Feeling a Bit Sheepish: A Bit of Glacial Mutton in the Sierra Nevada

Pothole Dome in Tuolumne Meadows at Yosemite National Park

Sometimes glacial terms represent some kind of logical relationship to reality. "Horns" that are sharp pointed glacial peaks, "cirques" that are circular-shaped glacial bowls, "glacial polish" referring to rock worn smooth by glaciers, or "moraine lakes" that are lakes dammed by glacial moraines. Then there are the others: tarns, eskers, kettles, aretes, and cols, the kinds of terms that cause misery among my first-year geology students. But one term seems to depart from all reality...roche moutonnée

Unnamed roche moutonnée just north of Lembert Dome in Tuolumne Meadows

The term roche moutonnée describes an asymmetrical glacially scoured rock outcrop that has a smooth slope on the side facing the flowing ice, and a steep cliff on the side where the glacier pulled away from the outcrop ("stoss and lee structure" is a related term). The scale can range from a few meters to many hundreds. They are common features in regions of bedrock that have been scoured by massive continental ice sheets, but are a bit less common in mountainous areas eroded by alpine glaciers. These glaciers are the kind that filled valleys, but didn't overwhelm the surrounding peaks, such as those that flowed through most canyons of the Sierra Nevada. Yosemite Valley is the most famous, but glaciers filled dozens of other major river canyons from south of Sequoia National Park all the way north to the Lassen Volcanic National Park region. At their height, these glaciers covered about 30% of the mountain range.

Hiking the gentle slope of Lembert Dome in Tuolumne Meadows

The Tuolumne Meadows region of Yosemite National Park was an odd exception to the usual Sierra Nevada glaciers. The area surrounding the meadows was covered by a 2,000 foot thick mass of ice that was far more like a continental ice sheet than an alpine glacier. The ice spilled over ridges into several other drainages, including that of the Merced River, which carved Yosemite Valley. The addition of ice from the Tuolumne drainage gave the Yosemite glaciers an extra bit of "oomph", allowing them to erode deeper and farther than they would have otherwise.

Looking down the western edge of Lembert Dome. Note the glacial polish on the summit ridge.

There are numerous roche moutonnées in the Tuolumne Meadows area, including the easily seen Pothole Dome (first picture of this post), and Lembert Dome near the Tuolumne Meadows campground (below). The summits of each are easily climbed (from the right direction, anyway), and both provide stunning views of the region.

The problem with roche moutonnée as a geological term is that we geologists can only barely agree on its meaning. It's derived from French, and the "roche" part isn't a problem. It means "rock". But "moutonnée" is the tricky one. It can be translated loosely as "sheep" (think "mutton""), but not exactly (French: "mouton"). Moutonnée (with the extra e's) translates to "frizzy", and is taken as a reference to sheep's wool. The term originated in the 1700s with a naturalist named Horace-Bénédict de Saussure (it would be decades before the term "geologist" was coined) who noted that the rocks looked like a type of wig apparently well-known at the time whose locks were held in place with mutton grease. Except that there seem to be few or no references to wigs that were actually called that (the closest version was a tête de mouton).

So we teachers are left with trying to define the term as meaning "rock sheep" based on the nebulous idea that the rocks look like sheep grazing in meadows. Which they really don't. But it's still easier than trying to describe obscure French wigs from the 1700s and mutton grease.

A few weeks back we were on our field studies trip to the eastern Sierra Nevada, and we descended from Sonora Pass into the drainage of the West Walker River at Leavitt Meadows. I was describing various glacier features and even mentioned the presence of a few ill-defined roche moutonnées upstream of the meadow. You can see them in the picture below. But then I noticed something else in the midst of the meadow. I saw them, and now you are hoping that Geotripper won't go there, but he will...

Yes. It was actual....

...rock sheep! In a meadow.

But I still don't think they look like eroded glacial domes.

For an excellent description of the derivation of the geologic term, check out this great story from the Atlantic.

