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.

Where the Rivers are "Upside-Down": A Hike to the Stanislaus Table Mountain

Around 10 million years ago, a lava flow surged from a volcano near present-day Sonora Pass in the Sierra Nevada. Composed of latite, but non-viscous like basalt, it flowed off the volcano and into the channel of the ancestral Stanislaus River, eventually flowing close to sixty miles. The river eventually eroded another valley and the lava flow was left relatively intact. The Sierra Nevada rose and tilted westward, and erosion removed the surrounding softer rock, leaving the former river valley as a ridge several hundred feet high. This was the origin of the Stanislaus Table Mountain, an inverted stream.

In this GoogleEarth image, the trail mostly follows the white line along the base, and then climbs the forested slope on the right.

Table Mountain forms a mesa-like ridge around Sonora and Jamestown in the Mother Lode of the Sierra Nevada. A lot of it is on private property, and as such is inaccessible for close investigation. But one portion lies within the boundary of the U.S. Bureau of Reclamation lands around New Melones Reservoir. The Bureau has constructed a trail to the summit, and that's where we were headed today on our Geology Club hike.

The trail is about 1.5 miles in length, and the first mile is a gentle grade through meadows and oak woodland. The grass was still green and wildflowers were reasonably abundant, but it isn't going to last. The soil felt bone dry, crunching under our feet. It's going to be a long, hot summer.

For the last half mile, the trail becomes increasingly steep. It's a climb of several hundred feet to the top of the lava flow. It was hot today, nearly ninety degrees, and I was appreciative that our trail was on the shaded north side of the lave flow. Oak trees provided shade, but also obscured the view but for a few choice moments.

The view provided us with confirmation of our progress up the mountain. But near the top the trail became a near scramble up the rocks. The short-cuts of use were hard to distinguish from the actual trail, as both were so steep.

The scramble was short, however, and we broke out into the barren surface of the top of the inverted stream. It was an alien landscape. Alien in the sense that it was covered mostly with actual native vegetation, unlike the grasslands below, which have been taken over almost entirely by European or Asian invasive grasses. The invasive species cannot compete in this harsh, mostly dry habitat. It was most certainly dry on this day, as we have had few rainstorms since February.

These rocky flats sometimes contain vernal pools and swales, and constitute one of California's rarest habitats. This section of the flow is one of the only protected portions of this type of landscape. The pools exist for only a few weeks at a time in the winter and spring seasons. At least one of the flower species here is found nowhere else in the world. In spite of the intense dryness, a few flowers persisted here and there, including on the shaded north slopes.

The top of the flow was a fine lookout for views in all directions.

The cliffs drop off steeply on both sides. The local casino lies directly below, and the abandoned open pit Harvard Mine lies just to the east.

To the north lies the nearly dry New Melones Reservoir.

It's a strange and wonderful environment on top of the lava flow, one that is quite different than any found elsewhere in the state. We explored the summit area for awhile, had lunch and started down the trail back to the road.

We visited Table Mountain in an entirely different time back in 2011. We arrived in a wet year, and hot on the heels of an overnight rainstorm. It's hard to describe just how different the scene was on that trip. There were pools and rivulets all over the summit area, and hundreds of small waterfalls cascaded over the cliffs. Flowers were everywhere. It was a totally different experience.

Here are a few scenes. For more, check out these posts from 2011:http://geotripper.blogspot.com/2011/03/day-in-fieldday-backwards-on-fun-having.html, http://geotripper.blogspot.com/2011/03/day-in-fieldday-backwards-on-fun-having_22.html and http://geotripper.blogspot.com/2011/03/day-in-fieldday-backwards-on-fun-having_23.html

What Is Your Favorite Volcanic National Park?

So many great choices! Would it be Rainier? Crater Lake? Lassen? Hawaiian Volcanoes? Haleakela? Yellowstone? Zion?

Wait a minute...Zion? Since when does Zion National Park have volcanoes? Since about 125,000 to 1.4 million years ago, actually. The western edge of Zion includes the Hurricane fault zone and a series of associated basalt flows and cinder cones. Firepit Knoll (above) and Spendlove Knoll (below) are two nearly symmetrical cones dating to around 220,000-310,000 years ago.

The park also has an outstanding example of an inverted stream. A flow filled North Creek and erosion removed the surrounding softer rock, leaving the former stream bottom as a prominent ridge.

It's a bit tricky to see, but cliffs drop off both sides of the road, which was built on the surface of the basalt flow. From higher up, the ridge is more obvious (below). The flow emanated from the Firepit/Spendlove Cones.

