A Beautiful and Precious Day: How it was today in Yosemite

OK, I'm waxing a little poetic today, but let me give you some background. Got to sleep late last night in the middle of a rainstorm, got up miserably early to see the rain continuing, and a check of the weather radar said it was still raining up in Yosemite National Park, which was the destination of our class field trip today. Drove to school during the gloom, gathered up the students into the bus, and we drove off into the rain. I've often said that Yosemite has a uniqueness that transcends bad weather, but not when I'm dragging a bunch of not-necessarily enthusiastic sleepy students.

And that's why today was precious...
By the time we reached Yosemite Valley early in the afternoon, the clouds were breaking up, and we were witness to a stunning view of rain-washed cliffs and roaring waterfalls. In many Octobers, the valley is dry and dusty, tired after a summer onslaught of millions of tourists. Above, North Dome and Washington Column are framed by bright blue sky. North Dome is a classic example of exfoliation weathering: as the rock was exposed by erosion, it expanded outward, cracking and forming slabs that ultimately slid off. The effect was to remove corners and edges.

Half Dome spent much of the day with a crown of clouds, but by afternoon it joined the skyline. Half Dome would have been best named Four-Fifths Dome, but I prefer the Native American name: Tis-sa-ack. Like North Dome, Half Dome was never covered by glacial ice, but instead was shaped by exfoliation. A prominent joint on the north face was quarried by glacial ice flowing along the base.

The rain this month was more than normal, and the rivers have started flowing early. Yosemite Falls looked more like early summer than the middle of fall. The falls are a classic example of a glacial hanging valley. Lower Yosemite Falls drop 318 feet almost to the valley floor. They are easily accessible by way of a short trail. Although it is not at all recommended, and people are constantly getting injured, climbing around on the falls is a common occurrence. At least they provide a sense of scale in the photo above.

From the Wawona Tunnel exit, the view is almost always indescribable, and I can never resist taking a bunch of photos. I try to bring out different perspectives, and today Half Dome was obscured by clouds, and shadows accentuated the flatness of the valley floor. I shot a bit to the right, removing El Capitan from the picture as well. Sentinel Rock and the Cathedral Rocks become the dominant cliffs from this perspective.

I had a chance to walk from Yosemite Falls to Happy Isles, a traverse of about 4 miles on the Valley Loop Trail. It was amazingly bereft of people, to the extent that I actually started to worry a little about mountain lions lurking atop the giant boulders that line the trail. The river crossings showed how much water was moving through the valley. Tenaya Creek (above) is usually almost dry.

My poor students, forced as they were to attend a required class field trip, dutifully took notes, and pretended to be interested. I think they had some fun while they were learning, though. For myself, I felt privileged to be in such a beautiful and stunning place...

From hell's heart I stab at thee: Accretionary Wedge #28

"...to the last I grapple with thee; from hell's heart I stab at thee..."

The Accretionary Wedge challenge for the month is to show off a favorite "deskcrop", those beautiful and significant rocks and minerals that have won a coveted position on the desk of a geologist/teacher. The convergence of the wedge with Halloween added a layer of complexity by asking for spooky items. Well, I have a lot of deskcrops on my desk, so many that there is precious little room for books and office supplies and that sort of thing. It was hard to choose.

That "spooky" thing made it even harder, but what is Halloween about anyway? The day was co-opted by the Christian religion from earlier Celtic traditions, but the upshot of the cultural intermingling of meanings is that on All Saint's Day Eve there are spirits from the underworld set loose on Earth. So I looked for the deskcrop that best exemplifies the underworld, and I realized it could be nothing else but the walls of hell itself: olivine dunite xenoliths from the deep mantle, brought to the surface of the Earth by volcanic eruptions.

Olivine is a beautiful mineral, one of my favorites, with a glassy luster and bright green and yellow color which varies due to a substitution of iron and magnesium in the crystal structure. It is also known as the gemstone peridot, the August birthstone. The streets of Heaven are paved with gold, but the walls of hell are apparently lined with gemstones! The mineral is relatively unstable at the Earth's surface, and readily alters to serpentine when exposed to water in the upper crust.