You Just Can't Take Sonora Pass for Granite: Travels Around the Sierra Nevada High Country

Sonora Pass in the Sierra Nevada is a sort of forgotten step-sister of the more famous Tioga Pass in Yosemite National Park. A paved road, State Highway 108, crosses the crest of the Sierra about forty miles north of Tioga, cresting out at 9,624 feet, but it sees far less traffic. There are, for instance, no national parks, no major ski areas (Dodge Ridge is thirty miles to the west), and no major resorts, and snowdrifts keep the pass closed for much of the year. But Sonora Pass is an incredible place to visit if you ever get a chance, and it makes for a nice loop trip out of the Central Valley, including Tioga on the way back.

The road is steep and winding towards the top, but the views are extraordinary. We are in the headwaters of the Stanislaus River, and the region was heavily glaciated during the ice ages, making for spectacular alpine scenery. But one thing seems to stand out as one approaches the summit. You can't take this pass for granite. At least not all of it. This bad pun can be understood in two ways: there is volcanic rock at the top of the pass, not granite, and what "granite" there is along the lower reaches isn't actually granite.

First off, there are lots of fine exposures of granitic rock below the pass, and along the highway for a considerable distance downstream to the west. But it is not technically granite. If one has had a basic earth science course, a person may have learned that there is granite, diorite and gabbro as the plutonic igneous rocks (light, medium, and dark). But if one takes physical geology or an upper division petrology course, one finds there are other rocks intermediate between granite and diorite. The differences come from the proportions of potassium feldspar, plagioclase feldspar, and quartz. As such, true "granite" is actually somewhat rare in the Sierra Nevada, perhaps accounting for 10% of the total. The most common rock is granodiorite, a granite-looking rock rich in plagioclase, but lacking in potassium feldspar.

I'm the first to admit that the difference to a layperson may seem trivial, but recognizing these differences allows researchers to draw many conclusions about the circumstances and timing of the intrusions that formed the backbone of the Sierra Nevada. And some of the other
"granitics" are truly beautiful rocks. At Sonora Pass, the common granitic rock is called the granodiorite of Topaz Lake, and it is one of the younger intrusions in the Sierra Nevada, having cooled about 89-93 million years ago. It has massively large crystals of potassium feldspar, 2-3 inches long, embedded in a groundmass (matrix) of smaller crystals of quartz and plagioclase. Such rocks with two distinct grain sizes are described as porphyritic. Glaciers have smoothed off the rock in many places ('glacial polish'), almost providing the effect of a polished countertop.

The other way that Sonora Pass cannot be taken for granite is the fact that the rocks at the pass are actually volcanic in nature. Between 11 and 9 million years ago, this area was an extensive volcanic center broadly similar to the one that exists today at Lassen Volcanic National Park, with andesitic stratovolcanoes, lava domes, and a modest caldera complex.

The volcanoes are long extinct, and glacial erosion has greatly modified their original outline. The mountains are striking if for no other reason than that they are different from many of the light-colored peaks of granitic rocks seen elsewhere. In a sense, they almost seem somber and dark. John Muir would no doubt have seen their beauty, but probably wouldn't have been inspired to call them the "Range of Light".

If you ever want to brag about your hiking exploits, but just don't have the energy, you can go to Sonora Pass, and wander a few yards up the trail that crosses the pass going north and south, perpendicular to the highway. That way, when people ask what you did last summer, you can truthfully say something like "I spent a bit of time on the Pacific Crest Trail"!

If you long to spend time in the Sierra Nevada, and just can't fathom the thought of fighting the crowds at Yosemite Valley and other parts of the national park, give Sonora Pass a look. I'm pretty sure you won't be disappointed. If you need a guide, my colleague Noah Hughes and I edited a field trip guide for the region for a meeting of the National Association of Geoscience Teachers in 2012, and the book we published is for sale (details are here; all proceeds support the scholarship program of the NAGT-Far Western Section).

Fall Colors in the Eastern Sierra Nevada!

We've returned from a field studies expedition to the eastern Sierra Nevada, and I can report that fall has arrived! We've had a very warm summer, and the heat waves continued into September, but then we had a sudden cold snap, and even some snow. Tioga and Sonora Passes briefly closed, but opened again in time for our trip. The aspens and cottonwoods have responded to the sudden cold conditions.

