Beginning about 15 million years ago, volcanism commenced just south of Sedona with outpourings of voluminous basalt lava. This is when the rocks on top of Mingus Mountain (near Jerome) were formed, along with those at House Mountain, a shield volcano near the Village of Oak Creek. At House Mountain the lavas only flowed to the west, south, and east. Detailed mapping of this curious distribution revealed that lava was precluded from flowing to the north by the presence of partially eroded but high-standing red rock buttes or the Mogollon Rim. Those rocks have now been further eroded and stripped back to the northeast.
The Verde fault near Jerome further depressed the Verde Valley such that the Verde River became ponded into a freshwater lake. This lake stretched nearly 35 miles from below Camp Verde to Clarkdale and from Jerome to Sedona. Fossils of mosses, tree roots, and giant herbivores such as Stegomastodons have been recovered from the white limestone of the Verde Formation. The lake was ephemeral with wetter periods and lake expansion interspersed with dry periods and lake contraction. The basin was breached about 2.5 to 3 million years ago.
Meanwhile, in the future area of Oak Creek, a second period of basaltic volcanism began lasting from six to eight million years ago. These lava flows filled an ancient valley that extended from the Mogollon Rim into the Verde Valley. Some of these flows entered the Verde lake area and black basalt lava is found occasionally interbedded with the white Verde Formation. Today, Interstate 17 takes advantage of the ramp built upon these valley-descending lava flows.
Beginning sometime after six million years ago, the Oak Creek fault became active and ripped through the sedimentary and volcanic rocks. Snowmelt on the Plateau percolated into the rocks where groundwater found the fault zone an attractive place to flow in the subsurface toward the south. This groundwater flow weakened the rocks and facilitated more vigorous surface runoff. Acting in concert, these two processes likely helped to carve Oak Creek Canyon. It is amazing that this half-mile deep gorge is less than six million years old.