If These Cliffs Could Talk: Leaning Tower and Bridalveil Fall in Yosemite

Bridalveil Fall and Leaning Tower on Sunday afternoon

Nature abhors cliffs as a general rule. Gravity dictates that cliffs will be a rarity in most parts of the world, because most rocks are simply not capable of resisting the intense stresses the cliffs cause. The cliffs of Yosemite are thus extraordinary, because they are among the highest in the world. The reason, of course, is because they are composed of extraordinary rock.

The subterranean world beneath an active volcano is a place from which the imagery of hell has been drawn. Seething hot magma pulses upwards through the crust, invading every crack and fissure, wedging rocks apart, and occasionally breaking through and producing catastrophic volcanic eruptions. But eventually the volcanic system becomes inactive, and the magma chambers eventually, and slowly, cool down. The silicate-based chemical stew starts to crystallize, and the interlocking crystals produce extremely strong rocks with names like granite, diorite, tonalite, and granodiorite. The rocks vary by (and are defined by) the percentages of quartz, plagioclase feldspar and orthoclase feldspar. Other accessory minerals, the dark ones like hornblende or biotite mica, give the resulting rock the characteristic speckled appearance. In a general way, the granite is lighter in overall appearance while the others are somewhat darker. These are the kinds of rocks that can stand in high vertical cliffs.

But other things have to happen to make cliffs. Normal river erosion tends to produce slopes, not sheer cliffs. Yosemite Valley was once a deep river-cut canyon, but the slopes would have been unremarkable, steeply sloping, but probably covered with talus (rocky debris) and vegetation. The lower Merced River canyon (below) provides a hint of how it might have looked: deep and rugged, but not the sheer cliffs of Yosemite. One of the factors is ice.

Conditions in the Sierra Nevada changed starting about two million years ago. The climate worldwide went through a series of cooling periods, and ice sheets covered much of North America and Europe. The climate would warm up again, the ice would recede, and then cooling would return. Perhaps a dozen ice ages took place, the most recent ending only around 12,000 years ago.

The ice did not cover the entire Sierra Nevada, nor did it cover the tops of the high peaks along the crest. The glaciers were confined for the most part to the valley floors, where the ice plucked boulders and abraded surfaces, eventually producing U-shaped valleys. There is direct evidence of at least three glacial episodes that scoured Yosemite Valley, the Pre-Tahoe (somewhere around 800,000 to 1 million years ago), the Tahoe (several pulses between about 160,000-65,000 years ago), and the Tioga (about 13,000-20,000 years ago).

And yet ice needs some help when attacking such hard rock. There have to be fractures and fissures for the ice to exploit before serious quarrying can take place. And that is where jointing and exfoliation come into the picture.

Intersecting joint sets in Tokopah Valley, Sequoia National Park

Granitic rocks cool at depths of several miles within the crust. Pressure is high at such depths, and so as the rocks are lifted towards the surface and exposed by erosion, they expand and fracture. Fractures that occur parallel to the surface are called exfoliation sheets. More or less vertical fractures, which may form from subtle extensional or compressional forces, are called joints. Joint sets can form in several directions as seen in the images above and below.

Intersecting joint sets in Joshua Tree National Park, southern California

Closely jointed rocks are readily excavated by glaciers, while more widely jointed rocks are not. Yosemite Valley, with ten different intrusions of granitic rock, exhibits wide variation in susceptibility to erosion, so that some units tend to form recessed slopes (i.e. the Diorite of the Rockslides), while other rocks tend to form bold cliffs such as El Capitan (the El Capitan granite).

I'm back from my first Yosemite trips in a number of  months, so I would like to share a couple of cliffs with you in the next couple of posts! I was going to call this mini-series "If These Walls Could Talk", but a short search revealed that naturalists at Yosemite are already using that title! So, I've modified it just a bit.

The cliff of the day is the one that extends from Bridalveil Fall to the Leaning Tower. Bridalveil Fall is a classic example of a glacial hanging valley that formed when the main Merced glacier cut a deeper valley than the smaller tributary glacier in Bridalveil Creek. The waterfall drops 620 feet, and runs pretty much all year, though in the autumn it is often just a trickle, as can be seen in the top photo of this post. 

In the spring runoff, the fall is spectacular. A couple of years back, in the blissful pre-drought days, I caught a rainbow in the spray coming of the base of the fall.

