Rim,” it is about 175 miles long and stretches from west of Sedona to near the New Mexico state line. The Rim is an elongated line of cliffs approximately 2,500 feet high and is named after the Spanish Governor of the Province of New Mexico Juan Ignacio Flores Mogollon, who served from 1712-15.
Some have difficulty pronouncing the word Mogollon, which is correctly pronounced muggy-OWN. There is much history and lore associated with this fascinating region and American writer Zane Gray did much to popularize this area in the early parts of the 20th century. But what about the Rim’s geologic origins? The story is no less interesting.
First of all, the Mogollon Rim contains a sequence of rock layers identical to that exposed in the upper half of the Grand Canyon. That is why the Rim and the Canyon appear so similar. However, there is one notable exception — the Oak Creek and Sedona areas have a layer of rock that the Grand Canyon does not, the Schnebly Hill Formation.
This formation is composed of 700 feet of red sandstone deposited some 280 million years ago in the Permian time period in a sandy desert that was adjacent to a shallow sea. This layer is exposed as you begin the hike up into the West Fork of Oak Creek. It extends north to underlie Flagstaff, but pinches out and disappears entirely by the time the same layers are exposed in the Grand Canyon.
The formation of the Rim as a modern landform is a story that begins about 80 million years ago. If one could go back in time and stand at the site of today’s Scenic Vista at the top of the Oak Creek Canyon switchbacks, nothing present on the landscape today would be visible. In fact, instead of standing on top of the Mogollon Rim gazing downward into Oak Creek Canyon, one could look south along a gently rising plain to the foot of a giant mountain range where Prescott would be founded much later.
Geologists have given the name Mogollon Highlands to this long-gone mountain range, suggesting a geologic connection between the Rim and the Highlands. These mountains once stretched from near Bisbee through Tucson, Phoenix, Prescott, Kingman, and on toward Las Vegas. Of course no human ever saw the Mogollon Highlands, but the evidence of their existence comes from the gravel that washed out of them and is preserved in small pockets on top of the Rim today. The gravel contains rock types that are only located in the Prescott area — a neat bit of detective work to understand the former geography of the region.
The Mogollon Highlands were still delivering gravel north to the Sedona area 25 million years ago but curiously this gravel only reached as far as the House Mountain volcano near the Village of Oak Creek. For years, the identical-looking gravels left on the Rim and those near the Village were thought to be the same, just separated into their disjunct positions by faulting.
I and other geologists conducted detailed mapping of the area and found that the lower gravel was halted on its northward journey by some imposing landform and I proposed that this was an early incarnation of the Mogollon Rim. It is now known that Mogollon Rim was “born” sometime between 30 and 25 million years ago, as the colorful layers were progressively stripped back to the north from the crest of the ancient Highlands (yes, rocks in the Mogollon Rim once stretched much farther south before they were eroded). Later, the Highlands became faulted and eroded away entirely, leaving a few of their gravel deposits and its offspring, the Mogollon Rim. Portions of the ancestral Rim are preserved beneath the House Mountain volcano.
In the past 6 million years, Oak Creek has carved its beautiful canyon back into the edge of the Rim. The next time you head to Sedona on Highway 89A, stop at the Oak Creek Vista and begin to view this landscape with new eyes!
Article excerpted from AZ Daily Sun in it’s entirity:
Wayne Ranney is a geologic educator who obtained two degrees from Northern Arizona University and has studied the geology of Northern Arizona for 44 years.
The NPS/USFS Roving Rangers volunteer through a unique agreement between the Flagstaff Area National Monuments and the Coconino National Forest to provide Interpretive Ranger walks and talks in the Flagstaff area each summer.
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