As for the quote, it comes to us from Moby Dick by Herman Melville, although it was also a memorable quote from the movie Star Trek II: The Wrath of Khan (and I hate to admit that I first knew of it from the movie).

Thanks to Research at a Snail's Pace for hosting the Wedge this month!

Liken' Lichens a Lot Lately...Fall Report from the Sierra Foothills

It was a beautiful sparkling clear day following yesterday's storm that dumped 3-4 inches of rain on many parts of Central and Northern California, so we decided to do our first fall reconnaissance in the Sierra Nevada foothills. We drove up Marshes Flat Road between LaGrange and Moccasin and found that fall really hasn't arrived yet at the lower elevations, just a bit of yellowing among the sycamores.
We did stop for a few moments to look over a rejuvenated brook, and I took a closer look at the rock surfaces, which consisted mostly of metamorphic greenstone and ultramafic serpentinite. My attention was drawn to the lichens on the rock surface, and I took some macro images. The lichens occupy a strange world we don't see all that often. They live on a different scale at the interface between rock and atmosphere. The acids they produce are one of the elements in the weathering and chemical breakdown of exposed bedrock.

Lichens are a complex symbiosis of algae and fungi. There are probably 500 different species in nearby Yosemite National Park, though only 100 have been so far cataloged. I have really only just begun to study these fascinating life forms closely.

Marshes Flat Road is a nice quiet avenue that crosses the high ridge east of Highway 49 between Coulterville and Chineses Camp. Paved, but mostly 1 1/2 lanes, it offers some nice views and excellent wildflower displays in the springtime.

It's an entirely different region, but I am also offering my favorite shot of a lichen, a display from a basalt boulder in the Mojave Desert Scenic Preserve in southern California. The colors are stunning to me. Lichens survive in a lot of tough environments!

A Quiz on Earth History: Can You Pass? A Comment on Science Education

Which is older, the dark dike, or the lighter sedimentary rocks?

How would you do on this test (taken from a chapter in Carlson, Plummer and Hammersly's excellent Physical Geology: Earth Revealed textbook)?

1. “ Geological processes operating at present are the same processes that have operated in the past” is the principle of a. correlation b. catastrophism c. uniformitarianism d. none of the preceding

2. “ Within a sequence of undisturbed sedimentary rocks, the layers get younger going from bottom to top” is the principle of a. original horizontality b. superposition c. crosscutting d. none of the preceding

3. If rock A cuts across rock B, then rock A is rock B. a. younger than b. the same age as c. older than

4. Which is a method of correlation? a. physical continuity b. similarity of rock types c. fossils d. all of the preceding

5. Eras are subdivided into a. periods b. eons c. ages d. epochs

6. Periods are subdivided into a. eras b. epochs c. ages d. time zones

7. Which division of geologic time was the longest? a. Precambrian b. Paleozoic c. Mesozoic d. Cenozoic

8. Which is a useful radioactive decay scheme? a. 238U-206Pb b. 235U-207Pb c. 40K-40Ar d. 87Rb-87Sr e. all of the preceding

9. C-14 dating can be used on all of the following except a. wood b. shell c. the Dead Sea Scrolls d. granite e. bone

10. Concentrations of radon are highest in areas where the bedrock is a. granite b. gneiss c. limestone d. black shale e. phosphate-rich rock f. all of the preceding

11. Which is not a type of unconformity? a. disconformity b. angular unconformity c. nonconformity d. triconformity

12. A geologist could use the principle of inclusion to determine the relative age of a. fossils b. metamorphism c. shale layers d. xenoliths

13. The oldest abundant fossils of complex multicellular life with shells and other hard parts date from the a. Precambrian b. Paleozoic c. Mesozoic d. Cenozoic

14. A contact between parallel sedimentary rock that records missing geologic time is a. a disconformity b. an angular unconformity c. a nonconformity d. a sedimentary contact

If you have a degree in geology, these questions on earth history should give you no problem; they represent basic principles in the science. If you are a student in a basic geology class, they would be challenging, but with a bit of study, you should get most of them right. And if given as an open-book test with no time limit, they should be no problem at all...except if you are a student in my distance learning class. They barely break 50% most of the time. It isn't that they are bad students; they do fine on most of the other chapters. It mystifies me why they do badly on this one chapter, year after year, but I suspect the reason lies in the student's previous K-12 education.