The Leaning Tower is composed of several kinds of rock, including the El Capitan granite and the Leaning Tower granodiorite. The granite is about 103 million years old, and the granodiorite somewhat younger, although a precise age hasn't been determined that I can find. The most remarkable feature of the Leaning Tower is...it leans! The cliff prominently hangs out over the valley below. El Capitan does this as well, but it isn't so easily seen. The cliff is one of the more popular climbs in the valley.

Next post: what's on the other side of Bridalveil Falls?

The Yosemite No One Sees in Summer...the Merced River Canyon

The drought in California is horrific. It is quite probably the worst drought in centuries, but we received a slight respite in the form of showers and snow during the last part of March and early April. It was a drop in the bucket towards relieving the huge water deficit that has built up in the last few years, but it gave a shot of energy to the seedlings of grasses and wildflowers. They perhaps should have sprouted and grown months ago, and they will be dried out in a few short weeks, but this week, the Sierra Nevada foothills were alive with color.

We were on our way to Yosemite Valley for our geology field studies course last Saturday, and I joked (only in half-jest) that the students had to "earn" the right to learn the geology of Yosemite Valley by first exploring the canyon of the Merced River. We had to prepare ourselves by undertaking a journey through the rocks that predate the granitic rocks that form the walls of the iconic valley. Our route took us through the foothill towns of Snelling and Hornitos, and then we drove up Highway 140 through Mariposa, over Midpines Summit and down into the deep gorge of the Merced River (originally the "Rio de Nuestra Señora de la Merced (River of Our Lady of Mercy)".

Highway 140 winds back and forth through a series of metamorphic terranes, bands of deformed and twisted rocks that traveled across the Pacific Ocean to be mashed into the western edge of the North American Continent. The rocks include slate, schist, marble, phyllite, quartzite and greenstone. These were the ancient rocks (between 600 and 200 million years old) that were intruded by the granitic rocks that eventually were exposed and eroded to form the dramatic cliffs of Yosemite Valley.

The Sierra Nevada is a huge westward tilted block. The slope has allowed the rivers draining the mountain range to carve deep gorges, some deeper than the Grand Canyon (Kings Canyon to the south is about 8,000 feet deep!). The depth of the Merced is more like 2,000 to 3,000 feet deep downstream of Yosemite, and in any other setting in most any other state, this would be enough for the establishment of national parks and the like. Instead, it is "just" the preliminary canyon one must pass through before arriving at Yosemite.

The carving of such deep canyons can have serious geologic consequences for society. The slopes of such canyons are inherently unstable, and mass wasting (landsliding) is a constant hazard. The building of roads and railways adds to the instability by undercutting the already steep slopes. Such is the case with Highway 140. CalTrans has struggled for years to prevent the collapse of a slope near the village of El Portal. In 2006, the slope flicked away the mitigation efforts and collapsed onto 600 feet of highway. The Ferguson Slide, as it is called, closed the highway for months and caused economic hardship for the surrounding communities. Two temporary bridges were constructed to cross the river and circumvent the huge slump. Decisions have yet to be reached regarding the permanent disposition of the highway.

The Ferguson Slide. 600 feet of Highway 140 is buried under the rocks.

As we drove deeper into the Merced River Canyon, the slopes seemed to come alive in orange and gold. The California Poppies have reached the zenith of their blooming cycle. In contrast to my previous gloom and doom post regarding fires, rejuvenation does indeed occur, as many of these slopes have been cleared of brush and chaparral by repeated wildfires, allowing for dense concentrations of wildflowers.

We passed El Portal, and then the Yosemite View Lodge. The name is not totally dishonest; the boundary of the national park does indeed lie a few yards east of the hotel complex, so the view of the granite slope beyond is indeed a view of Yosemite. But it is not yet the iconic valley with Half Dome and El Capitan and Yosemite Falls. It is pretty, though, and the Merced River makes a dramatic dash through the maze of gigantic fallen boulders.

 We arrived at the park entrance station, drove a few more miles, and made our last stop before Yosemite Valley. It was another site of mass wasting, but this time it was a rockfall, and it included granitic rock instead of metamorphic material. The so-called Cookie Slide took place in 1982, and involved around 100,000 cubic yards of material that bounced and fell down the slope onto Highway 140. Once again, the road was blocked for weeks.

