When you grow up in dry country, water in the wild seems almost supernatural. Visit one of those few places where permanent natural water sources exist, and you feel a bit like you've just crossed over into some sort of otherworldly realm. The ruins of long-vanished civilizations dot the desert landscape; ancient canals still flow from water source to field. Gigantic trees shade bare rock from relentless sun, their leaves whispering as they catch and tame the fierce desert breezes. These are places where geologic time and natural rhythms merge to create a place that seems to take you a step outside of time. I'm going to take you to one of those places right now.
To get there, we'll travel through millions of years of tectonic shenanigans and the remains of an ancient lake. Let's start at the top: we'll begin at Jerome, Arizona, high up in the Black Hills. From here, we can gaze down into the Verde Valley, a graben that began forming ten million years ago as the crust stretched and movement began on the Verde Fault. The Black Hills, a rather magnificent little mountain range, rose up on the southern side. A valley formed between them and the Mogollon Rim escarpment to the north.
See the mountains over there on the horizon? They're the San Francisco Peaks, and they stand atop the Colorado Plateau, the geologic province I grew up on. Behind you is the Basin and Range, where little fault-block mountain ranges march northwest like an army of caterpillars. You're looking over a province called the Transition Zone or the Central Highlands Physiographic Province, an in-between place, neither plateau nor basin but sharing features of both. You can find rocks here that also exist thousands of feet above on the Colorado Plateau. The mountains you're standing on and the valley you're looking down into are close cousins of the ones you'll find in the Basin and Range. It's an enchanting place, especially to those with a passion for geology.
Now we travel down the steep flanks of the mountains, down the rolling foothills, and take a quick detour over to Tuzigoot National Monument for a lovely view of the Black Hills and one of the rarest sights in Arizona. Look down from the hilltop ruins, and you'll see a thick green snake winding its way across the valley floor. This is the always-flowing Verde River. Take especial note of it: it plays many parts in our story.
We leave Tuzigoot and travel across the valley, exiting the highway near Camp Verde. The road twists and turns between cliffs of limestone.
We're driving through ancient Lake Verde. Back before the Pleistocene, in the roughly eight million years between the Verde Valley forming and the forming of the Verde River, this basin hosted an intermittent lake. Like many places in the Basin and Range, streams drained the surrounding highlands, but had no outlet, so they'd end on the basin floor. Here, when it wasn't hot and dry enough to evaporate their waters, a lovely lake formed.
The climate fluctuated several times over those millions of years. Lake Verde would sometimes be quite long, covering forty miles or so of the valley floor. In those wetter times, the limestone you're seeing formed in the center and along the margins. Unlike the thick marine limestones to the north and beneath this basin, it wasn't primarily made from critters with calcium-rich shells. You can find some shells from freshwater snails and other mollusks, but the greater amount of this limestone seems to have formed when groundwater dissolved some of the underlying Redwall limestone, a thick marine limestone deposited in shallow seas between 340-320 million years ago. The lime would precipitate out, forming thick deposits of tufa. A similar process is happening in the depths of the Great Salt Lake.
When the climate got drier, Lake Verde would begin to shrink and grow shallow, like the Great Salt Lake is now. Mineral salts became concentrated, and as the water evaporated, thick deposits of salt and gypsum formed, especially at the southern end. Streams draining the highlands deposited red muds and rounded river rocks. Basalt lava flows from the north sometimes flowed all the way into the valley, spilling fingers of igneous rock over the lacustrine and fluvial sediments. Then the climate would change, and the lake would rise again, and lay new layers of sand and limestone. All of these various lake deposits are known as the Verde Formation.
The boom and bust cycles of the lake laid down three hundred square miles of sediment that, as the Verde Valley continued to drop, reached a thickness of over three thousand feet in the center. Megafauna lived and died along the intermittent lake shores, leaving their bones behind to be buried in the sand and mud. Over nearly ten million years, deposition continued and eventually outpaced the slow descent of the basin, until the lake reached a high enough elevation to spill over the low pass between Hackberry Mountain and Table Mountain at the southern end. The breach drained the lake for good about two million years ago, and the Verde River was born.
This was during the Pleistocene, and the river, fed by abundant moisture, flowed broad and bold. It carved through the rocks of the Verde Formation, leaving behind the mesas we're driving through here on the valley floor.
Humans arrived about ten thousand years ago. They found perennial streams, abundant wildlife, and a warm, dry climate. And they found an oasis.
To be continued...
Abbott, Lon and Cook, Terri (2007): Geology Underfoot in Northern Arizona. Missoula, MT: Mountain Press Publishing Company
Blatt, Harvey (1980): Origin of Sedimentary Rocks. Englewood Cliffs, NJ: Prentice-Hall Inc.
House, P.K. and Pearthree, P.A. (1993): Surficial Geology of the Northern Verde Valley, Yavapai County, Arizona. Arizona Geological Survey.
Ranney, Wayne (2010): Sedona Through Time. Flagstaff, AZ: Wayne Ranney.