Our route took us over Sonora Pass, where fresh snow still lingered on the high slopes. It's a sight I'm not used to. We spent a lot of time in the Bodie area and in the White Mountains where there aren't many deciduous trees, but late in the weekend we moved into the canyons below the Sierra Crest.

One of the special places is Convict Lake, which is dammed by a moraine of the Tioga glaciation that ended about 13,000 years ago. Aspens and cottonwoods crowd the shoreline, providing vivid color.

On our last day, we took the June Lake Loop, and stopped to explore Silver Lake. The canyon of Reversed Creek and Rush Creek is a spectacular glacial valley with high peaks above, and a string of beautiful lakes.

As we wandered among the trees, we heard a rockfall on the slopes high above. It wasn't all that big, leaving little dust in the air, but it was exciting to listen to geology in action.

It's been a long summer, but the drought finally broke, and the forest was looking healthy. Our mountains might not have the color range of the eastern hardwood forests, but then again, those forests don't have the mountain backdrop of our beloved Sierra Nevada.

Pareidolia and Fall Foliage Art from the Sierra Nevada

Pareidolia is a trick of our minds that can cause us to interpret random images or patterns of light and shadow as faces. We were in the Sierra Nevada the other day ahead of Sunday's storm, looking for some fall colors around the meadows along the Clark Fork of the Stanislaus River near Sonora Pass.

I was mostly looking upwards towards the trees, but eventually my attention was deflected downward at the many aspen leaves that had already fallen to the forest floor. The bright yellow color of the leaves was giving way to brown, but the pattern was different on every leaf. The first one above immediately made me think of a face, or even a jack-o'-lantern (thus making a connection to tonight's candy obsession).

Conditions are changing rapidly up in the high country. We've been able to travel to the highest elevations all summer, but the latest storms have begun to drop snow over the passes. Sonora and Tioga closed Saturday in anticipation of the big storm on Sunday. They might reopen if conditions stay dry in coming weeks, but the writing is on the wall (and on the leaves): winter is coming.

The leaves were almost gone from the aspen trees up on Clark Fork. I hope the omens are good for a big snow year. We need a break from the horrific and continuing drought here in California and the Southwest.

The Sierra Beyond Yosemite: Donnell Vista and Sonora Pass

It's gloomy and foggy, and I haven't seen the sun for days. It's a few more weeks before field season (Death Valley!), but I can't help exploring the sunnier places from warmer times. I'm looking back at the pics from our fall semester where we explored a lot of places in the Sierra Nevada that aren't Yosemite.

That's the thing. Say "Sierra Nevada", and a lot of people will immediately think of the beautiful valley of the Ah-wah-nee, John Muir's favorite place on the planet, and in many ways mine as well. But the floor of Yosemite Valley is about 7 square miles. The national park covers 1,190 square miles (3,081 square kilometers). But the Sierra Nevada? It covers 39,612 sq miles (102,594 km²).  You could hide more than 30 Yosemite parks in the rest of the range. It is in fact the largest single range in the lower 48 states (large mountain systems like the Rockies and Appalachians are made up of numerous smaller sub-ranges).

So we are off onto a short exploration of some of the wonderful corners of the Sierra Nevada that aren't Yosemite Valley. We are following a week's worth of our trips last fall that took us over the range at Sonora Pass and down the east side of the range as far as Lone Pine and Mt. Whitney. We'll also explore the other national parks of the range, Sequoia and Kings Canyon, which we visited on a second trip.

We began our journey in some serious smoke from a series of fires burning through the western slopes of the Sierra Nevada. The drought and the fires have been catastrophic. We made a stop at the Twain Harte Lake exfoliation site. I wrote about it back then, and it was picked up on Reddit and IFLS, links that led to the post being the most read ever on Geotripper (12,400 hits and counting). We emerged from the smoke and climbed the upper reaches of the Stanislaus River, approaching Sonora Pass, which after Tioga is the highest paved highway over the Sierra Nevada at 9,624 feet (2,933 meters). Tioga Pass in Yosemite is 9,943 ft. (3,031 m.).

