An Aerial Tour of the Stanislaus Table Mountain

This is a short blog series of informational articles from my college faculty website that is soon to be extinct (arcane unsafe software, they say). Way back in 2002, the parent of one of my students offered me a flight of my choosing, and I knew it had to be Stanislaus Table Mountain. It is one of the more famous geological features of our region, and it is best appreciated from above. Please buckle your seatbelts, and comply with the no smoking signs...

Our flight takes us from Oakdale, a small town at the foot of the Sierra Nevada, to the Sonora-Columbia area in the Sierra Mother Lode. Our objective was to get a bird's-eye view of the Stanislaus Table Mountain, regarded by many as one of the finest examples of an inverted stream in the world. The "mountain" formed around 9 million years ago, when a latite lava flow streamed westward from vents in the vicinity of the Dardanelles, near Sonora Pass at the crest of the Sierra Nevada. 

The latite is dark-colored with phenocrysts (crystals) of plagioclase feldspar scattered throughout. It superficially resembles basalt, but is more closely related to more silica-rich lavas like andesite and dacite. The lava flowed down a river channel carved out of the tuffs and mudflow deposits of the Valley Springs  and Relief Peak formations, ending somewhere just west of Knights Ferry, having traveled some 60 miles. The lava resisted erosion better than the softer surrounding rock, and the flow eventually was exhumed, forming a winding, sinuous ridge, especially in the vicinity of Jamestown and Columbia.

Soon after takeoff, we approach Knights Ferry. The modern Stanislaus River makes a prominent loop, with orchards and agricultural fields on the left-side flood plain. The higher terraces are drier, and are used primarily for grazing. The exposed rocks in the barren areas are mostly Mehrten formation, which consists of volcanic mudflow deposits around 4 to 9 million years old.

North of Knights Ferry, exposures of the Gopher Ridge volcanics are visible. These are metamorphic rocks dating from the Jurassic period. These rocks formed as island arcs (volcanic islands like Alaska's Aleutian Islands) on the oceanic crust of the Pacific Ocean, but were scraped off against the edge of the North American continent as the rocks were subducted. The rocks have been metamorphosed, and turned almost vertical by intense east-west pressure. They are more resistant than the surrounding slates, and so stand out as a prominent ridge. The town of Copperopolis is barely visible in the upper right corner of the photo.

Farther along, we pass the site of the Harvard Mine. The open-pit mine was active from 1986 to 1994, producing about 660,000 ounces of gold from about 17 million short tons of rock. The lake in the pit is about 300 feet deep. Just prior to closing down, the miners recovered a huge mass of crystallized gold, weighing more than 40 pounds. The gold is on display at the Ironstone Winery in Murphys. The body of water in the background is New Melones Lake.

Our turnaround point was just east of the town of Sonora. The town had its beginnings in 1848 when gold was discovered by Mexican nationals who had lost their citizenship as California was signed over to the United States. The Mexicans were soon displaced ("invited to leave") by American miners.  The original town is visible in the center-right part of the photo, while the newer urban development is visible in the center and left-hand part. Scars from the on-going construction (completed long ago) of a bypass can also be seen.

Turning west, we start to see the inverted stream of Table Mountain. Highway 108 passes along the lower left hand part of the photo. Very little soil has developed on the top of the old lava flow (note the lack of trees, and widely scattered grassy areas). The sinuous nature of the flow is becoming evident.

A look out the back of the plane offers the best view of the flow. The barren-looking surface of the flow is actually a unique ecosystem of native wildflowers that are largely free of the invasive European grasses that have overwhelmed the original grasses over much of the region. It is a fascinating hike, especially in the spring after a few good rainstorms. A relatively recent hike to the top of the lava flow can be seen here: https://geotripper.blogspot.com/2015/04/where-rivers-are-upside-down-hike-to.html

Looking west, with Knights Ferry in the far distance. The flow is wider, especially where some of the lava backed up into some ancient tributary streams. Tulloch Lake on the Stanislaus River is visible to the right.

The rest of the flight was a bit more mundane, as we buzzed my house and returned to Oakdale.

My thanks to Ken Iwahashi, the pilot on our journey. 

Addendum: If you are wondering what the latite rock looks like, I went out today and got a few shots of it.

