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What's It Made Of?

Geology of the Mogollon Rim
by Wayne Ranney


With the exception of the Grand Canyon, no other physical feature of Arizona’s landscape is more prominent than the Mogollon Rim. Five of the six National Forest in Arizona contain or border this spectacular escarpment (the only exception is the Coronado).

But what is the Mogollon Rim? How did it form and when? The answers are only now becoming clearer as earth scientists complete detailed studies within Arizona’s Transition Zone Province, that area between the Colorado Plateau to the north, and the Basin and Range to the South. The Mogollon Rim lies in the heart of the Transition Zone and has undergone a fascinating evolutionary history.

The mogollon Rim is a high, forested escarpment, or line of cliffs that stretch almost continuously from Seligman, AZ to the Arizona-New Mexico state line. The heart of the Mogollon Rim goes from the small town of Strawberry to the developing communities of Pinetop-Lakeside. The name may have been derived from that of the colonial governor of Spanish New Mexico, Juan Ignacio Flores Mogollon, who administered these frontier lands between 1712 and 1715.

Geologically, the Rim has been forming since at least 35 million years ato. Its story begins with the birth of the Rocky Mountains, when a mountain range was uplifted in central Arizona. Geologists call this now-eroded uplift the Mogollon Highlands after the escarpment they help create. When the Mogollon Highlands were uplifted, the sedimentary layers which make up the rim were tilted down to the northeast. And this tilted stack of sediments, which are most spectacularly exposed in the Sedona area, have been eroding (retreating) to the northeast ever since. House Mountain, which is a shield volcano located southeast of Sedona in the Coconino National Forest, erupted between 15 and 13 million years ago at the very base of the ancestral Mogollon Rim. The distance can be traced from the edge of the lava flows to the present-day Rim and determine that the Rim has been retreating at the rate of one foot every 625 years. At this rate, which is only an average, the colorful cliffs north of Sedona will erode back to downtown Flagstaff in another 79 million years!

This escarpment, which provides Arizonans with a multitude of recreational opportunities and scenic vistas, is now being eyed by the thirsty desert cities to the south for its valuable water resources. The citizens of Arizona will be the ones who decide whether these resources are best utilized for continued urban growth or for a myriad of recreational pursuits. An informed educated public can best decide the wisest course of the Mogollon Rim’s future management.

NANHA Newsletter

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