In a series of ice ages, glaciers covered about 30% of the range, reaching as low as 3,000 feet or so in some of the deeper canyons. Yosemite is simply the most famous of the glacially carved gorges, and many others are of incredible and spectacular beauty. This was not always fully appreciated, and some of these wonderful wild canyons were dammed for irrigation storage and domestic use. Hetch Hetchy is the most familiar, but the canyon below Donnell Vista on Highway 108 has also been inundated. Still, the glacial heritage of Middle Fork of the Stanislaus is evident from the viewpoint. The steep canyon walls of granitic rock and the overall U-shape of the valleys are the result first of ancient river erosion and then modification by thick rivers of ice.

If you look at the second picture above, you can see some unusual looking mountain peaks. Their blocky flat aspect indicates they are composed of something different than the "expected" granitic rock. They are the remains of lava flows, ash flows and volcanic cones that once covered this part of the Sierra Nevada. Indeed, until 9 or 10 million years ago, the Sierra looked far more like today's Cascades Range than the lofty glacial peaks we see today. There were a number of snow-covered stratovolcanoes, but much of the remainder of the range was composed of lower hills. The upper reaches of Highway 108 where it crosses Sonora Pass cut through some of the volcanic rocks.

We stopped a mile or two short of the pass to get a detailed look at the granitic rocks. Depending on the relative proportions of plagioclase and orthoclase feldspar and quartz, rocks may be identified as granite, granodiorite, tonalite, diorite, or monzonite. The rock exposed just below the pass is called the granodiorite of Topaz Lake, dated about 89 to 83 million years ago, during the Cretaceous era. It was intruded in the deep crust about 4 or 5 miles down where it cooled slowly, forming visible crystals of feldspar, quartz and dark minerals like biotite mica and hornblende.

Glaciers scoured the surface of the granodiorite, polishing it and providing a nice view of the structure of the rocks. Some of the orthoclase (potassium feldspar) has formed huge blocky crystals easily visible in the shot below. Even better, during the intrusion process, blocks of the surrounding rock broke off and sank into the magma. Composed of minerals that had higher melting points, it didn't melt, but instead persisted as an alien mass in the granitic rock. Such inclusions are called xenoliths. They provide a peek at what existed here before the intrusion of the magmas.

We drove over Sonora Pass and headed into the barren lands beyond. Our destination was the site of a gold rush, but not the one that Californians are familiar with.

Geological Excursions in the Sonora Pass Region now available at Sunbelt!

Our field guide to the geology of the Sonora Pass region and the eastern Sierra Nevada region is now available for sale at Sunbelt Publishing for $24.95 (here is the link)! This was the roadguide for our recent meeting of the Far Western Section of the National Association of Geoscience Teachers. There are quite a few geological road guides out there for the eastern Sierra Nevada, especially around the Mono Lake area, but there have been fewer guides for the area to the north. It is a fascinating region, with widespread Miocene volcanism, faulting and Pleistocene glaciations (the largest glaciers of the eastern Sierra Nevada traversed the West Walker River gorge).

One of California's most intriguing ghost towns can be found at Bodie (above), and one can also find a strange "fluvial forest" in the West Walker River, a group of long-dead Ponderosa pines that provide evidence of a century-long mega drought only a thousand years ago (below).

The western slope of the Sierra along the Stanislaus River hides some geological treasures as well, including the Columns of the Giants and the Natural Bridges in the cave country north of Columbia State Park (below).

Here is the table of contents...

