Several years ago, before taking some students up there for field studies, an exceptional colleague (geologist Cara Thompson) asked me about the geology around Jamestown. Her question got me a little carried away. This sometimes happens when I find some time to wander into the amazing world of natural history and landscape mysteries. So, I will share this edited response by starting with my own observations based on my experiences up there. Here is a summary of the outcrops and geologic history of that region as I oversimplify it from the Central Valley up the Sierra Nevada slopes. If this discussion becomes too much geologic detail for beginners, you might start by viewing the images and consulting Chapters 2 and 3 of our book to build your foundation; if you are looking for more detail, check out the references listed at the end of this discussion.   

Sedimentary rocks that underlie the Central Valley have been down-warped, dipping toward the valley, and are often seen outcropping on its edges. However, these outcrops are far more common and dramatic along the west side of the Valley. For your trip, as you travel uphill from the Valley into the Sierra Nevada foothills metamorphic belt, you will notice those headstone-like outcrops rising above the grasslands and oak woodlands. They are often called tombstone or gravestone rocks and when you look at our photos, you can see why.

Metamorphic rocks in this region date back to the Mesozoic subduction zone when rocks from the Pacific Ocean floor were dragged toward the east and plastered against the continent, representing the original sources of the gold. These minerals would then be melted and incorporated into the edge of Mesozoic subduction zone magma chambers. Rich in valuable gold, silver, and other metallic ores, this remaining melt was finally squeezed through cooling cracks, joints, and fractures into the “veins” that would be the last to solidify and contain the gold along these contact zones. These vein-like intrusions can be seen throughout the area in both the metamorphic rocks and along the edges of the granitic batholiths that are mostly farther inland and upstream.

Now, here’s the key to the most interesting geologic features around Jamestown. Before the current Sierra Nevada Mountains were uplifted so high, the headwaters of their drainage basins extended much farther east into the Basin and Range. These gold-bearing outcrops were weathered and eroded west and deposited into ancient streams and rivers all the way to the Central Valley. These Tertiary deposits (with the gold) were then covered with lava flows from volcanic eruptions that also flowed west, following the path of those ancient streams and rivers. Dating shows that some of these rocks were still being deposited as recently as 9-10 million years ago, showing that today’s Sierra Nevada have been uplifted more than six thousand feet higher in that short time.

Differential weathering and erosion since has scoured the softer sediments that weren’t covered by lava flows in the stream beds and left the lava flows sticking up as the highest features in that area, creating their famous inverted stream canyons. Miners figured out that the lava capped those Tertiary gold-rich deposits and would dig through them (either by removing the lava stones and using them for building materials, or tunneling under the lava layers) to access the Tertiary gold deposits. What an interesting story of geologic history and human interaction!

You find this inverted topography in other Sierra Nevada stream canyons and hillsides such as north of Sacramento. Around what is now Oroville and Chico, the extensive Lovejoy Basalts erupted and flowed across the landscape during the mid-Miocene (more than 14 million years ago), evidently during a period of reversed polarity. We share an image in our photo essay here. This tough tableland erodes more slowly, mainly as chunks of the resistant volcanic rock finally break off after being undermined by the older but weaker sedimentary deposits below them. The tablelands remain standing above today’s surrounding landscapes. Look for the many “table” mountains labelled on your maps.   

Moving even farther east and higher, the Sierra Nevada Batholith dominates with all those granitic rocks that crystalized from the Mesozoic magma chamber within the subduction zone. Though there are several different dates, I usually use about 100 million years as a round-off figure to describe when they cooled and crystallized. These granites were more recently uplifted , causing overlying rocks to be eroded away and are now exposed so the Ice Age glaciers could scour them. Even farther up and east are those even older Paleozoic roof pendants, but we are now roaming beyond your Mother Lode field excursion.    

