The Mogollon Rim and the Colorado Plateau - A Quick Geological Summary
Sedona sits at the base of the southern edge of the Colorado Plateau, the huge wall that contains the red cliffs and canyons around us and is often mistaken for the slopes of a mountain range. This plateau edge is known as the Mogollon Rim, named for Juan Ignacio Flores Mogollon, an early governor of the southwest at the time Spain ruled the area from its New World capitol in Santa Fe, New Mexico. This wall or escarpment extends approximately 240 miles northwest to southeast across Arizona and ends at Mogollon, New Mexico. The Colorado Plateau is the second largest plateau in the world after the Tibetan Plateau and is approximately 140,000 square miles in size. It contains an incredible number of national parks. monuments. and archaeological sites within its boundaries such as Grand Canyon, Brice, Zion, Arches, Canvonlands, Lake Powell, Mesa Verde, Petrified Forest, and the Painted Desert. The famous canyons that cut into the surface of this geological layer cake are in effect merely tiny cracks running across small portions of the plateau.
About 80 million years ago, tectonic force deep within the earth’s crust uplifted the plateau over a period of millions of years to its initial height much higher than it is today. It subsided over millions of years and began the slow process of weathering along its edge through a combination of erosional forces. Geologists believe its original edge was at least 30 miles south of where we see it today in the vicinity of Prescott, Arizona. Four main layers of sedimentary rock are visible in these cliffs, rock that is compressed by its own weight or the weight of the layers above it. Sandstone is the primary rock although two layers of limestone are also evident. The color of our famous red sandstone is due to staining by iron oxide, commonly known as rust.
Each layer represents a distinct geological period with its own characteristic environment. Basalt, solidifed lava, is the most recent layer and is due to the intense volcanic events in the Flagstaff region in the last 10 million years. Over 500 extinct volcanos dot the landscape of the Flagstaff region and Sunset Crater, the most recent, erupted a mere 940 years ago in 1065-1066 A.D. Basalt acts as a protective layer on top of the plateau and slows the erosion process in the areas it covers.
The fantastic formations that stand above us in Sedona are erosion remnants classified as mesas, buttes, and pinnacles. They are, in effect, the harder, more resistant pieces left behind while everything else was washed away. Water, wind, freeze-thaw cycles, plant roots, and gravity all combine to create an erosional force that slowly eliminates the broken and fractured material and leaves these random shapes behind. A mesa is a flat, table-like formation; a pinnacle is a tower or column: both have a distinct shape. A butte, by elimination, is any formation that does not qualify as a mesa or pinnacle - buttes come in many shapes and sizes and are the most common feature of the landscape around Sedona as well as the other canyon areas of the southwest. As the Colorado Plateau continues to erode northward, these formations, too, will erode away as others are created and left behind in the landscape as free-standing monuments. At its current rate of erosion, approximately one meter every 2,000 years, the southern edge of the Colorado Plateau will have receded north to downtown Flagstaff in a mere 70 million years….
Jim Bergstrom, professional Jeep tour guide, NAI