A Geographical Sketch of the Central Sierra Nevada
A Brief Overview of the Basement Rocks of the Central Sierra Nevada
Trip 1: The Sierra Crest Graben: A Miocene Walker Lane Pull-Apart in the Ancestral
Cascades Arc at Sonora Pass by Cathy Busby, Alice Koerner, Jeanette Hagan, and Graham Andrews at the University of California, Santa Barbara
Trip 2: A Guide to the Geology of the Eastern Sierra Nevada between Sonora Pass
and June Lake, California by Garry Hayes, Modesto Junior College
Trip 3: Geology and Climatology of the Saddlebag Lake Region near
Tioga Pass, CA by Laura and Ryan Hollister
Trip 4: Sword Lake Debris Flow by Jeff Tolhurst, Columbia College
Trip 5: Unique Geology along the Stanislaus River, Western Central Sierra Nevada by Noah Hughes, Modesto Junior College
 Appendix A: The Flora of Central California: Central Valley to the Great Basin by Mary Cook

A reminder: Sales of this guide will fund the scholarship program of the Far Western Section of the NAGT, which supports geology majors throughout California, Nevada and Hawaii. The book can also be purchased directly from the Far Western Section at this link. Check it out!

Just Published! Geological Excursions in the Sonora Pass Region of the Sierra Nevada (and cute furry animal)

The newest guide by the Far Western Section of the National Association of Geoscience Teachers is now available for sale! Geological Excursions in the Sonora Pass Region of the Sierra Nevada, edited by Noah Hughes and yours truly, is a series of field trips at Sonora Pass and along the eastern Sierra Nevada as far south as June Lake and Saddlebag Lake near Tioga Pass. Other excursions include significant geologic sites on the Stanislaus River, including the unique Natural Bridges near the town of Columbia.

This is a fascinating region that hasn't always received the attention it deserves. A great deal of recent research has been done on the Miocene volcanism in the region and how it relates to the uplift history of the Sierra Nevada and the development of the Sierra Nevada microplate. The region is part of the Walker Lane, which in all likelihood is the future margin of the North American Plate. There are ghost towns, saline lakes, ancient metamorphic rocks glacial deposits and a strange "fluvial forest" in the West Walker River. There is the strange "Reversed Creek" near June Lake.

Chapters in the book include the following:

A Geographical Sketch of the Central Sierra Nevada

A Brief Overview of the Basement Rocks of the Central Sierra Nevada

Trip 1: Sierra Crest Graben: A Miocene Walker Lane Pull-Apart in the Ancestral
Cascades Arc at Sonora Pass (by Cathy Busby, Alice Koerner, Jeanette Hagan, and Graham Andrews at the University of California, Santa Barbara)

Trip 2: A Guide to the Geology of the Eastern Sierra Nevada between Sonora Pass
and June Lake, California (by Garry Hayes, Modesto Junior College)

Trip 3: Geology and Climatology of the Saddlebag Lake Region near
Tioga Pass, CA (by Ryan Hollister of Turlock High School and Laura Hollister of Pitman High School)

Trip 4: Sword Lake Debris Flow (by Jeff Tolhurst, Columbia College)

Trip 5: Unique Geology along the Stanislaus River, Western Central Sierra Nevada (by Noah Hughes, Modesto Junior College)

Appendix A: The Flora of Central California: Central Valley to the Great Basin (by Mary Cook, Modesto Junior College)

The book was prepared for the Fall 2012 meeting of the Far Western Section of the National Association of Geoscience Teachers held last weekend at the High Sierra Institute at Baker Station below Sonora Pass. All proceeds from the sale of the book support scholarships for geology majors in California, Nevada and Hawaii (details of the scholarship can be found here).

At this time, the book can be ordered directly from the Far Western Section at http://nagt-fws.org/publications.html for $29.95 plus shipping and handling (checks only). The web page also includes dozens of other guides for geological tours all over California and Nevada. The guide will soon be available from Sunbelt Publishing, which published the volume (http://www.sunbeltbook.com/). The ISBN number is 978-0-9606704-4-4.

Photo by Ryan Hollister

We are excited to be able to offer this exploration of a fascinating region! If you are interested in seeing some unique landscapes and want to catch up with some new Sierra Nevada research, check it out (and help some worthy students advance their studies in geology at the same time).

Photo by Ryan Hollister

Here is the promised furry animal. The pika is a rodent adapted to living at the highest elevations in the Sierra Nevada. Their habitat is being affected by global warming. They can be seen near Sonora Pass and in the region above Saddlebag Lake.

Vagabonding Across the 39th Parallel: Clicking my heels, because....

...well, there's no place like home...

