Geology
By Robert DeMayo
This first thing that you realize when you look at the earth here, is that it is old, really old. If you pick up a rock, and brush off the dirt, you may be surprised to find fossilized sea plants and sponges in it. Some would think it strange to be looking at exotic seashells in the desert, but this area has been covered by ancient seas as many as seven times. It was even a fresh water lake for thousands of years.
During prehistoric times—about 80 million years ago—massive earth upheavals created the Colorado Plateau. Just north of Sedona sits Flagstaff, almost 3,000 feet higher in altitude. If you look north from Sedona, in the direction of Flagstaff, you can see ten cake-like layers of rock that took 350 million years to form. The top edge of the plateau is known as the Mogollon Rim.
Once upon a time this was the coast—some in Sedona say someday soon it may well be the coast again—and over the millions of years that followed tropical seas came and went. Some overflowed, swamping everything, others retreated, leaving inland deserts in their wake. Inland rivers also deposited mud and debris on their journey to the sea. This was way before the time of dinosaurs! In fact, if you go down just a short distance below the streets of Sedona you will encounter Redwall Limestone—this layer was deposited on a tropical ocean floor during the Early Mississippian Epoch (330 million years ago).
The Redwall Limestone also houses numerous aquifers. Occasionally sinkholes develop when the water dissolves the underlying limestone and the ceiling layers collapse. One of these sinkholes—known locally as the Devil’s Dining Room—spooked local cowboys when they looked down into the newly created sinkhole, only to see ghostly clouds of red dust swirling near the bottom.
The red color in the rock comes from iron that was trapped in seawater; the rocks are actually rusting. Two of the main layers you can see from town are Schnebly Hill and Coconino Sandstone. Schnebly Hill Sandstone is comprised of mostly red rocks on the bottom half of the cliffs, the Coconino Sandstone is above it, and mostly yellow. The higher you go the less red you find because at this point the seas retreated and left dunes and deserts.
Near the rim of the plateau—over 2,000 feet above Sedona—is another level. This one is off-white limestone, and is comprised of seashells and dissolved marine organisms, left behind when the seas retreated a final time.
The top of the Colorado Plateau has a basalt cap, laid down by lava flows from nine to fifteen million years ago. Ancient seas, drifting dunes, lava flows and inland rivers have all contributed to the beauty of this area, but the main forces were always erosion and time. The current rate of erosion is about one foot every 625 years. If there had been no lava cap on this end of the plateau the erosion would have been much faster, and these canyons would be a lot closer to Flagstaff.
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