IMPERIAL VALLEY — If you had to give a date of birth to the Imperial Valley, most people would accept the date of June 21, 1901, when canal water from the Colorado River began to irrigate the first crops.
Native Americans were actually the first settlers, though none of them found any good reason to spend the summers. Instead they moved from the ocean to the mountains to the great Lake Cahuilla and to the Colorado River and beyond, following the seasons.
These people descended from those who crossed the Bering Strait from Asia into North America during the last Ice Age about 15,000 years ago. They are still among us today. Fossils indicate that humans like us emerged in central Africa around 330,000 years ago, so even the Native Americans are late arrivals.
We’re a very young area in geological time, too. I didn’t realize just how young until I took a class at Mission Trails Regional Park in San Diego and met Pat Abbott, a former San Diego State University geology professor. Since then, my drives between El Centro and San Diego will never be the same.
Standing behind the Mission Trails visitor center, Pat points north, where the hillside in Mission Gorge has been quarried. “See those layers? Those are remnants of the Santiago Peak volcanoes from 126 million years ago. Just to the right, you can see how the rocks are different, larger rounder, lighter in color. Those are plutonic rocks from two miles underground. They’re dated at 118 million years ago.”
You may remember from school about tectonic plates, the giant continental masses of the earth’s crust that float upon the molten rock beneath. About 175 million years ago, all of today’s continents were one giant supercontinent called Pangea. Even as far back as the 1500s people noticed how South America could fit nicely into Africa.
We also notice the same thing about Baja California. Moving Baja like a puzzle piece, Cabo San Lucas fits up against Puerta Vallarta 260 miles to the south. And that’s because Baja California, along with the west side of Imperial Valley, is drifting north as part of the Pacific Plate. The east side of Imperial Valley sits on the North American plate.
Back in Mission Gorge, Pat Abbott picks up a handful of rocks. “These rocks came from Sonora, Mexico,” he says. “Fifty million years ago, Sonora was about 10,000 feet higher than it is today. What we call the Ballena River carried these rocks downhill over 250 miles. We know it was a massive two-mile wide river because of the size and distribution of rocks deposited in east San Diego County.”
Imperial Valley didn’t exist then. This was long before Baja separated from Mexico and the Sea of Cortez was formed. Long before the Colorado River built up a dam of silt to separate Imperial Valley from the Gulf of California. What we call the Imperial Valley was born only 4 million years ago. And it is still forming.
There are only a few places around the world where the tectonic plates are spreading apart, and new crust is being formed. Mostly these “spreading centers” are out in the middle of the oceans. But we’ve got one here following the San Andreas fault that splits our Valley literally into two different gargantuan plates of earthen crust.
Over the last couple million years, the Colorado River has entered the Valley many times to fill it with the ancient Lake Cahuilla. Eventually, the river would find its way back to the Gulf. Each time Lake Cahuilla evaporated, the river’s silt remained, so much silt that in some places it’s 5,000 feet thick. And below that, a thin crust between us and the hot, viscous rock we call the mantle. That’s why we are so rich in geothermal energy. We’re so close to the heat source. Water from the river and local run-off have charged our basin with some of the best sources for geothermal energy in the world.
As you drive west on Interstate 8 past Ocotillo, you’re greeted by the familiar giant piles of rounded boulders. We learned from Pat that these also are plutonic rocks, uplifted 90 million years ago as one plate pushed under the adjacent one, like a cat thrusting its paw under a rug — 28 million years after those rocks in Mission Gorge.
Once on the surface, expansion and contraction causes rock to fracture into smaller and smaller boulders. Slightly acidic rain weathers and rounds the boulders over unimaginable stretches of time. Gravity and erosion keep pulling them down.
The earth around us seems solid and inert. But in fact, it’s a heaving, morphing sea of rock. We’re too young and too small to see it except through the insights of generations of geologists and our able imaginations.
Brian McNeece, a retired Imperial Valley College educator, is a member of the Internal Boundary and Water Commission Colorado River Citizens Forum.
This story is featured in the Jul 09, 2020 e-Edition.