Here is an excerpt from a field guide written by another exceptional colleague (Matt Ebiner) at El Camino College, where he mentions Jamestown:

1. “We leave Columbia and drive back 8 miles to Jamestown. Along the way we pass a long flat-topped ridge known as a table mountain.  This is basalt rock which flowed as lava down an ancient river valley, but through erosion of the surrounding hills (with softer rock) has resulted in a sort of topographical inversion (ridge where there previously was a valley).  The basalt cap kept a vast amount of ancient stream gravels in place, providing miners with more potentially rich pickings.
2. Past Jamestown we go south on Hwy 49 to Hwy 108 where we head towards Oakdale. We go south (left) on LaGrange Road (J59) and slowly lose elevation as we leave the Sierra foothills and approach the flat San Joaquin Valley.  There are fewer trees as the oak woodland becomes thinner and the grassland more expansive.  There is increasing agriculture with grape vines, beehives (the white boxes on the right side of the road), cattle ranches (notice the portable ramps for loading cattle when they are ready to go to the slaughterhouse.”

Below is an excerpt from what was an on-line BLM publication focused mainly on areas north of Jamestown; it also cited some other publications that might be helpful, but it might take some surfing to find it today:

ROADSIDE GEOLOGY AND MINING HISTORY OF THE MOTHER LODE 2008 PART 3: GRASS VALLEY TO WALKER BASIN PLUMAS COPPER BELT  by Gregg Wilkerson and David Lawler; U.S. Bureau of Land Management Far West Geoscience Foundation, Buena Vista

“The Smartville, Slate Creek, Lake Combie, Feather River, Shoo Fly and Calaveras Complexes are tectono-stratigraphic units of similar (but not identical) age. They are separate terrane blocks representing material from different parts of the ancestral Pacific Ocean which subducted and accreted to the North American Craton from Triassic to Miocene time. The Big Bend-Wolf Creek Fault Zone merges into the Bear Valley Fault south of Auburn. The Melones continues southward through the mining districts of Greenville, Garden Valley, Placerville, Plymouth, Amador, Sutter Creek, Jackson, Carson Hill, Jamestown, Coulterville and Mariposa.”

Here are some field guides (available through 2018) that will help with specifics:

• table mountain volcanics around Jamestown
• field guide and summary that includes around Jamestown
• geology field guide of the aforementioned region
• general landmarks and points of interest around Jamestown

Books and Research Articles:

• Allgood, G.M., 1990, Geology and operations at the Jamestown mine, Sonora Mining Corporation, California, in Landefeld, L.A., and Snow, G.G., eds., Guide to Yosemite and the Mother Lode gold belt: Geology, tectonics, and the evolution of hydrothermal fluids in the Sierra Nevada of California: Pacific Section, American Association of Petroleum Geologists, Guidebook 68, p.147-154.
• Allgood, G.M., 1990, The Jamestown mine–Its history, geology and operations: Society for Mining, Metallurgy, and Exploration, Inc., Preprint Number 90-400, 8 p.
• Alt, David, and Donald W. Hyndman. 2000. Roadside Geology of Northern and Central California. Missoula, Montana: Mountain Press Publishing Company
• Dohms, P.H., R.D. Hoagland and G.M. Allgood, 1984, Geology of the Jamestown mine area, Mother Lode gold belt, Tuolumne County, California: Unpublished report and fieldtrip handout, 6 p., maps and sections, scales 1:1,200, 1:600.
• Harden, Deborah R. 2004, California Geology. Upper Saddle River, NJ: Prentice Hall, 2^nd e.
• Haydock-West, M.D., 1988?, Mineralogy and petrogenesis of dikes associated with the Mother Lode gold-quartz vein, Harvard mine, Jamestown, California: California State Polytechnic University, Pomona, unpublished senior thesis, 42 p.

The following images serve as an introductory field guide to illustrate many of the rock formations discussed in our article.

The post Geologic History in Sierra Nevada Gold Country first appeared on Rediscovering the Golden State.

These eastern slopes that face inland are dominated by continental air masses. Summer’s daytime heat yields to some of the coldest temperatures in the nation at night, thanks to the thin, dry air. Temperatures at select locations (often high-altitude basins where cold air masses can pool) drop well below 0 degrees F during winter. Hardy species that can withstand such extremes mix together in resilient communities that may be more familiar to the Rocky Mountains. Add accessible water and you will find some of the finest displays of autumn colors in the state when trees such as cottonwoods (often at lower elevations), willow, and the rarer aspen ((Populus tremuloides) often found at higher elevations) lose their chlorophyll greens and turn dormant to signal winter’s approach. In some cases, these locations were within view of the magnificent glaciers that reworked eastern Sierra Nevada landscapes during hundreds of thousands of years of cooler and wetter glacial periods.

