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.

A Series of Fortunate Events: Climbing a Dome That's Not Exactly a Dome...It's Rock Mutton

There are domes and there are "domes". Yosemite National Park has a lot of domes of one sort or another, and there has always been a bit of confusion about their origin. From various points in Yosemite Valley, one can spy at least four them, famous Half Dome, less famous Sentinel Dome, North Dome, and Basket Dome. Although Yosemite is famous as a work of glacial action, the domes actually are not directly related to glacial erosion. As described in the previous post on Sentinel, these domes rose above the glacial ice. They formed instead because of exfoliation, the tendency of slabs of granitic rock to break off the corners and edges of monolithic chunks of exposed rock. In time they take their rounded shape.

Image from http://villains.wikia.com/wiki/Monstro

In the high alpine parts of Yosemite, the situation is different. Tuolumne Meadows is not a "U"-shaped glacial valley. It has an open aspect, with high mountain ridges separated by wide meadowlands dotted with granite monoliths that rise above the dark forests. The granitic knobs look less like rounded domes, and more like Monstro the Whale breaching at sea, with a gentle slope on one side, and nearly vertical cliffs on the other. Two of the most obvious are Lembert and Pothole Domes. Both rocks are popular hiking destinations, and both provide stunning views of the high country of Yosemite. Pothole Dome was our destination on this trip.

The open country of Tuolumne Meadows resulted from a different kind of glaciation. In Yosemite Valley, the glaciers only occupied the valley itself. At Tuolumne, the glaciers covered the entire landscape apart from the highest peaks. The depth of the ice exceeded 2,000 feet (~700 meters). Such glaciers are called icecaps or icefields, and they produce a different set of erosional features. Pothole and Lembert Domes are examples of Rock Mutton.

Oh, excuse my French! The domes are examples of a roche moutonnée. This has been translated variably as greasy mutton wig rocks, fleecy rock, or rock sheep, and none of these is a particularly apt description. The first was related to the sheep grease that was used to hold the hair in place in the old wigs worn by French and English dignitaries. The fleece referred to the "resemblance" of these rocks to the locks of hair in the aforementioned wigs, and the last was said to be a description of the rocks appearing as grazing sheep from distant vistas. I don't quite get it, but c'est la vie.

Whatever you might want to call it, Pothole Dome provides an intimate understanding of what rock is like when subjected to intense glaciation. The gentle slope, which faced the oncoming ice flows, has been smoothed and abraded by rocks, sand, silt embedded in the ice. The passage of the gritty ice has left grooves, striations, and polished surfaces that make climbing the dome a great deal easier than one might expect. Most of the huffing and puffing comes from the high altitude (8,000 feet/~2,700 meters) rather than steepness.

The steeper western side of Pothole Dome was "plucked". Ice formed in cracks and fissures of the granitic rock and was pulled loose by the glacier as it flowed away from the rock. This made for a decidedly asymmetrical dome structure if one wants to call it a dome at all.

An alternate term for these rocks is a stoss and lee structure. "Stoss" comes from a German term for "push" or "thrust", referring to the gentle slope facing the flow of ice (the glacier "pushes" up the slope). The "lee" refers to the trailing steeper plucked side. I tend to prefer this description, although it still isn't all that descriptive. How about a "scour and pluck" structure?

The other unmistakable evidence of the passage of glaciers is the sheer numbers of boulders that lie scattered across the surface of the dome. There aren't many good explanations for how these boulders could have gotten here without invoking the passage of ice (the boulders came from a different kind of granite exposed off to the east). These out of place rocks are called glacial erratics.

There are marvelous examples of glacial polish and striations all over the surface of Pothole Dome. The ice was last here around 12,000 years ago, so the rough surfaces surrounding the polished sections provide an idea of how slowly the granite is weathered in this cold alpine climate. At lower elevations, the rock would weather much faster and the polish would rapidly disappear.

From the top of the dome, one can see some of the terrain that stuck out above the icecap, including the Cathedral Range (below). The peaks are classed as horns when glaciers flowed away from the mountain, plucking away at the flanks, or nunateks when the glaciers flowed around the peaks, scouring away at the higher slopes.

