Tuesday, April 30, 2013

Some Resources on Yosemite Valley and the Mother Lode of the Sierra Nevada

Because we all know I almost NEVER mention Yosemite or the Mother Lode on Geotripper....
It's been quite a long time since I first ventured onto the Internet, about 15 years ago. I quite clumsily put together a web page for my geology department, which included a couple of resources on the Sierra Nevada Mother Lode and Yosemite Valley. It turns out that my original web pages are due to disappear into the mists of time as the school has taken up a different web platform. If you want to see my messy first effort at a web presence, it will be around until May 6 at this site: (http://virtual.yosemite.cc.ca.us/ghayes/). Two of the pages that I thought most useful were a virtual roadside geology tour of Yosemite Valley, and a review of the Mining History and Geology of the Mother Lode.

The virtual roadside tour of Yosemite Valley has been updated and posted at Geotripper Images (see it by clicking here). I've added more than 100 photos to the tour, and noted some of the changes that have taken place in the last fifteen years since I first published the guide (a dam has been removed, and there have been a few serious rockfalls). If you are headed to Yosemite this summer, give it a look!

The Mining History and Geology of the Mother Lode (click here to see the new version) still needs work. I found out the website was slated to disappear right in the middle of the last week of school, right in the midst of a move to our new Science Community Center, and just weeks before leaving on a total of three weeks in the field, part of which involves writing a field guide for Grand Canyon, Bryce Canyon, and Zion National Parks. So I don't have too much on my plate or anything like that.

In any case the Mother Lode history needs some serious updating and I didn't have much time this week to do anything about it (I put it up in a hurry in 1998 and didn't update it all that much over the years). If you see some inaccuracies in the descriptions of mining, I'd appreciate some constructive criticism. There is plenty of history about the Gold Rush out there, but not so much in the way of geology. 

Saturday, April 27, 2013

The Other California: The Biggest Living Things and the Deepest Canyon in the U.S. (maybe, almost, perhaps)

Sequoia and Kings Canyon National Parks were established 50 years apart (1890 and 1940), and they preserve different aspects of the Sierra Nevada, but they are adjacent and as such are jointly administered by the Park Service. My Other California blog series is an attempt to spotlight the lesser known parts of our state that don't always show up on the postcards that tourists buy, but which have incredible geological features. As a pair of national parks, Sequoia and Kings Canyon don't seem to fit the bill, but I include them because the two parks are less visited, and yet have some of the most spectacular geological scenery to be seen anywhere. How many places can boast the highest peaks, the deepest canyons and the biggest living things in the world?

A bit of perspective on my claims, though. Sequoia National Park includes the highest peak in the lower 48 states, with Mt. Whitney (14,505 feet; 4,421 meters). Denali in Alaska is much higher, and Mauna Kea and Mauna Loa on the Big Island of Hawaii are the tallest mountains in the world (if you measure from the seafloor). Kings Canyon has a reasonably valid claim to being the deepest canyon in North America, but the deepest part of the canyon lies a few miles downstream of the park boundary (though it is partly protected as Giant Sequoia National Monument). But the biggest living things? Absolutely.
There has always been a bit of confusion about the Giant Sequoias (Sequoiadendron gigantea), because California has two gigantic tree species. The other is the Coast Redwood (Sequoia sempervirens), which grows in a narrow coastal corridor between Big Sur and the Oregon state line. The Coast Redwood grows to immense heights (nearly 400 feet), but is usually slimmer. The tallest Sequoia trees usually don't exceed 300 feet (the tallest is 311 feet), but the trunk is more robust to a high level, so the shorter trees have the greater bulk, making them the largest living thing on the planet. The state legislature was confused certainly, as they made the "native redwood" into the state tree without realizing the two trees were distinct. The attorney general of the state eventually got involved, making a final ruling declaring both species to be the state tree. Both tree species live for thousands of years, but neither is the oldest living thing in existence. That honor goes to the 5,000 year old Bristlecone Pine, also a California resident species.
The trees are tremendous. They occur in about 60 groves between about 4,600 and 7,000 feet along the western slope of the Sierra Nevada. Such giant trees would seem irresistible to loggers, and many were cut, but the wood was actually of low quality for most purposes and was usually made into pencils, shingles or grapevine stakes. Today nearly all the groves are protected in the national parks (Yosemite, Sequoia, and Kings Canyon), a national monument (Giant Sequoia), and a state park (Calaveras Big Trees).