Thousands of years ago, as climates warmed and the glaciers retreated, plant communities were left to evolve in drier climates. We find their stranded remnants as relicts snaking mostly along riparian woodlands in protected canyons or clustered around springs where they have access to precious water. And if you were wondering why these landscapes aren’t blanketed with the seemingly endless fall colors that spread from New England to Appalachia each season, you forgot the magic limiting factor here: water. Only about 1% of Basin and Range landscapes are riparian habitats. Soils, fires, microclimates, slope exposure, and climate change and other human impacts also help determine the nature of these communities.

So it is here that we take you on a journey to find California’s spectacular autumn colors among native plant communities most people might not include in their stereotyped California landscapes. The time is late October 2018, when the majority of leaves have lost their chlorophyll, but haven’t fallen off the deciduous trees. Our search leads us toward the West Walker River and East Walker River headwaters near the Nevada border southeast of Lake Tahoe, where streams feed the upper Walker River Basin. And once again, we quickly discover that these landscapes have plenty of concomitant stories to share.

In this case, we see how two different drainages start in spectacular mountain scenery and then cascade off the eastern slopes of the Sierra Nevada to deliver precious water to high desert landscapes below. Both rivers turn north, separated on opposite sides of the Sweetwater Mountains before flowing out of California and into Nevada.  And as is common with so many streams in the West, human demands (mostly for agriculture) are often greater than the discharge from these rivulets.

The familiar results are less water for downstream ecosystems and users as the West and East Walker Rivers flow into Nevada and finally merge. By the time what’s left of the Walker River makes an abrupt right turn south and struggles toward Walker Lake in Nevada, there is little or nothing left, except during flood years. By then, free-flowing water must have survived abuse from a variety of stakeholders through several jurisdictions and land uses. Especially during dry years, downstream fish, wildlife, and entire riparian ecosystems struggle along with the humans who depend on these water diversions for survival.

So this starts out as a simple story about how a history of water, fire, and ice have combined to produce some of the state’s most spectacular fall colors. It quickly becomes a more complex drama that involves water diversion and water rights among multiple stakeholders and between bordering states and Indian reservations. Issues involving groundwater access and quality, irrigation for agriculture, domestic water quality, fishing and other ecotourism, watershed and ecosystem management and restoration, and sustainable land uses all float to the surface. It finally ends well beyond the boundaries of our California project in a threatened and protected Lake Walker, a terminal lake in Nevada’s high desert. During more than 100 years of human diversions, lake levels dropped 160 feet and total dissolved solids increased ten times up to 2010 in this water that often began as crystal clear Sierra Nevada snowmelt.

All of these undercurrents lurk among the relatively remote small town America landscapes and cultures near Hwy. 395 where the ever-changing weather and climate, the best fishing holes, and the tastiest homemade pies might dominate local conversations.         

Our images begin just south of and then around Twin Lakes near the headwaters of the East Walker River. We will then explore the West Walker River as it cascades through its gorge and then spreads into its Antelope Valley. 

We end by skipping south and away from these watersheds to find color in Round Valley just above and northwest of Bishop and below the lateral and terminal moraines left by glaciers similar to those mentioned earlier, a rangeland on the northern edge of an Owens Valley with a very different, but epic water rights story of its own.

Highway 395 is the remarkable thread that ties these images together, since all of these locations are conveniently accessible, especially by adding a short drive or hike up a local canyon. The colors get easier to find north of Bishop. Additional patches of fall colors can be found in canyons and on slopes near Hwy. 395 north of Reno, into more remote northeastern California. This would make a good excuse for another autumn excursion into the colors of the Warner Mountains.

Autumn Colors

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The post Searching for Autumn Colors in the Eastern Sierra Nevada first appeared on Rediscovering the Golden State.