Spring in the Sierra Nevada Foothills, A First Report (and a baby horsie)

It was the first day of April, and we decided to take a drive in the Mother Lode, the gold mining region that makes up the foothills of the Sierra Nevada. We weren't really expecting much in the wildflower department, as it has been a horrendously dry year, and the hills were still brown just a few weeks ago. March brought us a few serious storms so the ground is saturated and flowers are starting to grow, but few have bloomed. I included the picture of the Monkeyflower above because it was the ONLY one I could find. But at least it holds the promise of a colorful flower show in the next couple of weeks.

We headed up a road we've never followed before, Hunt Road out of Milton (and congratulations if you have ever heard of Milton). It was the kind of road so rarely traversed by sightseers that the ranchers look at you long and hard to see if you are capable of rustling their cattle. The road itself has been patched to the extent that the asphalt filling the potholes exceeds the amount of original pavement. But it was a beautiful drive. We climbed the escarpment underlain by the Gopher Ridge Volcanics, composed of the remains of island arc terranes that slammed into the western edge of the North American continent during the Jurassic and Cretaceous periods. Oak trees and the exposures of the metavolcanic greenstone were all around us. We could see the prairies of the Central Valley below and the Coast Ranges on the far side of the valley.

In the rugged terrain near the crest of the ridge we passed a stamp mill left over from the gold mining days. I don't know the name of the mine.

 After crossing the ridge, we entered into the vast meadow of the Salt Springs Valley. It looks like a flood plain filled with alluvium, but it is actually underlain by the Salt Springs Slate, which weathers quickly into a clay-rich soil.

Bedrock exposures stick up here and there, forming the famous "tombstone" rocks. The original mud and silt of the ocean floor has been compressed and hardened into the slate, but at heart these rocks are still clay, and break down quickly. The quartz bearing silt layers are more resistant to erosion.

Roadcuts into the slate reveal the inherent weakness of the rocks. Large slabs have broken off and slipped onto the roadbed. If you look carefully on the lower left, you can see some of the few poppies that we saw today.

We suspected we might find some flowers in at least one spot, so we drove south for 40 miles to the Red Hills "Area of Critical Environmental Concern" (and isn't that a mouthful?). The landscape there is underlain by serpentine and other ultramafic rocks, and grasses are so rare that other plants and flowers provide the dominant ground cover. Even the Red Hills were just beginning to show some color, with just Goldfields carpeting the rocks, and the one Monkeyflower shown at the top of the post.

In a few weeks, this spot will be alive with poppies, lupine, and a myriad of rare serpentine-tolerant flowers. We will be sure to visit in a bit and provide an update.

 Oh, as promised, a baby horse...

Springtime on the California Prairielands: Violence in a Gentle Landscape (and a gratuitous picture of a duck)

The gentle wind caresses the waving blades of grass...meadowlarks sing, frogs chirp...it's a serene day in the springtime of the Sierra foothills. Of course the science of geology holds no such sense of peace. Almost every aspect of this peaceful view speaks of violence, both in the recent past, and in the depths of time.

Take the layers revealed in the gentle slopes of the first picture above: a few terraces of dark rock on the distant skyline and some white exposures near the creek. These are two of the distinctive strata that make up much of the Sierra foothill slopes near the Central Valley, the Mehrten Formation and the Valley Springs Formation. The Mehrten layers resulted from numerous volcanic mudflows (lahars) and floods caused by eruptions of andesitic volcanoes in the vicinity of the Sierra Nevada crest in Mio-Pliocene time. Events that form layers like these are no picnic. Volcanoes of the 20th century like Mt. St. Helens and Mt. Pinatubo of the Philippines produced destructive mudflows that caused damage dozens of miles away from the volcanoes.

Likewise, the Valley Springs Formation speaks of volcanic violence. In Miocene time, around 20 to 30 million years ago, huge rhyolite caldera eruptions rocked the American west. A single eruption was capable of producing hundreds of cubic miles of volcanic ash that buried thousands of square miles under a stiflingly hot layer of what amounted to microscopic glass shards. The layers of the Valley Springs Formation record a number of violent eruptions, some relatively close, such as at the Sierra Crest, but some of them originated from ash explosions in eastern Nevada. Today, the differentially eroded layers provide shelter for burrowing owls and swifts along the small creek above.