The trees have a geological story. They once thrived across the northern hemisphere, and in North America fossils of the trees are preserved in Yellowstone and Petrified Forest National Parks (the direct ancestors are preserved in Nevada). The petrified trees in Yellowstone are several tens of millions of years old, but the trees in Petrified Forest are several hundred million years old! Ancestors to the Sequoia date at least to the Jurassic Period, so the trees were witness to the evolution of the dinosaurs and their extinction. Climate change seems to have been the tree's nemesis, and the Pleistocene ice ages probably eliminated them from most of their former range. The trees were able to survive in the Sierra Nevada in part because they could propagate downslope and upslope in response to the advancing and receding glaciers (see this National Park Service article for the details on the origin and distribution of the Sequoia trees).
I'm opening a can of worms by discussing the deepest canyon in North America. Hells Canyon on the Snake River along the Idaho-Oregon border is usually described as the deepest, but measurements vary, as well as the precise definition of canyon. I won't make a judgement other than to say that the two canyons are very close to being the deepest, only a few tens of feet apart, and that both of them are 2,000-3,000 feet deeper than the Grand Canyon (The Grand really is grand, though. According to the park service, with a volume of 4.17 trillion cubic meters, it is the largest canyon in the world). Spanish Peak, at just over 10,000 feet (in the photo below), looms 8,000 feet over the canyon bottom at Rough Creek.

How can this canyon be so deep? If you saw my last post, you would recall that the southern Sierra Nevada is topographically different than the northern Sierra. It is more a high plateau than a westward tilting block. The adjacent Central Valley is different as well. It's been sinking, so much so that some of the sedimentary fill has buried portions of the Sierra foothills. Strange things are happening in the southern Sierra, and it may be related to a process called delamination. The Sierra may have had a dense root of mantle material that was out of equilibrium with the surrounding hotter and slightly fluid mantle. The large mass broke away and sank deeper into the mantle. The overlying crustal rock, the Sierra, popped upwards like a ship losing an anchor. The Kings River, with an increased gradient, started cutting rapidly downwards within the last few million years. Spanish Mountain can be thought of a high secondary ridge like the Great Western Divide, but it was breached by the erosion of the Kings River.

What's ironic? The river that carved the deepest (or second deepest) canyon in North America doesn't flow into the sea. The Kings River historically flowed mostly into a large lake in the southern Central Valley (Tulare Lake) and evaporated away. Some distributary channels delivered water to the San Joaquin River (and then onto the Pacific Ocean), but today the river is fully utilized for irrigation, and even the lake is gone, replaced by agricultural fields.

When we visited a few weeks ago, the road into Kings Canyon was still closed. The problem isn't snow (there wasn't any to speak of). The canyon slopes are so steep and rocky that rockfalls are a continuing hazard when the ground is still saturated and subject to freezing conditions. We had to take a pass this time around, but we'll be back in the fall for a field studies class. Look for pictures around early October!

Oh, and there were some cute dogs hanging out at Grant Grove....

Friday, April 26, 2013

The Other California: Springtime along the Great Western Divide

Lupines along the Kaweah River gorge (Moro Rock and Alta Peak in the distance)
The Great Western Divide? Where's that?

My off and on blog series on the Other California is an exploration of the little-known places in my fair state with interesting, even fascinating geological features. Sequoia National Park might seem too familiar a place to be included as part of the "Other California", given that my own definition of the series is that it should include those places that don't normally show up on postcards, and Sequoia National Park certainly does.

