Cherishing Our Rivers: A Journey on the Water with the Tuolumne River Trust

Are you lucky enough to live near a river? For much of my life I didn't have that privilege. Southern California has creeks at best, except when they were flooding and otherwise causing havoc. The creeks often flowed through incredibly beautiful mountains and valleys, but they can't be a source of life for human civilization. We're too busy using what little water there is that there is barely enough to maintain a healthy ecosystem. Southern California has to import around 85% of the water that it uses.

People who live in Northern California or the Pacific Northwest might smile when I describe my Tuolumne as a river. Compared to the mighty rivers like the Columbia it is a middling stream. The thing is, it lies about that point in the northward journey in California where the rivers provide sufficient water for local use that it doesn't have to be imported from elsewhere (indeed, much of the water from the Tuolumne is exported to the Bay Area). We depend mostly on groundwater for domestic use, and the groundwater is replenished in part by the river. Most of the river goes to irrigation, but some is allowed to flow into the San Joaquin River and on to the ocean (not willingly, it should be noted; there are some who would just as soon use all of it).

The Tuolumne River begins high in the Sierra Nevada, flowing through Tuolumne Meadows in Yosemite National Park. It plunges into the Grand Canyon of the Tuolumne, a spectacular gorge that is as deep as that other canyon in Arizona. It is briefly stopped and diverted at Hetch Hetchy Reservoir, and then flows through a lesser-known gorge (unless you are a rafting enthusiast) to Don Pedro Reservoir. More water is siphoned off, but part of the river continues through the Sierra Nevada foothills and Mother Lode, and finally flows into the Great Valley, where it joins with the San Joaquin.

A swallow swings low over the river.

That's where I found myself today, rafting a section of the Tuolumne where it flows through the foothills between La Grange and Turlock Lake State Recreational Area. I had joined the Tuolumne River Trust on their By Land and By Water Canoeing trip, where I was serving as the resident geologist (literally 'resident', as I live just ten miles downstream). It was a fine adventure for the twenty or so participants, who were helping raise funds for the Trust.

What do they do? The Trust organizes river clean-ups, restoration of regions burned by the Rim Fire, and educational opportunities for children and adults. They also do political advocacy, as a voice for the river itself in the cacophony of agricultural and urban water interests. It's a great organization that deserves your support!

The river was alive with birds, bugs and amphibians. I glanced at a bug hovering a foot or so over the river, and it suddenly disappeared in the mouth of a swallow. The swallows accompanied us along the entire length of our journey. Maybe all the oars hitting the water was stirring up the bugs! I saw an old telephone pole in the distance, and immediately recognized the huge pile of sticks and random debris as an Osprey nest. Both parents were there, along with at least one chick (barely visible to the right of the birds in the picture above).

The Ospreys were not happy with the rafters and canoers below, and let them know in no uncertain terms. They relaxed when we floated by and headed downstream.The river is a perfect habitat for these grand birds. 90% or more of their diet is fish, and they are experts at landing feet-first in river to grab unwary ones floating near the surface.

Exposure of Valley Springs Formation along the Tuolumne River

There is some interesting geology on the lower river. The cliffs we passed had outcrops of white or tan-colored volcanic ash called the Valley Springs Formation. The ash was the result of gigantic eruptions of rhyolitic magma at calderas located in central Nevada! Given the media fascination with Yellowstone "super-volcanoes", people might be surprised to find that similar calderas erupted all across the American West between 20 and 30 million years ago.

A bit further downstream there are exposures of the Mehrten Formation. It originated from volcanic mudflows related to the Cascades-style stratovolcanoes that once existed near the present day summit of the Sierra Nevada around 10 million years ago. Sediments within the unit have yielded an array of fossils including ancient camels and horses, giant tortoises, and strange 8 foot long saber-toothed salmon! Younger rocks above the Mehrten have given up Mammoths, Mastodons, Saber-tooth Cats, American Lions, Short-faced Bears, and Giant Ground Sloths, along with many other ice age species.

The lower reaches of the Tuolumne River cannot be mistaken for a world class whitewater rafting locale (you have to go upstream for that), but it can present unique challenges to amateur canoeists like myself. There are thickets of willow and cottonwood trees along the banks, and sudden turns in the river course can drive boats into a maze of branches. We had half a dozen swampings that usually happened when both canoeists ducked in the same direction as they went into the thickets. No one was hurt, but it can be momentarily terrifying.

In some places the river was constricted to where it was only a few feet wide, and the water flowed very fast. If you look at the canoe above, you can see how fast it's plowing through the water as the people oared furiously to avoid the thicket just below. The water whipped them in a circle into a back eddy.