Earth history and evolution have always been a required part of the primary and secondary curriculum, especially in California, but I get the feeling they don't get a strong emphasis in the classroom, perhaps out of fear of controversy from creationist parents, or due to the beliefs of the classroom teachers themselves. Because we end up not teaching our students why science accepts the evidence for an ancient Earth, students are left with statements like "scientists believe the Earth is millions of years old" as if it were a 50-50 choice. It's this idea of belief in scientific findings that has brought us to this dismal moment in our country's history when we can't mobilize to fight global warming because politicians and their followers choose to believe it isn't happening. Because people like Rush Limbaugh, Glen Beck and Senator Jim Inhofe are accepted as climate experts. They aren't; they are appallingly ignorant or devastatingly cynical (or both).

There was an interesting moment last weekend at our Wild Planet Day celebration, though. A father was showing his daughter our skeleton of the sabertooth cat. She wasn't much more than 7 years old, but he said to her, "is this creature millions of years old, or thousands?" I kind of sat back, waiting for the assumed explanation of how scientists are wrong and that the earth is only 6,000 years old. But, to my surprise, he said "the dinosaurs lived millions of years ago, but the mammals like this sabertooth lived thousands of years ago". A small but satisfying moment to be sure. And too rare these days.

If you haven't had a class in geology, don't feel bad if you don't know the answers. I've listed them in the comments.

What do you think about the earth science education our children are getting these days? Am I totally off base?

Wild Planet Day at MJC: Science is COOL!

I get reminded every so often that the problems we have with science education in this country are not because kids aren't interested in science. It's maybe true that there is some sort of hormonal imbalance between about 6th grade and the end of high school that interferes with the natural curiousity that kids have about science, but even kids suffering from such afflicitions can sometimes be reached by intense therapy programs like NKLI ("No Kid Left Inside").

That's what made today an especially gratifying experience. Unknowingly coinciding with the birthday of planet Earth, our school held a Wild Planet Day Celebration as a fundraiser for our local natural history museum. This was the first time we had ever done such a thing, and no one knew what to expect: would families in our economically depressed region be willing to pay an admission fee to see exhibits of wild animals, programs in a portable inflated planetarium, hands on physics experiments and so on? Our anxiety was not helped by the gloomy skies that threatened rain. We set up and waited to see what would happen.

We were happily surprised by the response! It seems that there is a hunger for good science in our community, and families showed up in droves to enjoy a day of dropping eggs, petting farm animals, blowing giant bubbles, designing egg preservation contraptions (dropping them off a stairwell), building bird boxes, and meeting live bats. I heard that more than 500 people showed up.

In our little geology corner, we debuted our new museum specimen to the community, the sabertooth cat skeleton that arrived last April. We offered a very messy stream table excercise that was a hit with the kids, and laid out hundreds and hundreds of surplus rock, fossil and mineral specimens that we encouraged the kids (and their parents, as it turned out) to take home. The kids concentrated on their sample choices like they were in a candy shop, and they asked one question after another about their little discoveries

In a time when we have so much ignorance about science in the society at large, and among politicians in particular (especially the ones who determine school funding priorities), it was refreshing, even inspiring to see such enthusiasm and excitement about science, even on a gloomy, rainy day. It was a great day!

A Friday Mystery Photo from Kings Canyon National Park

No, I don't have endless supplies of mystery photos, but there are still a few in the files. This one is from our recent meeting of the Far Western Section of the National Association of Geoscience Teachers. We were at Road's End in the South Fork of the Kings Canyon looking at glacial moraines, when several of the group noticed the odd outcrop on the cliff above.

The rock is probably granodiorite (Lookout Peak or Paradise Pluton), and there are glacial moraines on the valley floor below. So what is going on here? Why the linear patterns on the rock face? Why the pockmarks? What do you think?

