You Can Never See This View Again. Ever.

The title isn't as ominous as it sounds, but it is true. This is the panorama seen from Tunnel View at the west end of Yosemite Valley, one of the most famous viewpoints in the world, in 2013. But the specific scene above can never be experienced by anyone ever again. Geological forces, instead of acting at a slow incremental pace, moved things tragically fast late last month. A chunk of granitic rock the size of a football field came off the cliff just east of El Capitan (cliff on the left), and the scar will be visible for centuries. The picture below is how it looked today.

The view is fundamentally the same, but the changes are occurring, and the appearance of Yosemite changes with it. See below for a zoom of the cliff near Horsetail Falls, first in 2013, and then how it was this afternoon.

There were a total of seven rockfalls from the site on September 27-28, 2017. One person was killed and two were injured. One report mentions the largest of the falls as weighing 30,000 tons. This wasn't the biggest rockfall in Yosemite's history, not even close. The largest in recorded history was the Middle Brothers slide of 1987, which totaled about 1.4 million tons. Prehistoric rockfalls in the Mirror Lake area were larger still.

Actually, if we were to look at photographs from a decade ago, there would be an additional difference. The 2009 Ahwiyah Point rockfall scar can be seen just right of center, in both of the pictures above (but especially in the upper one). It totaled 115,000 tons, but thankfully injured no one. Check below for a 2009 view of the same spot to compare the difference.

Although partly hidden in shadow, the rock face at the bottom, just right of center (below the snow) is composed of darker weathered rock instead of the white scar seen in the photos above. Geologic change is constant only in that fact that it happens. Sometimes it is imperceptible, and sometimes is rapid and tragic.

A Series of Fortunate Events: Glacier Point, Living Up to the Hype

Glacier Point on the rim of Yosemite Valley is hands-down one of the most spectacular places one can stand in the world. There's just no place like it. And it's noisy. It's crowded. There is a busy and often full parking lot. And despite that, it is worth putting up with, because there just aren't that many places where you can look straight down for 3,000 feet, and at the same time see the iconic rocks of the upper valley from a unique angle.

It takes a little effort to get there. One has to drive about 25 miles from the floor of Yosemite Valley up a narrow winding road. One has to wade through the crowds to get the edge of the precipice. But no matter how many times I've stood there, I can never fail to be in awe that such places exist. In one sweeping view one can see the north valley wall from the vicinity of the Three Brothers and Yosemite Falls, past Yosemite Point, on to North Dome and Basket Dome, across the deep trough of Tenaya Canyon to Half Dome. And below your feet there is the floor of Yosemite Valley, the meadows, the forests, the parking areas, the resorts, all from a nearly vertical perch at 3,200 feet (980 m).

It's from a high point like this one that I can best appreciate the work of the glaciers that altered a narrow river-carved canyon into the wide, flat floor we see today. The earliest glaciers filled the valley to the rim (the Pre-Tahoe stage around 800,000 years ago), while the later advances only filled the valley to one-third or so of the total depth (the Tahoe stage at 130,000-65,000 years, and the Tioga stage from 20,000 to 13,000 years ago). The domes, like North Dome, Basket Dome and Half Dome, rose above the ice at all times. Their rounded shapes are the result of exfoliation, the tendency of slabs to break off the edges and corners of solid monoliths of granitic rock as erosion took weight off the surface of the rock. Glaciers helped excavate the rock from the jointed face of Half Dome, but never overtopped the gigantic rock.

One might think that a view like this is unchanging on a human time scale, but that's not exactly true. It's a little tricky to see it in the shadows, but look at the whitish exposure of granite to the left of Half Dome in the picture above. That scar didn't exist prior to 2009. Every visitor prior to March of that year saw something different. The Ahwiyah slide involved thousands of tons of rock that fell 1,800 feet to the floor of Tenaya Canyon. The resulting explosion of rock destroyed hundreds of trees on the valley floor. To see some before and after pictures, click here for a look.

Our class finished at Glacier Point and headed back the vehicles, but we weren't done with standing in dramatic places. That will be in the next post!

The Dogwoods are Blooming in Yosemite Valley! And North Dome, the Stuff of Legend

The Pacific Dogwood (Cornus nuttalli) is a diminutive tree that forms some of the understory of Yosemite Valley's conifer forests. It seems practically invisible to park visitors (like me, anyway) most of the year except for two times: fall, when the tree becomes one of the most vivid contributors to the autumn colors of the valley, and spring, when the Dogwood flowers bloom. The flowers aren't all that showy actually. They are the small yellow sphere in the middle of the structure. But they are surrounded by large white bracts that look like flower petals. Bracts are actually highly modified leaves. Just the same, they add a bright splash of white to the forest understory in the spring. I was in Yosemite Valley just a week ago and I would swear there were no Dogwood blooms at the time, but there were many of them yesterday.