Where are these features, and how do they get missed by 95% of the park visitors? They can be viewed by following Kolob Terrace Road to Lava Point. The park brochure mentions the road, but I guess there are so many exciting things to see down in the canyon that folks don't set aside enough time to explore this road into the high country. It's both a shame and a blessing.

A blessing because I don't like crowds, but a shame because people are missing out on a very scenic and geologically interesting road. The road climbs 4,000 feet in 21 miles from the village of Virgin to Lava Point, one of the highest points in the national park. On the day we were there last week, the temperature dropped from 104 degrees in the valley to 78 degrees at the lookout. Aspen trees lined the last mile of the drive (most of the road is paved, except for this last bit, but the road is smooth).

The road begins in the Triassic Moenkopi Formation, and quickly ascends the lava flows to exposures of the Jurassic-aged Navajo Sandstone. This is the same unit that forms the spectacular cliffs of Zion Canyon. Without the vertical canyon walls, the Navajo weathers into a bewildering variety of beehives, castles, spires, domes, and whatever else your imagination conjures up.

The road crosses the park boundary several times, and passes through beautiful park-like meadows.

At road's end, we discover why the spot is called Lava Point. Columnar basalts form a cap on the edge of the plateau. At nearly 8,000 feet, the area is covered in a fir and aspen forest.

In a meadow near Blue Springs Reservoir, I caught a horse hanging out with a little friend catching flies on his back.

The cacti were in bloom here and there.

And then there is the view! From this vantage point, we can look into the deep canyons of Zion, and beyond to the Kaibab Plateau and the Grand Staircase. The view also extends to the high plateau country to the north.

There is a small campground set a few hundred yards from the edge of the plateau. There were a few people camping there, and all of six other cars at the viewpoint. It was cool, shady, quiet, and beautiful; a distinct contrast to the normal tourist haunts in Zion Canyon.

Give it a try next time you are in the region. And don't forget the Kolob Fingers, the other outlying, lesser-known part of Zion National Park.

Adventures in a Roadcut: It's All in Your Point of View

It's March in Central California, which makes for a really fine time for a geology field trip. We headed east into the Sierra Nevada foothills for a look at some Mother Lode history. For those of you not familiar with the history of California, the Mother Lode was the site of one of the world's greatest gold rushes, starting in 1848 with the discovery of gold flakes in the American River by James Marshall. Ultimately several hundred thousand people made the difficult journey to California to try and find their fortune. As is often the case with such things, only a few ever really prospered. We traveled through Hornitos, an off-the-beaten-track gold era town with some beautiful old ruins, and then followed Highway 49 from Mariposa to Coulterville and then to the hills near Jamestown.

The sun was getting low in the sky when we reached our last stop of the day on Peoria Flat Road outside of Jamestown. It's a neat little mystery for the students to work with, and a great lesson in the need for alternate points of view when problem-solving.

We had spent much of the day in typical rolling hills of the Mother Lode, and crossed several deep gorges, including the Merced River which flows out of Yosemite Valley. When we reached Peoria Flat, the landscape was different. The hilltops were flat and barren, even mesa-like. The flat hills were topped with some kind of dark brown rock that was unlike any of the the slate and serpentine outcrops we had been seeing all day. We found a spot where the road crossed through the flat-topped ridges and had a closer look at the dark rock and the sediments underneath it.

The students found that the dark rock was volcanic, and unweathered surfaces were nearly black. At their level (physical geology or historical geology), black volcanic rock usually equals basalt, though additional study would show the actual composition of the rock to be more along the lines of latite or trachyandesite. The rock underneath the ridge turned out to be conglomerate, with clasts composed mostly of a variety of rounded volcanic rocks. They were clearly deposited in a river. So we had something strange: a flat-topped hill composed of river gravels and topped by a volcanic flow. How did a riverbed and lava flow end up as a ridge?

The best clue to understanding this strange relationship is a different point of view. Several years back, a friend with a plane invited me on a flight over Sonora and Jamestown, and we flew right over the strange outcrop. I knew exactly what I was seeing because Table Mountain is a famous example of an inverted stream.

Nine or ten million years ago, the Sierra Nevada was a lower mountain range with a series of volcanic centers near the crest at Sonora Pass that would have resembled a region like Lassen Volcanic National Park. One lava flow was unusually long, some 60 miles, and as such had followed a stream canyon that had been occupied by the ancestral Stanislaus River. The lava displaced the river to something closer to its present pathway, and as the mountains rose and tilted to the west, erosion stripped away the softer rocks surrounding the lava flow. The old river bed became the flat summit of the Stanislaus Table Mountain; it had become an inverted stream.

You just have to look at the problem from another angle.

It was a beautiful day, and the flowers are ready to bloom. It's a really good time of year for geology problem-solving!