Oh, and there's a small secret in the comments...

Gotthard Base Tunnel Breakthrough: Why Go Through a Mountain When You Can Go Over It?

I've been kind of inundated by work, so I missed a few stories this week, but the news of the milestone in the construction of the world's longest train tunnel through Switzerland's Gotthard Pass caught my eye (see the rather incredible tunnel video at the bottom of this post). It's quite an accomplishment, digging a 35 mile-long hole through the Alps, to replace a much older 9 mile-long railway tunnel (dug in the 1880's at a cost of 200+ lives). There is also an exceedingly long auto tunnel (10.5 miles) that was completed in 1980. It is the third longest auto tunnel in the world.

I admit that tunnels fascinate me and I enjoy the concept of driving through a mountain, but I have noticed that they are dark, and the rocks are usually covered by concrete. I found going through the Chunnel to be kind of anticlimatic, for instance. A few years ago, I had the opportunity to make the journey from Italy through Ticino to Lake Lucerne, and our route called for the bus to go through the Gotthard Pass Tunnel. I looked at the maps, and saw that there were actually two highways that go over Gotthard Pass (6,950 feet). We were on a geology tour, so we politely requested (okay, we begged) for the bus to take the high road instead of the tunnel. We did, and it was spectacular!
The photo above is the view from near the entrance to the big tunnels. Our highway can be seen switchbacking up the slope of the peaks beyond. Gotthard Pass has been in use for centuries, with foot traffic beginning in the 1200's, and the first carriage crossing in 1775. Part of the cobblestone highway can be seen snaking up the mountain in the picture below.
Meanwhile, our highway also climbed higher and higher, and the views just got better. It was my first journey through the Alps, and I was mostly impressed. The one big complaint is that almost all the landscape was marked by human developments of one sort or another: railways and highways, power lines, avalanche control measures. Still, the mountains were beautiful, and we were so happy not to be driving through a dark tunnel. We stopped at the top of the pass, and were surprised to see a rock shop (the "Krystallgrotte") perched on the high cliffs! We explored a bit, and started down the other side. We soon realized why they wanted a tunnel. The road winds through a number of deep gorges, and passes over the Devils Bridge (the Teufelsbrucke), which allowed passage over a particularly dangerous part of the Reuss River in the Schollenen Gorge. According to the legends, the Devil offered to build the original bridge, as long as the first one over the bridge would give up their soul. The local villagers thought about it, agreed to the deal, and then made sure the first one over the bridge was...a goat. The Devil was a bit irate, and picked up a huge boulder to smash the bridge, but was confronted by a cross-bearing old lady. He fled...
I can understand the desire of the Swiss to shorten the trip across the Alps, especially when hauling goods, but there is certainly a lot to be learned from following the old highways. It was a stunning day that we could have missed entirely.