There were also hazards caused by fallen trees. Some of them reached almost across the river, and the one above occurred just after a fast-moving chute. Being short (which I'm not!) has advantages in such situations. The Trust folks put a premium on safety. You can see that everyone had a personal flotation device, and leaders were stationed at critical points in the difficult passages to help guide boats through.

We've abused our rivers. Gold dredges did incalculable damage to this stretch of the river by ripping out the riparian vegetation and destroying the soils along the river floodplain. Miles of river habitat were destroyed by the search for a few suitcases of gold, but money seems to trump all. Over the decades the river has started to recover, but it will be many more years before the river can be truly healthy for salmon and all the other life forms that historically lived along the river (I looked for River Otters today, but unfortunately didn't see any). There are many other challenges, like the invasive hyacinth, a water plant that clogs river channels, pollution (why do we use rivers as a dumping ground?), and low flows related to the continuing drought in California (the water is warmer than it should be).

It was a marvelous day spent with people who are working hard to protect and rehabilitate this precious resource. Rivers are truly a gift, and they need to be cherished and protected. What rivers are near you, and what can you do to help?

Volcanic Violence and California's Most Endangered Habitat: The Vernal Pool Prairie

The green hills of California's prairie barely conceal a horrific violent past. Around 20-23 million years ago, huge volcanic eruptions shook the western part of North America. Ash (pulverized volcanic rock) blasted into the atmosphere in unimaginable volumes, 100 cubic miles or more. Some of the ash was driven into the upper atmosphere, to be spread around the world with the wind currents, but much of it settled in layers that covered tens of thousands of square miles. Living things would have been buried at best, incinerated at worst. Any one of these events would have been a catastrophe. But there were many of them.

Some of the ash clouds rolled off of highlands that once existed in central Nevada and flowed west into California and down the western slopes of the slowly rising Sierra Nevada. These ash deposits are called the Valley Springs formation, and this rock still influences life in the present day.

The ash eruptions subsided, but new volcanoes developed at closer quarters, near or just east of the present-day crest of the Sierra Nevada. An occasional lava flow reached the lower foothills of the Sierra, but more often, it was a volcanic mudflow, or lahar, that came to inundate the landscape. The rocks today are called the Mehrten formation. Numerous animals lived on the resulting river floodplains and lakes, include tremendous six-foot long salmon (with "tusks"), and tortoises the size of their Galapagos cousins. Mastodons, horses and camels wandered the plains, chased by fearful carnivores, including the ancestors to the Sabertooth cat.

The geologic violence subsided, and erosion has subsequently smoothed off the hard edges of the volcanic rocks. In the semiarid climate, it is mostly grass and annuals that grow here, although the occasional oak survives in the barren landscape. The volcanic rocks contributed to the unique habitat that exists here today, providing through the weathering process copious amounts of clay.

Most of the minerals in ash will decay over time into one of the clay minerals. Soils that develop at the surface tend to be clay-rich, or clay may be present just below the surface, forming an impermeable barrier that prevents water from percolating into the ground. Because wind can blow out hollows, and because clay-rich layers can differentially settle, small pools of water may remain after major winter rainstorms. Because the water can't percolate, it doesn't disappear until it evaporates. The resulting ponds are called vernal pools. The pools are one of our more unique and unusual habitats in California.

A large number of species of plants and animals have evolved to survive in this environment that swings wildly from wet to dry within a period of weeks, and many of the species are endemic to the state. Colorful wildflowers bloom quickly in the drying rings of the "shores" of the little ponds, growing and setting seed before the water has completely dried up. The seeds can repose in the soil for years at time when drought dominates the landscape. Frogs and other amphibians may burrow into the soil and estivate (remain inert) for months on end, waiting for the rains to return.

We spent a delightful afternoon out on the prairie last weekend. It's not too hard because our village lies on the edge of the grasslands. We traveled Willms Road south of Knights Ferry near Highway 108. The spring flowers were just beginning to make an appearance following a warm dry February. There had been fears that this was the end of our rainy season, but a number of vigorous storms are on tap for the next two weeks, so the pools may be filling again for awhile.

Many birds call the prairie home. We felt especially lucky to see a pair of Great Horned Owls in a hollow along an ephemeral creek. The Valley Springs formation is soft and easy to excavate, but the holes are architecturally sound.

A Savannah Sparrow favored me with a pose on a fence next to the road. It was the first time I've had a chance to photograph one.

It was also the first time that I captured a Horned Lark. I thought at first that it was a Western Meadowlark, but I realized the markings were all wrong.

The Meadowlarks were certainly out and about, though. We could hear them all along our drive. At this time of year, they're looking for mates, and aren't at all bashful.

It was a beautiful afternoon in a serene "wilderness" (these are really private ranchlands), but we were also chastened by the sight of thousands of acres of new almond orchards that have been planted just to the west. The water for the trees is coming from irreplaceable groundwater aquifers, and when they have been sucked dry, the almonds will die away. In the meantime, hundreds if vernal pools will have been destroyed.