Pothole Dome's name reflects the presence of potholes on the south flank of the dome. These round holes formed when ice melted into the glacier through holes and tubes called moulins. The water flowing under the ice mixed with boulders and gravel in large swirling masses to grind out the holes.

The final glacial feature we noticed while climbing down were the chattermarks, concentric fractures caused when boulders being dragged at the base of the glacier skipped and chipped the surface. Usually the cusps of the crescents point away from the ice flow direction, so I found these a bit mysterious. The crescents open toward the east, presumably the direction the ice came from.

The day was passing and we had a few stops left on the itinerary. They'll be covered next time!

Out of the Desert and Back to the Green Hills of Home

Today wraps up the story of a week-long journey through the deserts of the southwest. I've been telling the story as if in real time, so it sounds like I'm just now arriving home, but actually we finished the trip just over a month ago. It's a time warp of sorts. I'm about to leave on another trip, this time to British Columbia and Alberta.

Coming home. You can live in a boring place, a place that is the worst in the country even, but coming home is coming home. There is the familiar kitchen, the television, the well-worn couch. The weeds. The office. To me, home is something bigger. It's the landscape that surrounds my home region. Basically those places within a day's drive that I've seen over and over. My home starts at the summit of Tioga Pass in Yosemite National Park. It's a three hour drive from my house, but a trip doesn't really begin until we've gone over the summit, and on the return trip we feel home the moment we get there.

Compared to the dry lands we had just explored, the mountains have a richness of moisture. It's not nearly what it should be, for we are still in the grip of a horrendous drought, but some late spring snow was still lingering in the high places. We felt such a relief at the sound of babbling creeks.

Tuolumne Meadows is the accessible part of the Sierra Nevada crest in Yosemite National Park. The meadows lie at 8,000 feet, and the surrounding mountains reach elevations of just over 13,000 feet. This region was the source of glaciers that scoured the deep U-shaped valleys of Yosemite and Hetch Hetchy. At one time there was an ice cap at Tuolumne that was more than 2,000 feet thick that spilled out in several directions, including Tenaya Creek which holds beautiful Tenaya Lake (top picture).

John Muir talked about the Sierra Nevada high country as the "gentle wilderness", and that's been my experience most of the time. I've seen violent storms across the Basin and Range and Colorado Plateau, and some have been terrifying because we were camped out there and had nowhere to hide or take shelter. If a storm blows in while I'm exploring Yosemite, well, we just jump in the car and go home. We can come back the next day.

We had already driven three hundred miles this day, so the sun was getting close to the horizon. Photographers say that evening and morning are the best times for good shots, given the nice contrasting shadows instead of the washed out colors from a noonday sun.

There was a storm over the Basin and Range that we had just skirted. We got a few drops, almost enough to turn on the wipers. From a distance it looked menacing. But all was calm for us in the meadow.

We made a last stop at Olmsted Point for a look at the sunset on Half Dome. We then headed down the hill, and returned to our home. It was nice to be back, but of course a few days of sloth and we wanted to get on the road again. We are restless souls sometimes!

"Fly-over Country" Moniker Doesn't Do this Land Justice: Crossing the Basin and Range

The Basin and Range province is one of the largest geological provinces in North America, covering all of Nevada, parts of eastern California, half of Utah, and large parts of Arizona. It is also one of the least populated, at least outside of the urban centers of Las Vegas, Phoenix, Reno, and Salt Lake City. The region was stretched in late Cenozoic time (during the last 30 million years) so that the crust broke apart and gigantic blocks slipped against each other like shelves of books tipping over. Several hundred horsts (ranges) and grabens (fault valleys) are the defining features of the province.