So why include Sequoia? The primary reason is that it actually is less known than other parts of the Sierra Nevada. Ask folks where they go in the Sierra, and Yosemite Valley or Lake Tahoe are often the first places mentioned. And people often come to the park not so much for the geology, but for the biology, mainly to see the trees after which the park is named. But the park has a rich geological heritage as well.
Moro Rock from the Kaweah River gorge
The shape of the Sierra Nevada is often described as a tilted block of granitic rock and metamorphic rocks. That's true in the Sierra Nevada north of Sequoia. Driving to Yosemite National Park from the west involves a gradual climb up the western slope that continues to the Sierra Crest before dropping precipitously into the Owens Valley or Mono Lake on the east. Sequoia and the Southern Sierra Nevada is quite different. When one arrives from the west (which actually is the only way in which the park can be approached by roads), the road climbs steeply to a plateau, and the slope continues to a high crest. But not of the Sierra Nevada. It's an entirely separate mountain ridge called the Great Western Divide. The actual crest of the Sierra Nevada lies farther east, across the deep gorge of the Big Arroyo and the Kern River.

The Great Western Divide is a spectacular mountain range. It rises nearly to the height of the actual Sierra Crest, with several peaks exceeding 12,000 feet in elevation (A dozen or so peaks on the Sierra Crest reach 14,000 feet). Despite being far to the south, the peaks were high enough to be scoured by the glaciers of the Pleistocene Ice Ages.

A few weeks back we had the chance to pay an early springtime visit to Sequoia National Park. We came in from the west, up Highway 198 along the Kaweah River through the town of Three Rivers, and then up the Generals Highway to the Sequoia groves for which the park is justly famous. The road is notably curvy, and climbs through a rugged canyon choked with giant boulders that have tumbled from the cliffs above.

We soon passed one of those kitschy things that Civilian Conservation Corps workers in the 1930s seemed especially fond of constructing: a drive-through rock (to go along with "drive-through" trees). They enlarged the opening under the immense boulder, and put the road through it.
If one wonders why the road today circumvents the rock tunnel, one need only look at the underside of the boulder. People driving in scenic national parks are not known for paying close attention to their driving, and in a time of massive recreational vehicles, this kind of thing just doesn't cut it anymore.
A hat tip to the arrival of spring in the Sierra Nevada...the redbuds were in full bloom up the canyon, providing a splash of vivid color on the dark green slopes.
Although not as colorful as redbud, the ceanothus shrubs added a wonderful fragrance to the air. The bees and other pollinating insects were in heaven.
As we went further up the canyon, Moro Rock loomed ever higher above us. The granite rock of the dome expanded as the rock was exposed at the surface. The plutonic rock tended to fracture parallel to the surface, which had the effect of removing corners and edges from the rock outcrop, eventually leading to the formation of the dramatic dome. The process is called exfoliation. It would be so cool to climb the dome, but we knew that most of the park access roads would be closed because of the winter snowpack.
Except that they weren't. When we reached Giant Forest, there were a few snow patches here and there, but the snowpack is currently at half the normal level. All the roads were open, so we headed over to the Moro Rock trailhead. As dramatic as the dome is, trail access is easy because the CCC put in a stairwell to the summit. Easy that is, if you don't have problems climbing 300-400 steps. The views in the clear spring air were stunning.

Immediately across the Kaweah Gorge were the Castle Rocks (9,000+). The spires and towers of granitic rock exhibit the other result of rock expansion: jointing. When the fractures that result from pressure release are vertical, they allow water to get into the narrow spaces. If the water freezes it expands, wedging the rock apart. The water also aids in the chemical weathering of the rock, so as time goes on the cracks widen, forming the prominent spires.
Moro Rock is best known for the wonderful perspective it provides on the Great Western Divide. The mountains are often hidden from view by other high ridges or thick forest, but Moro Rock stands out from the mountainside, and the view is tremendous.