 The size of some of the fallen blocks is astonishing. It's hard to see how a block like the one in the picture below held together during the downhill descent.

The rocks are part of an intrusive series called the El Capitan granite, which was intruded about 103 million years ago. A contemporaneous intrusion of dioritic rock lead to some co-mingling of magmas, including the enclave of darker rock in the granite (below).

I've always thought of granite as a particularly beautiful rock. The sparkling mass of quartz (glassy gray color), orthoclase feldspar (white grains), and biotite mica is pleasing to the eye, and tells an odd story: Yellowstone National Park, off in Wyoming, sits on top of a huge magma chamber that may one day explode again (but not tomorrow). In Yosemite, we are sitting within a former magma chamber, under the volcano. As such we are exploring the inner depths of an extensive magma system, brought to light by long periods of uplift and erosion.

In a coming post, we'll see how Yosemite looked on Saturday.

How it was Yesterday: Yosemite Jumps From Fall Right Into Winter

Mrs. Geotripper was a bit disappointed two weeks ago when I visited Yosemite Valley and she wasn't able to tag along. So it wasn't too much of a surprise that we ended up making our way to the valley yesterday, looking to take in some of the fall colors.

California may be a leader in many things, but the fall season is not one of them. When most of the deciduous leaves in the nation have long since fallen, our fall colors are just getting started. I saw precious few fall colors two weeks ago in Yosemite Valley, but one could see they would be changing soon.

We weren't disappointed. The oaks and dogwoods where at their brightest shades of yellow and red (and yes, my readers in the eastern U.S., I know our colors can't come close to the colors out your way!). But there was a twist. We arrived on the heels of the first real snowstorm of the season. There were still occasional flurries of snow and pogonip ice coming down as we arrived in the valley.

It was just stunning to me. Yosemite National Park is one of the most beloved of my favorite places in the world, and I go there every chance I get, but in twenty years, I'm not sure I've seen it prettier than this (although I admit I seem to say that with every new season). In the picture above, the oak tree is framed by the lowest of the three Cathedral Rocks (this is the side of the rocks opposite of Bridalveil Fall). The cliff is supported by the El Capitan granite, which lacks jointing (fractures) so it forms vertical cliffs.

In stark contrast to the cliffs of El Capitan granite, the Diorite of the Rockslides is riddled with fractures, and breaks down easily into talus slopes. It lies just west of El Capitan, and is relatively unremarkable unless new snow covers Bridalveil Meadow.

In the picture above, Yosemite Point is almost ignored for much of the year, because the attention of most people is drawn to Yosemite Falls, which normally plummet off the cliff just left of the center of the photo. The cliff is composed of El Capitan granite (102 million years old) and Sentinel granodiorite. The Lost Arrow is a jointed spire just to the left of the point.

Looking at Yosemite Falls, one can also miss one of the other prominent cliffs of Yosemite Valley, Sentinel Rock (above). It's on the south wall of the valley, opposite of Yosemite Falls. It is composed of Sentinel granodiorite, which is about 93 million years old.

Upper Yosemite Falls was not flowing, but enough melting snow collected in the basin below to provide a wispy flow of water to Lower Falls (320 feet).

In the photo above, one can see the cliffs near Taft Point. If you ever need to conquer your fear of heights, you can walk to Taft Point from Glacier Point Road (about a mile) and look straight down for thousands of feet through the Taft Fissures. Needless to say (but I always do anyway), you can't get there right now by road.

The prominent tower in the picture above (on the right side) is one of the Cathedral Spires. I used to get them mixed up; the Cathedral Rocks are just to the west, and are much larger, providing the frame for Bridalveil Falls.

Sentinel Rock looks quite different from another vantage point.

I have no idea if this storm was "it", the final closing down of the high country. If this is another drought year, the snow could all be gone in a few days, and we could be driving over Tioga Pass in January like we did last year. Or, it could be the beginning of the buildup of a nice snowpack that could ease our drought worries here in California. All I know is that it was an indescribably beautiful day (despite my efforts to describe it anyway...).

Next up: the birds of the day, and the sunset...

A Day at Yosemite in the "Spring": The Iconic Rocks

As I pointed out in yesterday's post, living near a spectacular, photogenic place like Yosemite means that putting a digital camera in hands like mine is dangerous, at least to hard drive memory storage. No matter how many times I stand at a place like Tunnel View (above), I take the same pictures from the same spot.