2.0% of the Tuolumne River Story, Courtesy of the Tuolumne River Trust. And 8-foot-long Saber-toothed Salmon!

Standing on the Dana Fork, close to the headwaters of the Tuolumne River in January of 2012 (when no snow had yet fallen)

Rivers often get used as a metaphor for life. Writers might compare the exuberance of youth with the dancing streams and waterfalls at the headwaters of rivers, and the calm deep waters of the big rivers denote adulthood. And of course, there is the "Old Man River". In a way, geologists once did the same thing, designating different stages of river erosion as "youth", "maturity" and "old age". On a planet that is so active, rivers rarely follow the script of such simplicity. Rivers undergo constant change, and some parts of river systems can be older than other parts. The watersheds can be altered by mountain-building events, lava flows, landslides, and other dramatic changes in a landscape. The Tuolumne River certainly fits that bill.

The Tuolumne River flows through the flat valley of Tuolumne Meadows, the location of a 2,000 foot deep ice sheet during the ice ages. The longest glaciers of the Sierra Nevada, 40 miles long, originated here.

The river seems "normal" enough. It has headwaters high in the Sierra Nevada in Yosemite National Park, it has carved a deep canyon along the middle stretches, it flows onto the plains of the Great Valley of California, joins up with other rivers, most notably the San Joaquin (although the San Joaquin hasn't contributed significant water to the system in many decades), and finally ends in the delta complex at the head of San Francisco Bay.

But there are significant deviations. In the high country of the Sierra Nevada, the river flows across a mostly horizontal landscape for a number of miles. Tuolumne Meadows could almost be described as a swampy stretch of river, especially during the spring runoff in wet years. Then the river plunges headlong into a deep gorge, complete with waterfalls and cascades that would be more characteristic of the headwaters.

Into the river at LaGrange Bridge

Along the ridges and slopes high above the present day river, there are stretches of river gravels from a long-gone ancestral Tuolumne River, one that may have had its origin in central Nevada. Other ancestral Tuolumne Rivers lie within the sediments of the Mehrten Formation that form bluffs around the present-day river in the foothills. These mudflow deposits (and occasional lava flows) record a time when volcanoes erupted near the Sierra Nevada crest only 9-10 million years ago.

I had a marvelous opportunity to explore 2% of the river last week, courtesy of the Tuolumne River Trust. The trip was a three mile long canoeing expedition (the river is 149 miles long, thus 2% of the river), from the Old LaGrange Bridge to the Old Basso Bridge. It was a yearly event, the "Paddle with the Salmon" trip. This part of the river flows among the lowest foothills of the Sierra Nevada, just a few miles upstream from where the river emerges onto the floor of the Great Valley at Waterford.

2% of a river doesn't sound like all that much, but there is a lot of geological history wrapped up in those couple of miles of river. Old LaGrange Bridge was built in a spot where the granitic and metamorphic rocks of the river disappear beneath sedimentary layers. There are three named formations in this part of the Sierra Nevada foothills: the Ione, Valley Springs, and Mehrten formations. They encompass around 40 million years of geologic history.

Around fifty million years ago, the region was very different. The Sierra Nevada, which had been an Andean-sized mountain range throughout most of the Mesozoic Era (the age of the dinosaurs), had been deeply eroded to what amounted to low hills (in many interpretations anyway; the issue is "under discussion" in research circles). Large rivers with a source at least as far east as central Nevada flowed across what would later become the crest of the present-day Sierra Nevada. These rivers flowed into a tropical coastal complex that included swampy estuaries and extensive sandy beaches. The resulting Ione formation has been mined for low-grade coal called lignite, for clay (used in ceramics), and the beach sand has been mined for glass-making (you can tour one of the old mines at Black Diamond Mines Regional Preserve near Antioch). We drove past exposures of the Ione where we got onto the river.

The river sediments, called the Auriferous Gravels were also mined, much to the detriment of the riparian habitat of the Tuolumne River. "Auriferous" means gold-bearing (Au is the chemical symbol of gold). Miners turned the river upside down in their search for the elusive metal. They used pans and other placering methods at first, but by the early 1900s they were using large floating barges called dredges. The dredges would be floated in a newly dug pond (filled by seepage from the high water table), and giant shovels ate away at one edge of the pond. They processed the sediments for the gold on the barge itself, and then used a conveyor belt to dump the waste rock at the other end. In other words, the dredges were sailing across the landscape, carrying their ponds with them.

The biggest terrifying rapid of the day! When one of the canoes got hung up on a rock, the trip leader hopped out of her raft, walked over the canoe, and moved it along. Yeah, the river was kind of low...