The Other California Goes Underground: Hella Hot Helictites at Black Chasm Cave

The guesses regarding the odd features in Wednesday's "Whatsit?" were all over the place, including a great story of the albino cave tarantulas, but a number of people remembered their obscure cave decorations and called them by their correct name: helictites. We were in Black Chasm Cavern near the towns of Volcano and Pine Grove in the Sierra Nevada Mother Lode. The state of California has a number of caverns offering tours, in the Sierra, the Klamath Mountains, and the Mojave Desert, but I find that Black Chasm, though small, is also one of the most spectacular. A big reason is the preponderance and variety of helictites that can be seen in the inner chamber.
Helictites are one of the stranger cave decorations because they seem to (and indeed do) defy gravity. Stalactites and Stalagmites form as water drips from the ceiling of a cave and splashes on the floor. Stalactites grow downwards from the top while stalagmites grow upwards from the floor. They occasionally grow big enough to link up, forming a column. In a nutshell, helictites are stalactites on drugs. Their origin is not clearly understood, but is thought to be related to water pressure and capillary action which can operate in opposition to gravity. Imagine a stalactite with water dripping form the the tip, but with water being squeezed outwards to the side rather than dripping straight down. The calcium carbonate precipitates out in an uneven manner, and the helictite grows in odd directions. Cave winds have also been suggested as an origin, but many caves that have helictites do not have strong wind currents.
Black Chasm has been granted National Landmark status because of the beautiful helictites that cover several walls. Why are they here in such abundance? One reason is that helictites are so delicate that they rarely survive the early discovery of a cave. Cave visitors in the 1800's and early 1900's tended to take cave decorations as souvenirs, and helictites were the easiest to break off (whether on purpose, or by accident). Black Chasm was discovered in the middle 1800's, and the outer rooms suffered a lot of damage, but there was a true chasm in the cave interior that kept out casual visitors. It is nearly 100 feet deep, and required ropes and climbing skills to cross. When the cave was developed in the 1990's, a stairway was constructed that allowed easy access to the inner chamber, but the operators of the cave have done a good job of keeping vandals from the delicate decorations.
Another reason I like the cave so much is that it is strategically lit to emphasize the best of the other speleothems (cave decorations), especially some beautiful drapery formations (above). Early cave explorers used smoky torches for light and were inclined to touch and muddy most of the accessible cave features. Through heavy visitation and abuse, most tourist caves have dirt covering the speleothems. This is not the case in Black Chasm, and the operators have made the most of it, by showing the beautiful translucence of the thin filmy draperies.

Caves are fragile, irreplaceable, and worthy of our protection. I wrote about Black Chasm last year, and ended with the following:

If you are interested in exploring wild caves, preservation and protection is the highest priority. As such, you should get in touch with the local grotto of the National Speleological Society if you want to be involved in protecting this special resource. If you have followed my blog for any period of time, you will know that I think that cave vandals are one of the lowest and most moronic forms of humanity in existence.

A Whatsit for a Wednesday: The Other California Goes Underground

Ohmigod! What is that coming out of that wall?! Heads for scale (they are standing about 10 feet from the wall). What is going on here? As always, first prize is a smirk of arrogant satisfaction, and extra points for outlandishness....we are still in California's Mother Lode region, as in the last few posts.

The Other California: It's a Real Grind...Chaw'se State Historical Park

It didn't take some folks very long to recognize Saturday's Mystery Photo...if you've been there, it's kind of hard to forget. The slab of rock at Chaw'se Grinding Rock State Historical Park has more than 1,000 mortar holes, and is one of only two places in the state where petroglyphs are found in close association (below).

The site is of great cultural significance to the local Miwok people and other Native American groups and there is an excellent museum at the park, as well as reconstructions of typical village architecture. The park even provides opportunities for camping in the structures; it must be a unique experience!

Our interest in visiting was geological; it was a field studies course after all. The slab of rock containing the mortar holes is not composed of granite, as many people might expect, being as we were the Sierra Nevada. As it turned out, we didn't see a single outcrop of granite during our journey between Chaw'se and Columbia State Historical Park 45 miles to the south. We were following the Sierra Nevada Metamorphic Belt which contains the Mother Lode that was the source of much of California's gold. Gold doesn't occur much in granitic rock, at least not in the Sierra.

We were traversing a stretch of the Calaveras Complex, a sequence of Permian-Late Triassic rocks that formed in a subduction zone, where the oceanic crust of the Pacific Ocean basin was forced beneath the western edge of the North American Continent. The rocks were churned up into a mess of slate and chert called a melange. The melange includes huge blocks of limestone (marble, actually), greenstone, and other metamorphic rocks that can be several miles across. It is one of the marble blocks that provided the huge slab that served as the base for the grinding stones.

Marble, like limestone, is composed of calcium carbonate (the mineral calcite), which many people associate with cavern development. Indeed, our previous stop that day was a tour of a beautiful little cave only two miles away. The region is pretty well honeycombed with dozens of caves that just beg for exploration (contact the local speleological society if you are interested in learning more about it). Pictures of our tour of the cave will be coming soon.

The Other California is my continuing series of the lesser known corners of my beautiful state, and the unique geology that can be found there. I don't know the entire state as well as I wish, and would welcome a guest blog on one of your favorite hidden places!

More Local Modesto News: Why We Need Wild Earth Day (and there's my new office!)

