Geology of the Verde Valley (Continued)

The erosion-resistant Kaibab Formation originated from the remains of sponges around 260 million years ago. This rock layer caps both the Mogollon Rim and the Grand Canyon.

Lee Mountain is part of the present day Mogollon Rim

The next 35 million years or so have no visible record on the Colorado Plateau, due to long erosional processes. The Verde Valley area shows no trace of dinosaur fossils. However, elsewhere on the Colorado Plateau many remains of these creatures have been found, such as at the Petrified Forest National Park.

The thin Moenkopi Formation was deposited in a tidal flat. Therefore it is not a regular layer, but occurs only in a few areas. North America was drifting away from the Pangaea supercontinent at this time, and colliding with another plate. This is when the modern Rockies were formed, called the Laramide event. During this period western and central Arizona was uplifted into what is called the Mogollon Highlands. This is also when the Mogollon Rim was formed. The Mogollon Rim is the southern edge of the Colorado Plateau.

Next came the deposition of the Rim Gravels 12 to 15 million years ago. Oak Creek began to erode the canyons that we now see in this area. House Mountain Shield Volcano in the Sedona Village of Oak Creek first erupted 14.5 million years ago, just under what was then the original edge of the Mogollon Rim. One of five active volcanoes in the Valley, The House Mountain lava flows were deflected by the Rim to the west, south, and southeast, covering the Verde Valley many meters deep in lava. River flows from the ancestral Mogollon Highlands were also deflected to the southeast, indicating the first evidence of the Verde River system.

The Mogollon Rim has retreated about 4 miles due to erosional forces. The red rocks seen against the eastern edge of House Mountain Volcano indicate the original southern edge of the Mogollon Rim, about two to four miles west of the present location.

House Mountain Shield Volcano

The Verde Fault was more active then, and over 6,000 feet of rock was downdropped to create the Verde Valley. Continued subsistence eventually caused the river to stop flowing, forming the ancient Verde Lakes. Two million years ago, the valley was filled with sediments which eventually completely covered House Mountain Volcano. The river and Oak Creek first flowed over the sediments, then began to cut into them. The top fifty feet or so of the mesa known as Tabletop Mesa, site of the Sedona Airport, was deposited by Oak Creek! The effects of the river erosion as well as ancestral Oak Creek began cutting deeply into the rock layers. House Mountain was again exposed to the elements through erosion of the river.

Since then water, ice, wind and rock falls have continued to erode the Mogollon Rim and the Verde Valley. What is viewed today are the remnants of these ongoing processes. As these erosional processes continue over the coming millions of years, the Mogollon Rim will eventually meet with the Grand Canyon, creating a really grand canyon covering most of northern Arizona.

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