With the covering of winter snow, one can imagine the glaciers that carved the horns, aretes, and cirques that are so well exposed here. Cirques are the bowl shaped basins on upper ridges where the glaciers accumulated. Aretes are the knife-edged ridges that divide glacially carved valley, and horns are the sharp pointed peaks that result when glaciers pluck rocks from the base of the cliffs in the cirques and aretes. The Matterhorn in the Alps is a familiar example, but there are many horns to choose from on the Great Western Divide.
I have not yet had the privilege, but the High Sierra Trail winds its way from Crescent Meadow in Sequoia to the summit of Mt. Whitney and the Whitney Portal trailhead. It is 60+ miles long, and crosses two major passes, and it must be a marvelous adventure. It is second only to the John Muir/Pacific Crest Trail in popularity.

The picture below illustrates the difference between glacial erosion (the cirques and horns in the upper part of the photo), and the weathering and exfoliation that happens at the lower elevations (the domes both right and left of center).
Moro Rock also provides a wonderful view west towards the Sierra Nevada foothills and the usually invisible Central Valley. You are looking at the most polluted air in the United States: Bakersfield and Fresno. It's not entirely their fault, as they don't necessarily produce more pollution per capita, but the towns are surrounded by high mountains, so they can't blow their pollution into someone else's area the way other municipalities are able to. The photo below is as clear as I've ever seen it from Moro Rock.

Next, we took a look at some trees with a unique geologic history...

Monday, April 22, 2013

Thoughts for Earth Day 2013: There is a price to pay

Andrew Alden at About Geology notes today that he hosted an Accretionary Wedge on Earth Day in 2008. I had only been blogging a few months at the time, but I contributed, and in reading over my post from back then, I see that things haven't changed all that much. In just two months, I'll have the chance to return to my favorite spot on the planet at Cedar Mesa, and hopefully it will look as pristine as it did five years ago. Thanks, Andrew, for the reminder!

Here is my post from April, 2008:

This month's Accretionary Wedge (Accretionary Wedge #8 ) is hosted by Andrew Alden at About Geology, and the entries involve our responses to Earth Day. Mine is a bit late in coming; I reward myself with a blog entry if I get my other work done, and this has been finals week. It's been a bit hectic!

I offer today a picture of my most sacred spot on the planet (so far): the edge of Cedar Mesa at Muley Point. The cliff below the rocks drops 800 feet straight down to a flat plain which is then carved by the San Juan River into intricate canyons some 1,200 feet deeper still. Monument Valley and the Raplee Anticline lie in the distance, as well as the towns of Cortez CO and Farmington NM. It is a precious place to me, full of mystery, beauty and solitude. Ghosts of the Ancestral Pueblo people lurk here, and the fossils of Permian reptiles as well.

But...hidden in the bottom of the deep canyon is the greatest upstream extent of the artifical evaporation pond of Lake Powell. On a 1964 topographic map of the National Recreational Area, there is a notation at Muley Point: "slated for development". In the distance, at Mexican Hat, oil wells pump the black liquid from the ground, and around Farmington, a GoogleEarth view reveals hundreds or thousands of gas wells. Coal is mined from Black Mesa, off to the south, and evaporite minerals are torn from the ground to the north of Canyonlands National Park. Power lines criss-cross the region. My favorite place is under siege.

Of course, we need all these things to live, but the point of my entry today is this: there is a price to be paid. The price takes many forms, from high prices on commodities, in foul air, polluted rivers, extinct plants and animals, and in the almost never recognized loss of the wild places of our planet, the gauntlet in which our ancestors survived and thrived. We have lost touch with the earth that gave us our birth, and which continues to nurture us, despite our abuse. And our abusive ways are about to come to an end, one way or another: we will finally destroy the last of the wild places, drill the last drops of oil and shovel the last lumps of coal, we will melt the last glaciers, and deplete the last soils. Or, we will choose not to do these things, and exist on our planet in a new way: a sustainable existence that finds a way to give something back to our planet.

