- Kathleen Scavone
- Posted On
The Living Landscape: Geologically speaking
LAKE COUNTY, Calif. – The science of geology presents itself to us daily via Lake County's contrasting landforms of mountains, valleys, streams and lakes.
As you drive south between Hidden Valley and Middletown great tracts of serpentinite, our State Rock lay exposed.
Middletown's Rabbit Hill offers stunning geologic features in its boulders and rock outcrops where serpentinite and a rock made up of microcrystalline quartz known as chert are exposed.
Serpentinite comes in an array of colors from shiny green to tans and browns. If you've driven through the Lake 29 Expressway project, which is, as reported on Aug. 6, 2020, in this publication, "The largest highway project in the county in 50 years," you may have marveled at the geologic features being unearthed by monster machines.
Along the stretch of highway, acres of glistening black obsidian lay exposed, then, farther along the highway red volcanic soils paint another picture of Earth's bold geology, while next along the panoramic highway brilliant white soils are in evidence.
The white features that lay exposed are Pozzolans and Dacite, which are also volcanic in origin and sometimes occur in the centers of old volcanoes.
The volcanic Pozzolans and Dacite are the materials being mined at the big white mountain at Kelseyville's S-Bar-S Quarry.
Since our own volcano, Mount Konocti, is nearby we don't have to look far to find the origins of all of these volcanic materials. But Mount Konocti is but one relatively small volcanic feature in the grand scheme.
Spread around Lake County and its environs are numerous hot springs, along with the Geysers Geothermal Field in the Mayacamas Mountains.
The Geysers' boundaries are the Collayomi and Mercuryville Faults. All of this geothermal activity is due, scientists say, to a large magma body beneath the surface.
The activity from Earth's plates and the continuous changes it incurs happens all along the California coast, with many volcanic fields tied in.
Our own volcanic field is called the Clear Lake Volcanics, and includes the Geysers and Cobb Mountain areas extending down to Lake Berryessa, but not Mount Saint Helena.
The Clear Lake Volcanics is at the top of the chain of volcanic fields. The various volcanic fields that are active over time eventually go extinct. Our Clear Lake Volcanics is still considered active.
Now we can reflect upon our Geology 101 courses of our pasts and remind ourselves that plate tectonics is a large force in our landscape since Earth's crust is made up of moving plates, with each plate moving in varying directions at any given time.
In the Jurassic Period, around 135 million years ago all of the west coast of our continent up to the Sierra Nevada foothills – and that includes Lake County – was submerged beneath the ocean. Geologists tell us that if you look carefully, possibly in a chunk of chert rock from a nearby creek, you may find shell fossils.
Also seen from Highway 29, between Lower Lake and Kelseyville in Manning Flat, is a large channel that runs throughout the flat where a sign once stated it was "Guy's Fault."
Geologist Dean Enderlin tells the story behind this geologic feature: “The bottom of Thurston Lake is considerably higher in elevation than Clear Lake level, so the two aren't directly connected. Seasonal runoff into Thurston Lake does recharge groundwater, however, which eventually enters Clear Lake as springs in the vicinity of Jago Bay. The water in Thurston Lake is remarkably murky (turbid), because it is fed by clayey runoff from Manning Flat on the ridge to the southwest. This is a man-made effect. Manning Flat was originally a valley with no natural outlet (called a closed basin). Water would have originally accumulated in the basin as seasonal lakes (vernal pools). In an effort to drain the valley, landowners attempted (and eventually succeeded) in cutting a channel through the ridge that separates Manning Flat from the Thurston Lake watershed. Since then, the fine-grained sediments in the Manning Flat basin have eroded rapidly, creating a deeply incised channel that is actively cutting through the soils in the flat and is threatening State Highway 29. As a prank, the channel was once posted with a sign, reading "Guy's Fault," but the channel has nothing to do with faulting. It is purely erosional, but its path across the flat may have followed portions of old roadways and a race track that was once used there."
Our county is full of geologic wonders that await discovery. According to Lake County Marketing's website, www.lakecounty.com , other rocks and minerals that can be found here include, "An abundance of obsidian, onyx, agate, jasper, chert, quartz, travertine, jade, petrified wood and clamshell fossils – many of which are the result of the Clear Lake Volcanic Field. "
When the world opens once again after the pandemic, they invite you to “visit the Lake County Historic Courthouse Museum, where you’ll see a 94-carat faceted Lake County Diamond!"
Kathleen Scavone, M.A., is a retired educator, potter, freelance writer and author of “Anderson Marsh State Historic Park: A Walking History, Prehistory, Flora, and Fauna Tour of a California State Park” and “Native Americans of Lake County.”