The dredges remained active through the early 1970s. The dilapidated remains of an abandoned dredge can be seen just a couple of miles from our section of the river, off LaGrange Road. The remains of a dredge maintenance camp can be seen on a bluff above the river at Old Basso Bridge. One of the old dredge fields has been preserved as a county park called the Joe Domecq Wilderness.

Many of the efforts of the Tuolumne River Trust involve the rehabilitation of the riparian habitat along the river, to assist in the recovery of the population of Chinook Salmon and other native species. There is a lot of work to do. The dredges were terribly efficient, with the emphasis on "terribly". The heritage of the mining efforts are barren fields of boulders. The fertile soils are long gone.

Near the end of the rafting trip we passed buff-colored exposures of the Valley Springs formation. The layers contain volcanic ash and tuff from gigantic eruptions of rhyolite calderas (so-called "supervolcanoes"). Incredibly, the eruptions took place in central Nevada. These ash layers traveled hundreds of miles! There were Native American grinding stone holes in some of the exposures.

In the hills above us we could see ledges and cliffs of the Mehrten formation. From about 11 million to 7 million years ago, eruptions from andesite volcanoes near the crest of the present-day Sierra Nevada produced huge lahars, or volcanic mudflows that choked old channels of the ancestral Tuolumne River. The river would shift in response to the disruption and establish new channels. The rivers flowed across low plains populated by various species of camels, horses, antelope, and mastodons. There were also large Galapagos Tortoise-sized turtles, and strangest of all, 8 foot long salmon with large teeth variably described as tusks or saber-teeth. Despite the fearsome teeth, they probably fed on small prey. If you would like to learn more about the salmon and the turtles, check out the new exhibit at the Great Valley Museum at Modesto Junior College.

We only saw a few salmon during our float trip, but that was okay. We know they're out there. What we did see was 40 million years of Sierra Nevada history exposed along just 2% of the river's length. What a marvelous adventure in the past (and in the present day as well)! I deeply appreciate the kindness of the Tuolumne River Trust for allowing me to join in on their journey.

Driving Through the Most Dangerous Plate Boundary in the World: A Gentle Landscape Belies a Fiery Past

As we leave the Great Valley behind on our journey through the most dangerous plate boundary in the world, we finally enter the world of the Sierra Nevada. Many may think of Yosemite Valley or Lake Tahoe when the Sierras are mentioned, but the mountains rise modestly from the west side. The transition from the flat Great Valley to the gentle rolling terrain of the Sierra Nevada is not always obvious. Because most of my Sierra journeys begin there, we'll start in the central part of the range, the drainages of the Tuolumne, Merced, and Stanislaus Rivers. The picture below is the new bridge over the Tuolumne River near Old Basso Bridge upstream of Turlock Lake State Recreational Area.

This quiet gentle landscape belies a violent past in several ways. The initial rock outcrops along the rivers look sedimentary, given that they are layered and are composed of gravel, sand and silt, but the origin of some of these rocks was in fire. They are volcanic. Secondly, the lowermost rock layer found here contained gold, and miners ripped into these rocks with a ferocity that would humble today's heavy equipment operators. Thirdly, the rocks underlying these sediments record the intense deformation related to terrane collisions in an earlier time, when the Ancestral Sierra Nevada was forming.

The Sierra Nevada begins as prairie, or near the rivers, forests of cottonwood, sycamore, oak and the occasional Gray Pine. It's dry country, a far cry from the cool pine forests and alpine peaks that people usually associate with the Sierra Nevada. One can choose to follow the high-speed roads like Highway 108 out of Oakdale, or Highway 140 out of Merced, but I suggest some of the quieter avenues, like Lake Road or Highway 132. There are ranch roads that provide an even more serene journey through the foothills. The picture below is from Warnerville and Willms Roads east of Oakdale.

The basal sedimentary rock is called the Ione formation. The gravels, sands and clays of the Ione were deposited in a distinctly different environment than we see today. The sand was deposited along a beach strand, the clays (and associated low-grade coal deposits) in coastal estuaries and swamps, while the gravels settled in large rivers flowing into the coastal delta complex. Fossils in the Ione indicate tropical conditions. The Sierra of 40-50 million years ago was a coastal jungle, not unlike the coast of the Yucatan Peninsula of Mexico today!

Things changed around 25 million years ago. The climate cooled, the sea retreated to the west, and intense rhyolite eruptions began from calderas off to the east, near the present-day Sierra Crest, and farther away in central Nevada. These eruptions produced not lava, but volcanic ash, the pulverized remains of rock that exploded rather than flowed. These eruptions are the most violent known, killing all life over hundreds or thousands of square miles. The Sierra Nevada, a mountain range that had been laid low by erosion, was stirring again. The mountains were about to rise again...

Next up: mud. Lots and lots of mud...

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....

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.