Here's the reason we need a fundraiser like Wild Planet Day: we are building a wonderful Community Science Center that will house a planetarium, an observatory, and a brand new, expanded Great Valley Museum. The building will house the Geology, Earth Science, Chemistry, Physics and Biology departments for Modesto Junior College as well. This facility reflects our local community pride, as it was funded by a local bond rather than the state or national stimulus spending. I will be occasionally posting updates on the progress of the building, which we expect to occupy in 2012. I can see my office at the northeast corner, although, being on the third floor, it is kind of hard to imagine so far (picture angle is towards the northeast).

The round foundation will be the planetarium, which will be the only one in a forty mile radius. Our community has been working for this since the 1970's...

Our Wild Earth Day is not a fundraiser to build the building, but to fund the exhibits and expenses of having a quality museum with programs to increase awareness of science and environmental issues in our region. If you are in the Modesto area, please come on October 23rd!

For My Modesto Area Readers: Wild Planet Day is October 23rd!

"Wild Planet Day" Science Fest to be held at MJC

It will be the Earth's birthday, at least according to Bishop Ussher...if you are in the Modesto region, come and check it out. The MJC Geology Club will be giving away free pieces of the earth and showing off fossils and other good stuff. It's for a good cause, raising money for our local natural history center, the Great Valley Museum.

From the press release:

The Modesto Junior College Great Valley Museum and Science, Mathematics and Engineering Division are hosting a family oriented Science Festival with the theme “Wild Planet Day” on Saturday, October 23 from 10 a.m. to 3 p.m. The event will take place in and around the Science Building on the corner of College and Coldwell Avenues on East Campus. Admission for the day is $8 per person or $25 per family, with no admission charged for children under 3 years old.

This fun-filled fest will include live animal presentations, hands-on science labs, telescope and planetarium viewing and geocaching, a popular hide-and-seek game using global positioning system (GPS) devices. Participants can observe Steve Sutton’s Reptile and Amphibian Show, a presentation by the Bat Conservancy or wild animals from the Stanislaus Wildlife Care Center. There will be squid, pumpkin carving and 4-H animals. Science enthusiasts can enjoy fascinating hands-on science labs including DNA extraction and make-your-own-lung.

The event is a fundraiser for the Great Valley Museum, a non-profit foundation dedicated to providing science and natural history information to adults and children of all ages through classes, programs and exhibits. The museum serves the families of Stanislaus, San Joaquin, and surrounding counties. For more information on the Science Fest call (209) 575-6196.

Video of Yosemite Rockfall from...a Unique Angle

When I noted the rockfall off the wall of El Capitan last Monday, I had no idea one of my former students was climbing on the cliff nearby. She and her partner had a video camera and filmed the rockfall from one of the most unique of vantage points, from on the cliff itself. This is a Facebook link, so I don't know if it will work for everyone, but check it out; it is spectacular!

http://www.facebook.com/video/video.php?v=443539259734

A Saturday Night Mystery Photo

Back home from today's field study course in the western Sierra Nevada. The rock is marble of the late Paleozoic Calaveras Complex...what's going on here??

Balancing Boulders and Erratic Rocks!

Sunday's mystery photo produced a pretty good response of hypotheses, both of the scientific sort, but also of the fanciful sort (and sometimes the boundary gets blurred!). The rock in question is known as Globe Rock, and it can be seen along the Sierra Vista Scenic Byway, an 85-mile long loop road in the high forest country between Yosemite and Kings Canyon National Parks. The area is drained by the San Joaquin River. The elevation is about 7,000 feet.
As noted on the geologic map by N. King Huber of the Shuteye Peak quadrangle (USGS Geological Quadrangle GQ-728; get all of the central Sierra Nevada geologic maps here), the rock underlying the Globe is the Mt. Givens granodiorite, a medium to coarse-grained hornblende-biotite granodiorite and granite that is locally porphyritic with tabular K-spar phenocrysts with a U-Pb age of about 90 million years (my casual non-geologists readers are forgiven for not following this, but my students had better understand it; there's a midterm Tuesday on this stuff!). The rock is labeled on the geologic map, near the top and to the left.