The environmental movement on our planet has been demonized, trivialized, and marginalized, because, I suppose, it has always threatened the perceived profits of somebody. What have environmental groups tried to do since the hey-day of the 1970's and the first Earth Day? A short list might include:
  • Increase mileage standards and encourage the use of mass-transit
  • Encourage the development of alternative energy resources
  • Decrease emissions from our vehicles, including greenhouse gases and ozone destroying compounds
  • Encouraged laws to protect our water, air and soil
  • Protect the wild places that still remain on our planet
Environmentalists have been the visionaries and the prophets for our planet. They have seen the things we do today to abuse our planet, and offer an alternative path for the future, one based on sustainability. Such ideas run counter to the profit motive of particular industries, and thus environmentalists are attacked as elitists and flakes, while the money continues to flow into the coffers of the energy companies and the developers. But the bills for all of us are coming due...

The oil is running out, and thus the price spikes. We will never see cheap oil again. The mass conversion of agricultural fields to the growth of biofuels is causing grain prices to spike, and we are becoming less and less able to feed the hungriest people on the planet (Malthus is in the air; "Running Out of Planet to Exploit," ). The prices of metals are climbing.

Change is possible, and I sometimes see hopeful signs, and part of my optimism comes from Earth Day, and the works of good people to bring awareness to those who are waking up to the spectre of high prices and resource limits. We have a choice though...we can let the decisions about the future to be made by energy companies and their political lackeys, or we can demand a future based on sustainability. It will take education, and an end to the corporate media's obsession with Britney and Paris, and kidnapped white women, and American Flag Lapels. People, when given the right information, can make the right choices.

Those are my thoughts this week. You are welcome to comment!

Saturday, April 20, 2013

How it Was Today: Reflections in Yosemite Valley

What is my favorite season in Yosemite? Hard to say, but I tend to think that it is whatever season it is when I happen to be there. Except maybe for hot dusty August. I just had the happy opportunity to spend two straight weekends in the valley introducing my students to the fascinating geology to be found there.
 And a few living things as well. Can anyone tell me what kind of butterfly this is?
The spring melt, very unfortunately, is coming early this year. We had a great beginning in our precipitation totals through December, but the skies suddenly dried up, and we had the driest January to March precipitation on record, with a paltry 26% of normal. The early snows are gone, and the few additional snowdrifts are melting rapidly. But there is water in the valley right now, and the falls were full and booming. It was a beautiful day.
Iconic Half Dome was nicely reflected from a pond in the Cooks Meadow area. Half Dome, or more correctly Three-Quarters Dome, stood high above the icefields that filled Yosemite during the ice ages, and achieved its shape from exfoliation and jointing.
Sentinel Dome is sometimes missed, as it lies on the opposite side of the valley from Yosemite Falls. It is a spectacular edifice that would be the focus of a national park if it existed any place besides Yosemite Valley.
We went to some of the usual places. From Tunnel View we had a perspective of Half Dome from near the spot where the valley was first discovered by Americans of European descent. It must have been a stunning sight in 1851, although the significance of the awesome valley was lost on most of the militia members, who were more intent on capturing Miwok Indians. The medic for the crew, Lafayette Bunnell, was deeply impressed and his writings were part of the inspiration for making Yosemite a national Park. The Cathedral Rocks were sharply etched into the cloudless sky, and Bridalveil Fall was full. Thousands of people see this every day. What was it like to be the first person to see it, whether in 1851, or 4,000-5,000 years ago?
In springtime, Yosemite is graced with a multitude of incredible waterfalls, some of which don't last into the summer. One example is Ribbon Fall, which at 1,612 feet is actually higher than Upper Yosemite Falls (1,430 feet). It is just west of El Capitan and is easily viewed by visitors at Bridalveil Falls across the valley. It will probably be dry in a few short weeks, which is why it is less known.
Yosemite Falls was nothing short of spectacular. With a total drop of 2,425 feet, it is the fifth or sixth highest waterfall in the world. Its roar could be heard all over the center of Yosemite Valley.
I hiked to the lower falls as well. At 320 feet, it is dwarfed by the others, but a trail ascends quite close to the base, and it both wet and exhilarating to draw close. I had chosen to walk to the upper end of the valley, realizing that I could get there almost as fast as the tramways (lots of tram stops getting from one end of the valley to the other).
 The dogwoods were starting to bloom out too. It's one of the nicest signs of spring.
There aren't many other flowers on the valley floor yet, but our drive through the foothills afforded a great deal of color. There were white lupines....
 And a small magenta flower I didn't recognize. Can anyone identify it?
 Blue lupine could be seen all over the place...
And finally, a robin posed for me.
And that's how it was today.