I might make the argument that every moment at these places is different, and that would be true; the presence or absence of clouds, the season, the time of day, these are all factors in the interest of a picture. So it is, that for the 38th time, I took digital shots of the iconic rocks of Yosemite Valley. Enjoy!

The most famous view of Yosemite Valley quite probably is that from Tunnel View, where Highway 41 emerges from the darkness to reveal a sweeping panorama of El Capitan, Half Dome, the Cathedral Rocks, and Bridalveil Fall. I liked the view on Saturday because Half Dome was obscured by clouds. It's not that I don't like Half Dome; it's just that when expected parts of the scene are missing, the emphasis of the photo changes.

Half Dome spent the whole day Saturday peeking from behind clouds. I like it that way; drifting clouds provide a sense of the immensity of the dome. It rises more than 4,000 feet above the valley floor. Half Dome is the product of the release of pressure as the rocks were exposed by erosion. The rock expanded and fractured parallel to the surface, a process called exfoliation. Edges and corners break away over time, leading to the dome-like shape. The steep cliff formed along a vertical fracture called a joint. For the record, when viewed from other angles, it becomes clear that the dome is somewhat misnamed. If anything, it should be called Four-fifths Dome.

Sentinel Dome is the result of vertical jointing. I liked the way the snow flurries were whirling about the cliff faces, giving the cliff a more mysterious aura.

Storm clouds covered the eastern end of Yosemite Valley, but the sun was occasionally breaking through at the western end. In the picture above, we have Bridalveil Fall and the Leaning Tower. The source of its name should be obvious.

We are three weeks into spring, but it is obvious that winter still governs the landscape. There had been several inches of snow the previous night, and our van was constantly clobbered by clots of snow falling off the trees.

El Capitan is a three thousand foot high cliff of relatively unjointed granite. El Capitan granite is composed of quartz, potassium feldspar, and biotite mica. A slide a few years back dropped some chunks of the granite onto Highway 140, allowing us to see what the rock looks like up close. The casual visitor almost never sees such clean fresh surface because it turns out that there is an entire other world within a world in Yosemite, as well as almost everywhere else. That will be the subject of another post.

Taking the High Ground: Yosemite as you (maybe) haven't seen it

If the last few entries in the Yosemite series have left you with a fear of cliffs and steep drop-offs, I suggest today a slightly different point of view. Between Taft Point (7,500 ft) and Glacier Point (7,214 feet) stands the barren expanse of Sentinel Dome, which at 8,122 feet is one of the highest easily accessible points on the rim of Yosemite Valley. The normal hike (a bit over a mile) begins at the Taft Point parking area, although anyone who is savvy with a topographic map will note the presence of a service road about 2/3 mile northeast on Glacier Point Road that vastly shortens the hike (maybe a half-mile or less). Unlike the previous viewpoints, the exfoliated dome is set back from the rim, and although there is a short stiff upgrade at the end, there is no place to fall off of. The summit of Sentinel Dome seems to be an outlier of El Capitan granite, although it is surrounded by the granodiorite of Kuna Crest and the Sentinel granodiorite.

The view from the summit of Sentinel is superb, although it reveals little of the valley floor. Instead, it provides a sweeping view of the valley walls and the surrounding region of glaciated peaks, cirques, aretes, domes and cliffs. It stands only 700 feet shorter than Half Dome. It is visible from the classic view of Yosemite Valley, just behind the lowest of the Cathedral Rocks, and just to the right of Sentinel Rock.

Today's picture is looking west towards the portal of the valley, framed by the Cathedral Rocks and El Capitan. The granite that forms both of these edifices is mostly unjointed, and the glaciers that scoured the valley were unable to quarry deeply beneath the cliff faces. The sediment between the portal cliffs is only about 300 feet deep. At the upper end of the valley, in the vicinity of the stables and Stoneman Meadow, the sediments may be as much as 2,000 feet thick!

One may notice the near lack of cliffs beyond the portal formed by El Capitan and Cathedral Rocks. These are the Rockslides, and are composed of diorite of the Rockslides, which is Jurassic-Cretaceous in age, somewhat older than the other rocks in the valley. The rocks have been intruded by mafic dikes and joint more closely than other plutonic rocks in the valley, causing them to easily break down and form talus slopes. Roads built across this slope have been abandoned over the years due to constant rockslide damage.