Of course this is mostly a geology blog, so I couldn't help but notice that I was using North Dome as a backdrop to the blooming trees. North Dome is one of the less heralded sights in Yosemite Valley, a place with so many gigantic cliffs and waterfalls, that otherwise spectacular features get lost in the shuffle. It sits to the east of Yosemite Falls and across Tenaya Canyon from the much more famous Half Dome. It is a marvelous example of an exfoliation dome, which developed as the rock was exposed by erosion. The granitic rock, which formed miles deep in the crust, expanded as it reached the surface and the rock slabbed off, removing the corners and edges and forming the spherical outline (although from above it is a more linear ridge).

According the writings of Galen Clark, North Dome is tied in with Half Dome in the mythology of the Native Americans who inhabited the valley. It is called To-tau-kon-nu'-la, referring to the cranes that could be seen around the base. Half Dome, across the canyon is Tis-sa'-ack. According to one version of the story, the great chief Choo'-too-se-ka', whose name was later changed to To-tau-kon-nu'-la after he built is home on the dome, fell in love with the woman Tis-sa'-ack who had come out of the south to help teach the people to weave beautiful baskets. She did not return his love, saying she needed to return to her people, and she left in the night. The chief set out to search for her and never returned, leaving the people behind to suffer droughts, floods, rockslides, earthquakes and other calamities. One of the earthquakes caused Half Dome to split and half of it fell into the valley. Eventually the Great Spirit had mercy on the remaining people and returned the land to a bountiful state. An image of Choo'-too-se-ka/To-tau-kon-nu'-la appeared on North Dome, and Tis-sa'-ack is visible on Half Dome.

There is a second story about quarreling spouses that I don't like as much...

Of course those old myths don't reflect reality...like rockfalls, floods, and earthquakes. They never actually happen in Yosemite, right? Oh...

Half Dome and the Ahwiyah Point rock fall of 2009 (center)

Reflections on Yosemite and Mass Wasting

OK, not philosophical reflections, but literal reflections. One of my favorite spots to visit on the floor of Yosemite Valley is the Mirror Lake area, due to the relative peace and serenity brought about by the lack of cars and other urban distractions. Just people walking about quietly. Mirror Lake is not exactly a lake, and not exactly a river, and for a good part of the year it is entirely dry.

Tenaya Creek is a relatively minor tributary of the Merced River, but Tenaya Canyon is nearly as deep as the Grand Canyon. That would seem to be a bit of a mystery, since the size of glaciers in a canyon can be roughly comparable to the size of the watershed, and it looks like there was a very large glacier here. Studies of the glacial topography help to explain the disparity: large masses of glacier ice spilled over the divides from the Tuolumne River drainage, adding to the ice load in Tenaya Canyon. As big as they were, glaciers never overrode the summit of Half Dome, which looms thousands of feet above the lake. They instead undercut part of the base of the dome, taking advantage of joint patterns in the granitic rock to quarry rock away from the 700 foot-high face.Mirror Lake developed several thousand years ago when a large rockfall collapsed off the north wall of the canyon, damming up Tenaya Creek. The lake mostly filled with sand and was turning into a meadow, but heavy riverflows during spring runoff scoured out the river channel and left some very wide pools that are excellent for photographing (and just appreciating) the cliffs and domes that are reflected from above.

Lest one thinks that the rockfalls in Yosemite are a thing of the past, a large rockfall occurred here just a few years ago (see some posts on the Ahwiyah Point fall here and here). Ahwiyah Point is the prominent summit just east of Half Dome (below). The rockfall destroyed hundreds of trees, and a hiking trail, which is still closed to entry due to continuing danger from falling rocks.

Mirror Lake is not hard to get to (a tram stops just about a mile away), and one can even ride a bicycle most of the way up. There is a small network of trails around the pools, and the main trail climbs out of Yosemite at Snow Creek. It is a very beautiful corner of a very beautiful valley. Don't miss it if you get a chance to visit!

NPS Web Page Chronicles Yosemite Valley Rock Falls

Via Yosemite.blog.com, here is a link to a new web page on the rockfall history of Yosemite Valley. Besides a report on rockfalls in 2009 (there were 52), the page includes a wealth of links to other resources about mass wasting phenomena in the region. The map above chronicles the location of all the major rockfalls from 1857 to 2009.

It was a big year in the valley for this sort of thing. The Ahwiyah Rockfall actually changed the appearance of Yosemite as seen from the Wawona Tunnel and Glacier Point forever, and was the largest rockfall in 22 years. Luckily no one was hurt or injured.

The Ahwiyah Point Rockfall: the View I Wish I Had

Photos courtesy of the National Park Service. Photographer: Greg Stock, Park Geologist

The National Park Service now estimates that the March 28 Ahwiyah Point Rockfall had a volume of 43,000 cubic meters, or 115,000 tons. This means that the event was almost 50 percent larger than the 1996 Happy Isles Fall (30,000 cubic meters), and is the largest event since the 1987 Middle Brothers slide (1.5 million tons). The airblast was reminiscent of the Happy Isles event.

These pictures, taken by Greg Stock, the park geologist, show the views I wish I had when I was there last weekend. The top photo is from the switchbacks to Snow Creek, and the other is obviously from the edge of the destruction zone.

For some strange reason, rock climbers have been inordinately interested in this event, and the effect it may have had on some of the Half Dome climbing routes. Via Geology.com, here is a comment string replete with photos from a number of climbers.