Friday, April 19, 2013

Interested in Earth History? This is where you should learn about it...June 15-29

North Rim of Grand Canyon National Park (yes, Gaelyn, we'll be on the North Rim, hope to see you there!)
There is no place on this planet like the Colorado Plateau. It's hard to find anyplace else on Earth where the crust remained relatively stable for upwards of a billion years, accumulating several miles of horizontal sediments, only to be lifted up rapidly in the last few million. The Colorado River and her tributaries then stripped away much of the sedimentary cover, and cut deep into the underlying metamorphic rocks, which record a violent geologic history of colliding landmasses and mountain-building. The resulting landscape is one of the most beautiful places imaginable.
Angels Landing Trail in Zion National Park, Utah
The plateau country is a training ground for geologists and earth scientists, and has been since the days of John Wesley Powell and Joseph Ives, who were the first to lead research parties into the region (they didn't "discover" the plateau, of course; Native Americans have known the region for thousands of years). If you are curious about learning geology in this incredible region, you might consider joining us as a student (of any age) on our geology field studies course Geology 174, offered under the auspices of Modesto Junior College in Modesto, California.
Goosenecks of the San Juan River, Utah
Our field course will be a grand loop through the plateau country, with investigations of the Mojave National Scenic Preserve, Grand Canyon, Zion, Bryce, Arches, Canyonlands, Capitol Reef, Black Canyon of the Gunnison, Mesa Verde and Great Basin National Parks, as well as many monuments, including Natural Bridges, Navajo, Hovenweap, Colorado, and state parks like Kodachrome Basin, Goblin Valley, and Berlin-Ichthyosaur. It will be an unforgettable two week trip from June 15-29, beginning and ending in Modesto. Information can be found at my school website at  http://hayesg.faculty.mjc.edu/Geology_174_Colorado_Plateau.html.
Black Canyon of the Gunnison National Park in Colorado
It's not a comfortable trip...we travel in school vans (which of course are known for their luxuriousness!), we camp every night, and the days can be hot, windy, cold, stormy, and we are out in the middle of anything that happens. But we are staying in beautiful places each night, and there are even showers and laundry available every third day or so! Extensive hiking is not required, but there will be many chances to explore the parks and monuments that we are visiting.
Double Arch in Arches National Park in Utah
Geology 174 is a 3 semester unit course. By end of the course, you will be able to see the landscape the way geologists do: by identifying rocks, minerals and fossils, and interpreting the geological history of an area by working out the sequence of events as exposed in outcrops. If you are a science teacher, you will come home with a collection of photographs that illustrate most of the important principles of geology, and a selection of rocks, minerals and fossils that will make a great classroom teaching tool (legally collected, of course; there are many localities outside of protected parks from which to collect samples).
Canyonlands National Park, Utah
The cost of the trip is $650.00 plus the cost of tuition (Currently $46 per unit for California residents, and $222 per unit for out-of-state residents). The cost includes transportation, food, camp fees, and entrance fees. Participants would want to bring a few dollars along for showers, laundry, and souvenirs.  The food is tasty and plentiful (everyone helps cook and clean!), and the school vans...are vans.

For those of you who live in the Modesto region, we are having an organizational meeting on Monday, April 22 at 7PM in Science 132 of the East Campus at Modesto Junior College. Attendance is not mandatory (and not binding, either), but will be a chance to learn more about the trip and the class. If you can't make it to the meeting, I will send the class materials to you.
Antelope Canyon, Navajo Nation Tribal Park
If you are not in the area, we will be glad to arrange for transportation from nearby airports and train stations (we actually have an Amtrak station in town). Enrollment can be completed online once you are registered with the college (http://www.mjc.edu/index.html). Please contact me through the class website if you have any questions.
Bryce Canyon National Park
Hope to see you out there, back of beyond!