A Day in the Field...Day Backwards On Fun Having, Part Three. Stanislaus Table Mountain from above

The conclusion of my backwards mini-series on the Stanislaus Table Mountain starts with an earlier trip that will serve to explain the origin of the backwards geology of our hiking destination from last weekend (parts one and two here and here). The picture above reveals our path from a slightly different point of view, mainly from several thousand feet above. Back in 2002, the father of one of my students called me early one morning and said "Let's fly my plane. Where do you want to go?" I immediately thought of Table Mountain, because it is best seen and appreciated from the air.

I've been saying "backwards" a lot in these posts, but the actual term we want to use here is "inverted", as in "inverted stream". Our hike was to the top of a lava flow that sits hundreds of feet above the surrounding landscape. On first glance, this may not seem to make much sense. The key to understanding this oddity is to realize that the lava flow did not start out on top of a ridge.

Around 6 to 20 million years ago, volcanic activity was producing ash and lava flows in the summit region of the growing Sierra Nevada in the vicinity of Sonora Pass and the Dardanelles. The earliest volcanic activity included violent explosions of light colored rhyolite ash that coated the Sierra foothills (the Valley Springs Formation), but later eruptions mixed with water and snow to form volcanic mudflows (called lahars). These mudflows and river deposits coated the foothills region hundreds of feet deep, a series of layers called the Mehrten Formation. In the picture above, the layers form contours on the sides of the hills. For those familiar with the Knights Ferry area, the Mehrten forms the cliffs of Lovers Leap.

In the midst of this activity, about 10 million years ago, a single flow of latite (a sort of orthoclase bearing andesite) traveled more than forty miles down the western slope of the rising mountain range. Having flowed far beyond any volcanic slopes, the lava followed a riverbed, an ancestral path of the Stanislaus River. The picture below reveals the winding path the lava followed (our hike was located right in the center of the photograph). The surrounding rock was easier to erode than the latite, so when the mountains continued to rise, the lava flow remained while the other rocks were carried away. The bottom of the river became the top of the ridge: a textbook example of an inverted stream.

Farther "downstream" the lava flow widened considerably, forming a more plateau-like landscape. The Stanislaus River ended up carving through and around the lava flows. Some of the steep gorges became idea sites for modern-day reservoirs, like New Melones and Tulloch, seen below. The Table Mountain lava flow disappears beneath the slopes around Knights Ferry.

I mentioned in the previous post looking over towards the Harvard Mine from the summit ridge of Table Mountain. 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.

It was a lot of rock that was removed and piled up to get at the gold. So not only is the local landscape inverted, it has also been turned inside out.

I hope you've enjoyed these vignettes of my pair of journeys to a backwards landscape! The flight was made possible by Ken Iwahashi of KKI Corporation in Modesto. I have a web page with additional pictures and descriptions of the flight at Geotripperimages.com.

For more on the volcanism of the Central Sierra Nevada, check out:

Busby, C.J., et al., 2008, The ancestral Cascades arc: Cenozoic evolution of the central Sierra Nevada (California) and the birth of the new plate boundary, in Wright, J.E., and Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: A Tribute to Cliff Hopson: Geological Society of America Special Paper 438, p. 331-378.

A Day in the Field...Day Backwards On Fun Having, Part Two

Continuing our previous blog entry about exploring part of the California Mother Lode, we reach the summit of Table Mountain, near New Melones Reservoir. We had made our way across some grassy meadows and oak woodlands, and then climbed up a steep talus slope through brushy thickets to reach the edge of the summit. We walked into a strange forbidding landscape. The summit of Table Mountain at first looked barren and practically lifeless, but it took only a moment to realize it was full of life, especially on this particularly strange day of rain and storms.

I walked quickly over to the other side of the ridge and looked down on Highway 108 and the Jamestown area, including the mothballed Harvard Gold Mine. The ridge is long (miles long) and winding, and flat. But it is only a few hundred feet wide in most places. Boulders of lava were scattered all across the surface. The 'backwards' theme of these post has the most to do with the strange location of the lava flow at the top of the ridge. I'm going to fully explain that in the next post, but first I wanted to mention something outside my expertise: the biology...

As I wandered across the summit plateau, I realized that something was missing: fields of waving grass. It was striking. In the meadows below, exotic grasses were growing profusely everywhere. Up on top, grass was present, but as can be seen below, it is dominated by other plants; especially small flowers and mosses. Clearly the soils were very thin due to the nature of the lava, but these non-grass plant species were doing just fine, from the looks of things.

From what I could understand from some of the environmental studies of the volcanic tablelands, the exotic grasses have never really established themselves on the plateau the way they have in the pasture lands and meadows below. It's too tough of an environment. Apparently, most of the plants up here are natives.