Roadside runoff provides a bit of extra moisture supporting a thicket of monkeyflowers along Willms Road near Knights Ferry (above). But again, the landscape reveals a bit of geologic mayhem with the black boulders of the Table Mountain lava flow in the distance.

A favorite stop of ours on our seasonal treks through the foothills along Willms Road is a small stock pond a few miles south of Knights Ferry. At this time of year the flowers and birds are plentiful. The cattail marsh on the far shore was filled with chattering birds of all kinds. A peaceful, if raucous scene, but once again the present-day scene reveals a history of violence. The bits of rock sticking out on the lakeshore and far hillside are exposures of the Jurassic (yes, that Jurassic, the dinosaur period) Gopher Ridge Volcanics. These are andesitic and rhyolitic rocks that formed volcanic islands along subduction zones in the Pacific Ocean many miles away. Not only was there the violence of the original eruption, there were also the tectonic movements that carried the island arc into the western edge of the North American continent. The rocks were jammed into the continent, deformed, and metamorphosed into greenstone and slate.

The timing doesn't have to be measured in the millions of years. A look at the grass stuck in the fence (above) reveals that the road was inundated three and four feet deep during one of our intense rainstorms a month or two ago. The flooding obviously didn't do the road a whole lotta good either.

The frogs in the creek seemed fine, but unfortunately they reveal a certain level of violence, this time to the ecosystem of the foothills. Bullfrogs are an introduced species. They are voracious predators of other frogs and the native species have not been able to compete. They have driven the native species into a few refugia where the bullfrogs can't survive (even there, the native frogs have problems; they are eaten by the introduced species of fish as well).
I would almost be depressed by all the violence, but geologic processes are the very essence of drama, and the earth would be a boring (and probably unliveable) place if we didn't have earthquakes, volcanoes, plate movements and mountain-building. Just the same, I am ending with a picture of a duck that I saw hanging out along the water. As far as I know, the duck is not a terrifying animal unless you happen to be a snail in the creek....

The Other California: Spring Arrives in the Prairie Lands

California has a rather sullied reputation these days; a lingering depression (at least in our Central Valley), high unemployment, urban nightmares, Hollywood, and a legislature that lacks the will to support the education of its children and young adults. And yet, 38 million people choose to live here. They have a lot of reasons, and some think they want to leave, but frankly California is a nice place to live. Especially if you have any interest whatsoever in geology. There is hardly any place in the world that packs so much geographic variety within its borders: coastlines, mountains ranges, deserts, and valleys. My Other California series is an on-again, off-again exploration of the geologically interesting areas that don't normally show up on our tourist postcards. I started this series with an exploration of the prairie lands of California that surround the state's vast Central Valley. Because I live on the fringe of these prairies, I venture out some afternoons to see what's happening. Friday was an exploration of Merced Falls Road in the Sierra Foothills, a short drive that included a surprise vista of Yosemite's Half Dome and Clouds Rest.

Merced Falls Road connects Highway 59 at Merced Falls to Highway 132 near Don Pedro Reservoir. It provides access to a number of ranches and the recreational areas along McClure Lake. It also includes numerous exposures of the metamorphosed sediments and volcanic rocks of a Jurassic island arc system that was slammed (in a geologic sense) into the west coast of the North American continent in late Mesozoic time. The rocks have been upended and deeply eroded, producing a landscape that looks like a neglected cemetery (the outcrops look like tombstones). The rocks include the Mariposa formation and the Gopher Ridge volcanics.

Friday was a fine day for seeing the early blooms of what promises to be a fine wildflower season. More on that in the next post. When we reached the little village of La Grange downstream on the Tuolumne River, we could see the preparations being made for an exceptional spring runoff. The snowpack in Yosemite is somewhere around 150-160% of normal after three years of drought, and the water authorities are emptying out Don Pedro Reservoir to make room for the much needed onslaught of snowmelt that begins soon. Below is the artificially induced mild flood being conducted to avoid an out-of-control flood in a few weeks.

What else did I discover? I found out my hay fever is alive and well...sniff...