A lot of the answers to my challenge on Sunday noted the possibility that this was a glacial erratic sitting on a pedestal. This is the most likely answer based on my cursory examination, although the possibility must be considered that it may also be an unusually weathered exfoliation slab (in other words, the rock formed in place, not transported from elsewhere). There are glacial deposits shown on Huber's map only a mile or so away, and the larger pre-Tahoe glaciers may have covered much of the region. In either case, the pillar would have developed because the large boulder protected the underlying rock from weathering, while the surrounding rock was eroded away. In this setting, the amount of erosion suggests that the boulder has been there for a very long time. The odd spherical shape of the boulder itself would also be an indicator of age; corners and edges tend to be weathered away first.

Glacial erratics are very often composed of a different kind of rock, and the black coloring on the boulder certainly causes the rock to appear to have a different composition, but a close look at the base of the boulder (above) shows the rocks to be actually quite similar, and they are both certainly granitic. The dark color is provided by dark oxides and lichens growing on the surface. Even if the rocks are the same, it quite probably could still be an erratic, as the Mt. Givens granodiorite is widely exposed in the region.

Rocks on pedestals are quite interesting for a number of reasons, not the least of which is their contribution to the scenery. If they can be tied to a particular glacial episode, they can be a clue to the rate of erosion in a region. If they are truly precarious, they provide some data about the likelihood of severe shaking during earthquakes. If they haven't toppled in the thousands of years they have existed in a particular spot, it would suggest a limited degree of ground movement (although I have to wonder...this rock is close to major faults in the eastern Sierra Nevada).

The other rock I photographed this weekend illustrates what probably amounts to a younger pedestal boulder, on the flanks of Sentinel Dome in Yosemite National Park. It is less rounded, and has not developed as high a pedestal. This could be a function of age, although differences in composition and local soil conditions could also be a factor. This site is described as being outside the limits of the Sierra glaciations, so it is either a rockfall 'erratic', or it is part of a slab that formed in place.

Balanced rocks are found in many different circumstances and many different places. One of the commenters provided a link to a balanced boulder in Kansas, and I've included a shot of the famous Balanced Rock at Arches National Park in Utah.

What other balanced rocks do you know about? Got any pictures?

It's Fossil Day during Earth Science Week!

It's the first ever National Fossil Day, and I feel inclined to repost my blog entry of March 27, 2008...
This is another of those maybe not so fascinating pictures, but it was kind of exciting for me. Off in the Sierra Nevada foothills for a class field trip last weekend (see Picture of the Day - Sierra Nevada Foothills ) we had a bit of extra time, and we made an unscheduled serendipity stop in a section of the Mariposa Slate along the Melones fault zone at Don Pedro reservoir. I've not kept up with the latest paleontology of the metamorphic belt of the western Sierra Nevada, and mostly I remember statements in the old guidebooks that the fossils are rather precious and rare, due to the effects of heat and pressure on the former oceanic sediments.

Just the same, one of the recent guidebooks mentioned that some sparsely distributed fossils might be found here, so we stopped and gave it a shot. As it turned out, the only person to find anything at all was yours truly, the instructor, but I was quite thrilled to be able to pick out a few pelecypods from the Mesozoic-aged metasediments. It was the first time I have ever found any in these mountains.

Still, even though no one else found anything, my students seemed to find my enthusiasm catching, as they gathered around to see the small discovery. It is this kind of moment that makes teaching geology so satisfying. After the last few days of discussion about high starting pay in the petroleum and mining industries (Get Rich Being a Geologist! But...? ), it is good to remember that some kinds of work have other rewards. After teaching for twenty years, and committing to huge overloads every semester, I almost make what some undergrads are apparently being offered these days for starting pay. But I wouldn't trade a minute of my career for the money, and I look forward to doing it for the next 20 years (if they let me!). I love seeing the moment when one of my students sees Yosemite Valley or the Grand Canyon for the first time, and I love to hear the squeals of excitement from a bunch of cynical adults find a shark tooth or a fragment of dinosaur bone for the first time in their lives.

