Before we begin to understand Sedona’s red rocks, let’s have a look at what lies beneath them, the so-called basement or foundation of the Verde Valley. These rocks are hidden from view in the Sedona area but are uplifted along the Verde Fault and can be seen in the Black Hills (Mingus Mountain area) near the old mining town of Jerome. They are part of the oldest rocks in the American Southwest and formed in a vast submarine volcanic field some 1,750 million years ago. In this setting, volcanic lava and ash was erupted violently onto the sea floor and as the eruptions ensued they infused the rocks with copper, gold, silver, and zinc minerals. Hot springs ringed the volcanic complex and it was along these thermal fractures that the minerals would accumulate. Later, when the eruptions ceased, the rocks became deeply buried and were subjected to metamorphism (altered by heat and pressure), giving them their rugged, ancient appearance. Evidence gleaned from within the rocks reveals that they were buried under many miles of overlying rock in a large mountain range. As material was naturally eroded off of the mountaintop, the confining pressures were removed and the rocks incrementally rose closer to the surface in a buoyant fashion. By about 525 million years ago, the mountains were completely eroded flat and the rocks were once again located very near sea level.
This time the sea encroached upon the landscape from the west and as the shoreline slowly transgressed to the east, it left a veneer of coarse beach deposit known as the Tapeats Sandstone today. This dark brown, durable rock, about 525 million years old, was quarried and used in the construction of many of Jerome’s historic buildings. It sits atop the older meta-volcanic rocks, creating the Great Unconformity that is so well known from the depths of the Grand Canyon. More than 1,200 million years of earth’s recorded history is missing at this contact in the Verde Valley. After another significant period of erosion lasting 135 million years, another sea inundated the area and left behind two rock layers, the Martin Formation and the Redwall Limestone. Both of these deposits were important in the building of Jerome as they were also quarried and used for building stone and used in the making of local cement. In fact, both the Martin and Redwall formations are used today to make cement at the Clarkdale Cement Plant, located on the eastern flank of the Black Hills. Fossils within these deposits show that the sea was located in a tropical climate belt, with clear, warm water that was teeming with coral, sponges, and other marine creatures. Arizona was located close to the equator at this time as the earth experienced a warm interlude. All of these events left deposits that make up the foundation of Sedona’s red rocks.