We were on our way to an exploration of the Colorado Plateau, but needed to make our way to Las Vegas to meet our group. We could have taken the freeways through the Central Valley and Mojave Desert, but I've been down that road several hundred times over the last quarter century. We elected to cross the Sierra Nevada at Tioga Pass and cross the high ranges and deep basins of the region referred to by many as "fly-over" country. It's barren on some ways, but it is a treasure in so many others. How many places will you visit in your life where you have a decent chance of being the only human being on a particular mountain range? That can happen out here.

I could be accused of false advertising in showing these pictures of the alpine scenery of Yosemite National Park, but the presence of the Sierra Nevada is what forms the character of the Basin and Range. Many consider the Sierra to be the westernmost of the Basin and Range mountains, and the high 400 mile long mountain crest blocks the majority of precipitation from ever reaching the region. During the ice ages, the glaciers of the Sierra had a profound effect on the province, primarily in the formation of vast freshwater lakes in some of the driest places on the planet, Death Valley being an outstanding example. It once contained a lake 100-plus miles long and 600 feet deep.

A few of the lakes persist, barely. After we crossed Tioga Pass, we had an outstanding view of Mono Lake, which is huge, but it was once much larger and deeper. Evaporation (and water diversions by Los Angeles) have left behind saline water in which only small fairy shrimp and brine flies can survive. Not a complex ecosystem, but millions of birds depend on the rich protein.

Those birds include species like the gull below, which I suspect of supplementing its diet with scraps of sandwiches from the Whoa Nellie Deli!

The White Mountains, the next major range east of the Sierra Nevada, are spectacular, with a high point of 14,252 feet at White Mountain Peak, and several others exceeding 13,000 feet. The north end of the range extends into Nevada where Boundary Peak forms the highest point in the state of Nevada at 13,147 feet. We headed south along the east flank of the mountains in Fish Springs Valley.

The valley supports some ranching and alfalfa fields, but it's also very dry. We saw some immense dust devils that looked a bit like tornadoes.

We climbed to Lida Summit and looked back across one mountain range after another to the Palisades Crest in the Sierra Nevada.

A number of wildflowers were in evidence, including many Prickly Poppies and Globe Mallows. It's dry country, but lots of plants have adapted to the harsh conditions. The extreme range of elevations from 14,000 feet to below sea level have caused the province to have a stunning number of plant species.

We headed down from Lida Summit and started south towards Las Vegas on Highway 95.

As we rolled slowly (lots of speed traps) through the town of Beatty, Nevada, we encountered dozens of wild Burros. They are the descendants of animals abandoned by the early miners, and they have become a pest in many ways, contributing to overgrazing, trampling rare plants, and fouling springs, but darn it, they're also terribly cute. In hopes of controlling the populations of these creatures the Bureau of Land Management captures as many as they can and puts them up for adoption.

We made to Sin City, and today we met our fellow travelers for the coming week. We had a great day touring the geological environs of Las Vegas, and pictures will undoubtedly appear soon here on Geotripper!

The Right Place at the Wrong Time: A January Sunset at Olmsted Point

Yesterday I saw sights I never thought I would see: Tioga Pass in mid-January, short of snow but full of incredible beauty. There was so much to see in few hours I had, not the least of which was the sunset. We drove west from the summit of Tioga Pass back down through Tuolumne Meadows and Tenaya Lake.

As we saw in yesterday's post, Tenaya Lake is covered with a thick sheet of ice. As the sun approached the horizon, the colors on the peaks above intensified. Being able to walk out onto the lake allows for unique photo angles that are more difficult during the summer.

We drove to Olmsted Point to watch the evening drama play itself out. I climbed up on the smooth granite slopes above the parking lot and snapped pictures in several directions. For those who have only seen Yosemite from the valley floor, Olmsted is an incredible sight. To the south, Half Dome rises above the ridge. Yosemite Valley is hidden deep in the shadows below the ridge.

The dominant mountain to the south is Clouds Rest, the highest peak visible from the floor of Yosemite Valley. It is a massive piece of granite that has been scoured by repeated snow avalanches, forming the unique chutes visible from Olmsted.