Thinking about teaching for a career? There is a lot to be said for enriching the lives of others!

Just a Regular Day on Sentinel Dome

Just a picture today, of an entirely normal day hike up Sentinel Dome on the brink of Yosemite Valley. Well, ok, not an ordinary day. My wife and I got to witness the largest rock fall in Yosemite this year, and then as we made our way down the granite dome, we saw a huge bird (I presume a Redtailed Hawk; I'm open to corrections) perched in a tree at the edge of the cliff. It seemed rather unconcerned about our presence, and I guess was waiting for an unwary chipmunk to come wandering about. I've photographed hawks before, but not from twenty feet away. It was a beautiful creature!

El Capitan Area Rockfall Probably Biggest in Yosemite Valley This Year

A follow-up to yesterday's post about the rock fall in Yosemite Valley: the rockfall is still being analyzed, but according to Greg Stock, Yosemite's park geologist, the event yesterday near Horse Tail Falls is probably the largest mass wasting event in the valley this year. They are using a LiDAR scan to calculate the size of the chunk of rock that fell. A preliminary report on the fall on a climber's site has a much better picture of the event itself, and a before/after shot of the release point. There were a total of three rock falls, a small event at about 11:30 AM, a much larger fall about three minutes later, and another smaller fall at about 1:00 PM. Thanks, Greg, for the info.

More updates as they become available! The scar and impact zones can be seen in the photo above.

Update #1:
Greg Stock, park geologist reports:

Several large rock falls occurred from near the East Buttress of El Capitanon October 11, 2010. The first rock fall occurred around 11:30 am, and was followed about three minutes later by a much larger rock fall. A third smaller fall occurred at around 1 pm.

All three rock falls detached about halfway up the far eastern side of the southeast face of El Capitan, roughly along the path that Horsetail Falls takes when flowing. Rock debris hit a prominent ledge beneath the cliff and fragmented into smaller boulders, producing substantial dust; the dust cloud produced by the second impact was visible throughout Yosemite Valley. Boulders did reach the base of the cliff, but did not impact any trails or roads. Although there were many climbers on El Capitan at the time, there do not appear to have been any injuries associated with these rock falls. However, climbers are cautioned that future rock falls from this area arepossible.

Geologists are still investigating these events and are mapping the size of the failures in detail, but preliminary estimates suggest the volume exceeded 1000 cubic meters, making this the largest rock fall thus far in 2010...

...More information is also available on the Yosemite NPS website:
href="http://www.nps.gov/yose/naturescience/rockfall.htm">http://www.nps.gov/yose/naturescience/rockfall.htm

Also, an excellent shot of the dust cloud is posted on Planet Mountain: http://www.planetmountain.com/english/News/shownews1.lasso?l=2&keyid=37636#

Rock Fall Near El Capitan in Yosemite Today

Your on-the-spot geological reporter was on the scene and on the job today when a rock fall occurred on the cliff just east of El Capitan in Yosemite Valley. Well, ok, I was almost on the scene, and almost on the job...to be honest, I was near the top of Sentinel Dome on the south rim of the valley, eh, dozing on the smooth granite slope. I was listening to what I thought was an odd-sounding jet off in the distance, but when the sounds abruptly stopped, I thought "rockfall!". I ran to the edge of the cliff just in time to see the dust rising from the tumbling boulders.

There has been little in the way of official information as of yet, but it seems to have originated in the vicinity of the cliff at Horse Tail Falls, the high vertical wall just east of the main cliff of El Capitan. I only had time for a cursory examination, but I think the impact point is the white rock on the cliff just above the tips of the trees.

There are white streaks on the dark gray cliffs above the white rock that may be impact scars(click on the photo below for a larger image), and lots of fresh looking boulders on the slope below. I can't tell if trees were snapped off or not. I would appreciate any reports or additional from climbers, hikers, or anyone who was in the meadow below. I will gladly post any pictures, too!

Once a big chunk of rock breaks off a cliff, nearby rocks may be destabilized, and subsequent rock falls are possible. Clearly the climbers know this...