NorCal Rocks: The Fascinating Geological Story of Northern California

Lassen Peak

Geology is not considered terribly exciting, probably because most of it happens in ultra, ultra, ultra slow-motion. But there have been a few moments in the prehistory of Northern California that I would have paid a lot to witness.

Compared to geologic time, watching grass grow would seem like a non-stop, high-octane, action-thriller. If you could “fast-forward” geologic time you’d likely be flabbergasted by how much the California landscape has changed and is changing all around us. To appreciate the apex moments in the building of California, you need some background on the three forces that do most of the work: tectonic plates, magma making its way to the surface, and water.

Out of sight and out of mind off the coast of Eureka, the Gordo Plate is creeping under the North American Plate. As it scraps underneath California, muddy sediments from the ocean floor stack up forming the Coastal Range. When the old seafloor gets deep and hot enough it melts becoming magma, which then rises back to the surface forming our largest mountain range, the Sierra Nevadas. Meanwhile the San Andreas Fault continues to jerk and wrench parts of Southern California northward and eventually out to sea.

All that melted seafloor becomes magma, which builds volcanoes, as well as forming craters, calderas, hot springs and geysers. When volcanoes erupt, magma becomes lava flowing over the surface of the earth. When that lava cools it solidifies and builds layers of rock (basalt). Successive volcanic events can stack up in layers like those of a birthday cake.

Water comes in rain, snow, ice, rivers and glaciers. What plate movements and magma builds up in the landscape, water erodes, moves, and tears back down. Water is at work 24/7, 365 days a year.

When these three forces work in concert, mountains and volcanoes form, and erode away. Plates collide and rip apart in geologic time like cars in a demolition derby. Land masses meander around the globe like so many toy sailboats adrift on the ocean. Continents rise up out of the sea and become dry land, others sink to become new seafloor. Here are a few tidbits about geologic NorCal that I would like to have seen.

Water Above, Water Below

Flickr/James St. John

Ever wonder why the Central/Sacramento Valley is so flat? That’s because 80 million years ago much of it was the floor of a vast inland sea. In fact, fossils of a sea creature aptly named “Shastasaurus” have been excavated from the limestone rocks around Shasta Lake. When you look at a topographic map of California, it’s easy to imagine the Central/Sacramento Valley full of water, because it was.

Of course, nature rarely leaves anything alone for too long. From the days of that inland sea the landscape changed over time, sea levels receded and the land itself was uplifted. The dry land we see today has only been free of salt water for maybe 1.5 million years. Even before there were people around to name our major rivers, they were hard at work dumping sediments from the tops of the highest mountains onto the valley floor. Today those sediments are thousands of meters deep, which accounts for why it is some of the richest agricultural land on earth.

Speaking of water, there was also water underneath that ancient inland sea, or at least there is today. Farmers have been drilling wells in the valley floor since the 1920s in search of precious water to sustain their crops, especially in those drought years California is so well-known for. Some parts of the valley are now as much as 28 feet lower in elevation for all the groundwater that has been removed. 

Moving Mountains

Sierra Nevada mountains. Photo by Sébastien Goldberg 

Faith isn’t the only thing that moves mountains. The Sierra Nevadas may be California’s biggest mountain range, but they’ve also been sliced and diced a good bit. First, let’s talk about what happened to the northernmost Sierra Nevadas.

It might have been one colossal earthquake, or it might have happened in a succession of smaller quakes, but I would love to have witnessed the top end of the Sierra Nevada range breaking away before slowly trudging north and west.

Today we call the mountains just north of Redding the Klamath Mountains, and they extend up into southwestern Oregon. Included in the Klamaths are the Siskiyous, Marbles, Scott, Trinity and Salmon Mountains, and they all share one unique feature. They were once the northern end of the Sierra Nevada mountain range. 

What specifically caused the Klamaths to break away from the Sierras millions of years ago we may never know entirely. But the rocks of the northern Sierras and the southern Klamaths are nearly identical geologically, like matching pieces of a jigsaw puzzle. Today the gap between them spans about 60 miles. In the time since the ranges began moving apart, more plate movement, erosion and layers of basaltic lava from volcanic eruptions have filled in the gap between Chico and Redding. Since the Klamaths broke off and moved apart NorCal’s two signature volcanos, Mt. Shasta and Lassen Peak were formed to dominate the landscape.

The southern Sierra Nevadas have done a bit of traveling too. What was once the southern tip of the Sierra Nevada range, granitic rocks called the “Salinian Block,” also broke away (thanks to the San Andreas Fault) some time in prehistory, and has since trudged more than 300 miles north and west. These rocks began their journey north of Santa Barbara, and now can be found close to San Jose. 

The Real Mt. Shasta

Castle Crags with Mount Shasta in the background. Photo by Ande Baldwin 

Though we think of NorCal’s largest volcano as a single entity, it contains four different major eruption cones plus one more from even earlier days. Mount Shasta’s last eruption was about 240 years ago (a fleeting moment ago in geologic time) from the Hotlum Cone. But other cones on the mountain indicate Shasta has had a fairly active eruption history. 

The oldest eruption cone on Shasta is the Sargent’s Ridge Cone, which is 100,000-200,000 years old. The next is the Misery Hill Cone which came into being 30,000-50,000 years ago. The Shastina Cone and the Hotlum Cone (the last one to erupt) are at least 9,500 years old. Each cone is evidence of other ancient eruptions.

One additional cone nearby, the Red Fir Cone, is even older (at least 590,000 years old), a collapsed compound stratovolcano. It is considered part of “ancestral” Mt. Shasta, so the old volcano has been volcanically active for a very long time.

Lassen Peak

Lassen is the largest lava dome volcano on earth, has zero glaciers on it and boasts the highest known winter snowfall amounts in California. The mountain is about 27,000 years old, and arose from the northern flank of the even-older volcano Mt. Tehama. It last erupted 1914-1921.

Although the eruption history of Lassen Peak before 1914 is somewhat murky, there was at least one explosive eruption that formed a 360-foot deep summit crater.

What if?

OK, this is where we admittedly leave hard science behind and enter a world of conjecture. And it isn’t exactly geology either, but when you try to imagine what NorCal was like a long time ago, other questions just “bubble” to the surface. Could humans and woolly mammoths have coexisted in California? It’s certainly possible.

Not nearly enough is known about when the first humans showed up in the Golden State. Officially, the remains of “Arlington Springs Man” is the oldest human remains discovered in what is now California. They were excavated on Santa Rosa Island (part of the Channel Islands) in 1959 and were dated as being around 13,000 years old. But when you Google “First Humans in California” you can’t help stumble over a National Geographic study that claims humans may have been in California 130,000 years ago. If this study was based on hard science, the archeological community probably wouldn’t find it so controversial. There is a lot of skepticism, and many authorities reject the study completely. 

So let’s stick with the “official” oldest human remains found in California at about 13,000 years old. Mammoths arrived in North America around one million years ago and died out around 12,000 years ago. That leaves about 1,000 years where human beings and woolly mammoths could have coexisted in what is now California. It’s intriguing to think about.

So often we limit our thinking of “history” to the times when human beings were around. But there is a lot more to the story of how northern California came to be, and what it must have been like in those distant times. The rocks can tell us much, and raise a lot of intriguing questions to consider.

Chip O'Brien

Chip O'Brien is a regular contributor to California Fly Fisher and Northwest Fly Fishing magazines, and author of River Journal, Sacramento River and California's Best Fly Fishing: Premier Streams and Rivers from Northern California to the Eastern Sierra. He lived in Redding, California, for eighteen years, where he was a guide, teacher, and regional manager for CalTrout.

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