Wednesday, April 17, 2013

Accretionary Wedge #56: Because Every Picture of the Earth Tells a Great Story

Andrew Alden at About Geology is hosting this month's Accretionary Wedge, and the topic is landscape topography through the eyes of a geologist:  

Once upon a time, you took a picture of something that lots of people photograph. However, because you are a geologist, it didn’t turn out the way it does for most people. Show us that picture, tell us what you see in it, and tell us about the way you take pictures.

No matter where you go, no matter where you are, there is a story to be told by the rocks. And ultimately, no matter how dull the scene might appear, the story is fascinating. These are the first words that I utter in every class I teach, and I cannot look at a landscape, any landscape, without wondering what lies beneath. And sometimes the scenes are far from dull.

To many, the story to be told by a picture may be "I was standing in front of some scenery". Few geologists can ever see a landscape that way. The picture above is one of my favorites, because such a rich history is encapsulated in the small frame of the photograph.

We are standing in a small arch located in Arches National Park in Utah, which actually happens to be called Frame Arch. The view extends across the fault graben of Salt Creek Valley to the distant La Sal Mountains. The reddish rock is the Entrada Formation, a sandstone deposit that developed in the tidal zone of a coastline in Jurassic time. Dinosaurs once climbed coastal dunes in this spot where I stood snapping a picture.

The rocks are tilted because there is a huge body of salt beneath the surface. Water trickling down into the fissures and joints dissolved away the salt near the surface, and the Entrada layers collapsed into the void.

The La Sal Mountains in the distance are volcanic, or to be more correct, laccolithic. The high peaks are made of the more resistant rock that squeezed between and inflated the space between sedimentary layers. Later on, the rocks were lifted up and the overlying layers were eroded away. The eroded mountains are the hearts of ancient volcanoes.

How did this photo "turn out different"? It's because I was more or less ignoring the reason this opening in the rock was called Frame Arch...just to the left in the distance, you can see the slightest opening to what may be the most famous arch in the world, Delicate Arch. It's the symbol of the park, and almost everyone climbs up to Frame Arch uses it to frame Delicate (below). But the geologist's eye takes in more than icons.
Thanks, Andrew, for hosting the Accretionary Wedge this month!

Sunday, April 14, 2013

A Day for Dam Big Rocks

Some of my students have a penchant for misusing a certain expletive when they want to describe something related to reservoirs. I occasionally take advantage of oral presentations by referring to "dam engineers" in reference to some of the big mistakes made in reservoir engineering (i.e. Vaiont Reservoir in Italy and St. Francis Dam in Southern California). So I can't resist a bit more today.

I was out in the field again today, taking my students to Yosemite National Park, looking at some dam big rocks (DBRs). The first two DBRs were in the Merced River gorge near Elephant Rock. They are but two pieces of a very large rock fall that took place in 1982. The material in the slide mass (termed the Cookie Slide) totaled around 100,000 cubic meters. The slide closed highway 140 for months, and permanently closed the old Coulterville Road that once existed on the upper slope. I guess I could call these DBRs because if the rock mass had been any larger, it would have dammed the Merced River.
Just the same, I found myself wondering how such giant boulders could have survived the fall down the steep slope without breaking into small pieces.

Speaking of DBRs not breaking up, take a look at Leaning Tower near Bridalveil Fall in the picture below. The rock actually leans more than 200 feet over the valley floor. What the heck is holding up that dam rock?
And finally there is the biggest dam rock of all, El Capitan. That's a heck of a big blank wall of granite, nearly 3,000 feet of sheer rock. Can I make a case that these are DBRs too? Sure: when glaciers were scouring out Yosemite Valley, primarily during the Pre-Tahoe (Sherwin glaciation) about 800,000-1,000,000 years ago, the ice couldn't make much headway downward through hard ridge of granite on the valley floor between El Capitan and the Cathedral Rocks. This ridge, only about 300 feet beneath the valley floor, prevented the glaciers from cutting the valley any deeper, and thus served as a sort of dam.
Oh, but it was a pretty day for learning some geology in one of the most beautiful places on the planet. More later...