To the north is one of the most famous views of Yosemite, Tenaya Lake with the backdrop of Mt. Conness on the Sierra Crest. The glacial geology is a bit unusual here. Tenaya Lake's canyon barely has any kind of creek, much less a river. It doesn't seem like a big enough drainage to have had glaciers, but glaciers it had. It turns out that nearby Tuolumne Meadows was covered by a massive ice cap more than 2,000 feet thick. Most of the ice flowed down the canyon of the Tuolumne River, but some of the ice flowed over the low divide into the drainage of Tenaya Creek which flows into Yosemite Valley. The consequence is that Tenaya Creek had a much larger glacier than it otherwise would have had. It was Tuolumne River ice that carved a major portion of Yosemite Valley on the Merced River.

During the summer, the sun sets more to the northwest. Today, just a few weeks after the winter solstice, the sun was setting far to the southwest, and whatever light and shadows are present at that time are quite different from what one would see in the summer. I'll just have to come back up next summer and see how different things are at the opposite solstice (any excuse works for me!).

 The color deepened, and I took another set of images as the sun settled into the horizon...

It was a real privilege to be in the High Sierra in the midst of January. With the lack of snow, I worry about our drought and the fires that are sure to come this summer, but today it was strange and wonderful.

The Right Place at the Wrong Time: A January Day at Tioga Pass

If anyone had told me anytime in all the years that I have lived here in Central California that I would be where I was today, I would not have believed them. It just doesn't happen. Tioga Pass in Yosemite National Park always closes in November or early December. Always (ok, in 2000 it was open for one day in January). Everyone in the Sierra knows that the pass closes and no one gets to go up there except for the hardy cross-country skiers until May or June. But that's the kind of year we are having here in California; water managers should be terrified. The snowpack is at 15% of normal, and after today, I'm saying "What snowpack?"

So off we went today, to see what we could see. There have been few storms yet this year or last, but it is plenty cold. Cold enough that Tenaya Lake and all the other high country lakes and ponds are completely frozen over. If you click on the photo above, you'll see ice-skaters out on the lake. It is a strange sight to see; frozen lakes, but no snow to speak of on the surrounding peaks.

Being a Californian born and bred, I admit to not knowing much about the dynamics of lakes freezing over, but I am pretty sure that these are pressure ridges caused by expansion of the surface of the lake as it freezes. It was a fascinating thing to see.

Oddly enough, the temperatures were in the forties and fifties, and I wore a short-sleeved shirt all day. We drove past Tenaya Lake and made our way to Tuolumne Meadows. Again, there was ice everywhere, but no snow other than scattered patches in the forest. The moon rose over Cathedral Peak (or is that Unicorn?)

Tuolumne Meadows was pretty as always, maybe even more so. I was happy to be able to visit in January, an absolutely new experience for me. I just worry about what this means for our water supply this year. These are Texas kinds of numbers, drought-wise, and no storms are in the long-term forecast.

We stopped at the top of Tioga Pass. On January 5. It was beautiful. And weird. Standing at 10,000 feet in the dead of winter and not dying of hypothermia. The last time my ancestors did something like this, very bad things happened (hint: my ancestors included the Donner family...look it up if you don't know the story).

Heck, we came this far, why not cross the Sierra? We headed a few miles down the road towards Lee Vining. We were tempted to hit the Whoa Nellie Deli for dinner, but we realized that an incredible sunset might be developing. We turned around and headed back towards Tioga Pass, but not before snapping a picture of Ellery Lake (also completely frozen over).

We stopped to walk on the Dana Fork of the Tuolumne River (below). The silence of the frozen landscape was eerie. No water was flowing anywhere.

 A lenticular cloud formed over the Sierra Crest. Or was it a UFO?

One more oddity. We did have an unusual storm earlier this year. It involved no rain or snow, but it did include hurricane force winds. Driving down from Tioga Pass I noticed hundreds of Lodgepole Pines that had been literally ripped from the ground. They were all pointing west. Again, an eerie day.

We headed back to Tenaya Lake and Olmsted Point to see how sunsets differ when the sun is setting far to the south of where it sets in summer or fall. Look for it in my next post.