About 237 million years ago theMoenkopi Formation rocks were deposited in a coastal plain that covered what is now northern Arizona and surrounding states. The plain ended at the shoreline of a sea in Utah. Seasonal stream beds crossing that plain deposited thick sheets of sand. Over millions of years, “another 10,000 feet of Mesozoic sedimentary rocks accumulated.”
All of these sedimentary layers have been warped, twisted and eroded, ending up being uplifted by molten lava between 70 million and 30 million years ago as a huge chunk of thick crust. This was the origin of the Colorado Plateau, a 130000 square-mile block. In time, that molten base began to erupt in the San Francisco volcanic field, creating thousands of cinder cones, long lava flows, the stratovolcano known as San Francisco Mountain, and lava domes like Mount Elden near present day Flagstaff. Volcanic activity continued, with the most recent eruption occurring in AD 1064. At that time, the Sinaguan people found that the loose cinders made a good mulch for growing crops, and they created Wupatki. They used the red Moenkopi sandstone exposed in the area along with Kaibab limestone and black basalt from the volcanic field to build substantial buildings.
Centuries later, when the railroads necessitated the creation of the town of Flagstaff, the buildings were made of wood. However, over the years the citizens learned the same lesson that many cities of the day learned, as fires consumed the town. When reconstructed, the materials of choice were identical to those of the ancients, Moenkopi sandstone, Kaibab Formation limestone and volcanic basalt.
Used copies of the Stone Landmarks are available for purchase at various booksellers. Photos are my own.
Arizona is a geological laboratory because the natural forces in play are so visible. Erosion is a general term for the weathering of rocks and the transportation of them. The forces of erosion are gravity, ice, water and wind.
The San Francisco Mountain in Flagstaff is a strato-volcano in the midst of a volcanic field of over 600 smaller volcanos. This largest volcano is now 12, 633 feet in elevation, but it is estimated it reached 16,000 feet before a sideways eruption or glacial erosion, or both, collapsed or carved out the northeastern side of the mountain.
You can see green lichen growing on this large basalt boulder at the base of Mount Elden in Flagstaff. Weathering and erosion from the lichen, rain, snow, and freezing winter temperatures have and cracked the boulder in half.
In the mesa shown below, you can see how the soft sedimentary layers at the top have been weathered away by wind and water and gravity has piled it around the bottom.
Wind is a powerful force in erosion as you can see in the photo of Tsegi Canyon below where it hollows out the sedimentary rock.
Water helps to carry away the debris weathered from the higher elevations and the grit and sand in the water help to erode more rocks as it passes.
The classic Arizona erosion example is the Grand Canyon of the Colorado River.