It is a mostly dry and hot environment for much of the year, but for a few weeks or months, rain fills a number of low areas called vernal swales, which provide a unique growing environment for endemic species. Invasive exotic species cannot tolerate the extreme conditions. The vernal swales are broadly similar to the vernal pools found on the floor of the Central Valley (in the few undeveloped areas), but are more rocky with less clay. I saw quite a few unfamiliar flower species.

As I got closer (to the ground, that is), more and more species of plants appeared. I had to step carefully, ponds were everywhere! Most of the flowers were very small, reminding me of the toy flowers a child might use to decorate the outside of dollhouse. A garden of miniatures....

The colors were vivid, despite the grayness of the skies. Actually, as I looked around the skies were not just gray, but threatening. Rain was falling all around us, and a bitter cold wind was sweeping across the summit plateau. I wasn't quite ready to retreat just yet.

I got on my hands and knees and realized I was looking at an entire ecosystem on the surface of a single boulder (below). It was an incredibly colorful palette of living things, and a wonderful confluence of geology and biology.

I generally don't make a practice of quoting long passages of someone else's work if I can do a decent job of paraphrasing. but I'm not much of a biologist, and I have a feeling some of my biology-loving readers might want some of this info. It was buried deep in an environmental impact report by the Bureau of Reclamation about the Table Mountain flora and vernal swales (found here, starting at page 4-29):

"Vernal pools are an ephemeral wetland vegetative community with predominantly low-growing, ephemeral herbs. Germination and early growth occur in winter and early spring, often while plants are submerged, and pools dry out by summer. Flowering is often in bands at the margins of the pools. This community type occurs in shallow depressions, ranging from a few meters to tens of meters in diameter. Characteristic plant species found in vernal pools are Pacific foxtail (Alopecurus saccatus), common blennosperma (Blennosperma nanum), Cleveland’s shooting star (Dodecatheon clevelandii var. patulum), toothed downingia (Downingia cuspidata), spinysepaled button-celery (Eryngium spinosepalum), hedge-hyssop (Gratiola ebracteata), Fremont’s goldfields (Lasthenia fremontii), Douglas’ meadowfoam (Limnanthus douglasii var. rosea), white-headed navarretia (Navarretia leucocephala ssp. leucocephala), adobe popcorn flower(Plagiobothrys acanthocarpus), miniature popcorn flower (Plagiobothrys stipitatus var. micranthus), Sacramento pogogyne (Pogogyne zizyphoroides), Delta woolly marbles(Psilocarphus brivissimus var. multiflorus), greater duckmeat (Spirodela polyrrhiza), and Wildenov’s clover (Trifolium willdenovii) (Stone et al. 1993 in Reclamation 1995). Special status plant species that may grow in the planning area vernal pools include Sacramento orcutt grass (Orcuttia viscida), slender orcutt grass (Orcuttia tenuis), Bogg’s Lake hedge-hyssop(Gratiola hetersepala), and legenere (Legenere limosa).

Within the planning area, intermittently-formed pools appear after rainfall or snowmelt on top of Table Mountain between 1,200 feet in elevation in the south and 2,600 feet in the north. Although these pools share some of the characteristics of some vernal pools in the Central Valley, they are not true vernal pools in that they do not have a clay underlayer that prevents percolation. Instead, they form in swales in the rocky surface of Table Mountain. The soil is poorly drained and the parent material on Table Mountain is a Pliocene lava flow (andesite). Intermittent pools occur on Table Mountain in seasonally wet to saturated rocky meadows that have slight soil development (Evens et al. 2004). They are interspersed within the annual grassland (Reclamation 2006b). Intermittent pools at Table Mountain do not support the range of species found in vernal pools in the Central Valley, possibly due to differences in substrate(primarily shallow, rocky substrate versus clay substrate in valley vernal pools). Although vernal pool habitats are very delicate and easily disturbed in general, this is even more pronounced on Table Mountain where soils are poor, shallow, and loose.

To date, vernal pools have resisted invasion by exotic plant species, probably due to their ephemeral nature (USACE 1997). However, the scientific community is concerned that exotic plants may colonize vernal pool communities, possibly displacing the highly specialized native vernal pool species (USACE 1997). Despite these concerns, there is no supporting evidence that this change is occurring in vernal swales found on Table Mountain (USACE 1997).

So much of the time I am showing people a place that is new to them. It is always fun to show someone the treasures that exist practically in their own backyards. On this hike, I got that experience of seeing a place for the first time. I loved every moment!

In the final post of this mini-series, I will take a look at Table Mountain from a slightly different point of view...