After a two week journey across the 39th parallel exploring the geology of Colorado, Utah, and Nevada, we were finally to the last stage, the last mountain range between us and home in the Central Valley. It was the Sierra Nevada of California

It's no small barrier: the Sierra Nevada is the largest single mountain range in the United States, and for 400 miles it rises as a nearly inaccessible wall of solid rock. It has been a barrier to human and animal travel for thousands of years. Along the highest part of the Sierra Crest, from Sequoia National Park to Yosemite National Park, a distance of  least 150 miles, only one throughgoing road crosses the crest, at Tioga Pass where we started our journey two weeks earlier. We crossed Sage Hen Summit, and the mountains came into view. Even though it was late July, snow still covered large parts of the high country. It had been that kind of year (compare to the extreme dry conditions we are seeing now).

The Sierra Nevada is such an extraordinary mountain range that it can dwarf other incredible mountains nearby. In any other region, a hundred mile long mountain range exceeding 14,000 feet in elevation would be world renowned. Instead, the White Mountains are barely known to most people, even in California. Likewise, a chain of thirty or so active volcanoes, some only a few hundred years old, would be getting far more attention in 2012 than good ol' Yellowstone. But they don't appear in the news very often, and not on the Discovery Channel either. As far as I can tell, there aren't even any conspiracy nuts weaving stories about UFOs and harmonic convergences at all.

We came across Highway 120 and wound our way around the north side of the Mono Craters (above), and stopped to enjoy a panorama of the Sierra Crest on the east boundary of Yosemite National Park. The prominent peaks in the picture below are Ritter and Banner, two spectacular mountains that were left outside the boundaries of Yosemite. I suspect the reason involved possible mineral sources in the metamorphic rocks (they are protected from development by their designation as a wilderness area).

A little further to the north we could see the Lyell Crest and the countryside near June Lake. The glaciers wreaked havoc with the topography there. In one spot the river flows towards the mountain range rather than away from it (by some incredible coincidence it is called Reversed Creek).

And then Mt. Dana, the second highest peak in Yosemite (below). We were close to our home territory, and it might have been quicker to go home the way we came two weeks earlier, but we still had a sense of curiosity about the other pass, Sonora, a little north of Yosemite National Park. We were tired, but we still wanted to explore...a little bit.

Something caught our eye in the foreground. There is almost nothing that can live on a surface of recently erupted pumice ash. Water simply percolates through and what little clay that forms in a few hundred years dries up within a few weeks of the last snows. But that was the kind of summer we had. Late July and spring was only beginning. The pumice flats east of the Mono Craters were alive with colorful wildflowers.

They certainly weren't tall. I was down on my hands and knees trying to get pictures of the common ones, like the magenta flowers above, that I assume is some kind of Monkeyflower (corrections are most welcome).

There were a few spindly lupines hanging on as well...

And some delicate yellow daisy types (again, any id is welcome!). We stopped for a sandwich in Lee Vining, and headed north on Highway 395, and then east in Highway 108 over Sonora Pass (9,624 feet). We stopped for a few minutes at the Leavitt Falls Overlook to see the snowmelt-swollen cascade pouring out of the hanging valley below the pass.

The road climbs steeply and becomes a narrow byway. If you have one of those RV's don't try this one! We passed meadows and waterfalls that had an air of familiarity. We were almost home. We had had a good trip, and seen a great many new places, and seen familiar places in a new way. But it was time to wrap things up.

What did we find? There's no place like home. Not the Dorothy's "We're not in Kansas anymore" kind of 'no place'. Literally: there is no place like home. One of the joys of teaching geology is the ability to say that you can never escape it (in a good sense, most of the time). Wherever you go there is a geological story that is distinct from everywhere else, and the story is almost invariably fascinating. We could have picked any line across the earth's surface, and we would have found something unique (but I must say the richness of the American West is unparalleled).

We crossed the pass and started down the canyon of the Stanislaus River. In two hours the vagabonds were relaxing once again in their own home. The dog and the cat were certainly happy to see us again. It was good to be back, but it wasn't a week before we wished we were on the road again.

For those of you who have been following this series, I hope you have enjoyed the journey!