El Capitan Rock Avalanche Article by Greg Stock


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Chris McNamara

SuperTopo staff member
Topic Author's Original Post - Dec 6, 2008 - 02:49pm PT
I'm forwarding an interesting article on the El Cap rock avalanche, one of the larger ancient rock falls in the Valley.

You can download the pdf file by clicking on this link:


Cumberland Plateau
Dec 7, 2008 - 03:07am PT
Interesting read, thanks!
Ed Hartouni

Trad climber
Livermore, CA
Dec 7, 2008 - 03:18am PT
great read!
was that what Greg was doing up there this summer before being plucked off? collecting samples to date? that would be very cool!

Big Wall climber
Seattle, WA
Dec 7, 2008 - 03:38am PT
Interesting indeed. It evokes some intense imagery.

Trad climber
El Portal, CA
Dec 7, 2008 - 10:32am PT
Really interesting stuff. I always liked that essay by Muir.

Greg, random question: which sample isotopes did you use?

Trad climber
A place w/o Avitars apparently
Dec 7, 2008 - 10:35am PT
Interesting read.



Dec 7, 2008 - 11:23am PT
"If for a moment you are inclined to regard these taluses
as mere draggled, chaotic dumps, climb to the top of one
of them, and run down without any haggling, puttering
hesitation, boldly jumping from boulder to boulder with
even speed. You will then find your feet playing a tune,
and quickly discover the music and poetry of these magnificent
rock piles - a fine lesson."
dee ee

Mountain climber
citizen of planet Earth
Dec 7, 2008 - 11:43am PT
I'm curious how big that earthquake was 3600+ yrs. ago!

Trad climber
Truckee, CA
Dec 7, 2008 - 09:05pm PT
There is a plausible conclusion to be reached from the facts stated in the article, if one let's one's imagination run a bit. Stock states that the Taft granite, which forms most of the avalanche debris, is found in the summit area of El Cap (although he doesn't also say, not found much on the face itself). Therefore it's perfectly plausible that there was an enormous overhanging section, or even a monumental roof, at the top of the crag before the 3,600 ybp earthquake. Imagine if PO or Mescalito reared back in an awesome overhanging section or 100-ft roof right at the top. What routes there could have been on that chunk of rock!
Captain...or Skully

Social climber
Where are YOU from?
Dec 7, 2008 - 09:12pm PT
For a while, anyway.
I was talking to Walt one day, & he was telling me about the big chunk of Surgeon General(that he & Klaus climbed on!!)that peeled off. I think Klaus led that.
A piece , or pieces of rock that size, in flight.
Yowza!(hey, I like the word.It fits.)

Trad climber
The state of confusion
Dec 7, 2008 - 09:21pm PT
Pretty darned amazing!

Dec 7, 2008 - 09:29pm PT
This can make you wonder about any similar places in the Sierra that have extensive talus below huge escarpments.

"Now who you jivin' with that Cosmik Debris?"

Ricky D

Trad climber
Sierra Westside
Dec 7, 2008 - 09:36pm PT
I'm curious - if the debris field is composed mostly of "Taft" granite that for the most part does not appear on the face...does this imply that the flow originated as an overlying cap piece on top of of El Cap?

Yosemite Valley
Dec 7, 2008 - 11:18pm PT
Thanks for posting the article, Chris. I was thinking that climbers might be interested to know more about that heap of rocks adorning the Nose approach. The exposure dating method utilizes beryllium-10, a rare isotope produced in quartz only by cosmic rays. Concentrations of beryllium-10 are usually very small, so measurements must be made on an accelerator mass spectrometer.

I climbed Mescalito this fall (well, a lot of it anyway) as part of an effort to determine how extensive the Taft Granite is on the SE Face, because, like mongrel, I wonder whether there might have been a big roof at the summit that collapsed. At first I didnt think this was possible given the sheer volume of the deposit, but as the mapping has progressed it seems that scenario is back in play. No sample collecting involved, just photographing the rock at each belay with a label and a scale bar easy! At this point I have photos from the East Buttress, Zodiac, PO Wall, Mescalito, and the Nose (many thanks to Jesse McGahey, Lincoln Else, Roger Putnam, and Holly Beck for helping with this effort). Actual petrologists (including Minerals) are now helping to identify the rock types and map them on high-resolution photos (http://www.xrez.com/yose_proj/Yose_index.html);. Im hoping to map New Dawn in the spring, and possibly the NA Wall as well, but if anybody is planning on climbing a route on the SE Face and would be willing to shoot a few photos at the belays, let me know.

Greg Stock
Park Geologist
(209) 379-1420

Captain...or Skully

Social climber
Where are YOU from?
Dec 7, 2008 - 11:22pm PT
Will do, bro.
What could be easier..?
I got room on the card, after all...
Brian Hench

Trad climber
Seattle, WA
Dec 8, 2008 - 02:05pm PT
What attempts have been made to use cosmogenic dating techniques on rock still on the cliff face? It would seem that all you have to do is to match the date of boulders from the rockfall to in situ rock on the face.
scuffy b

On the dock in the dark
Dec 8, 2008 - 02:11pm PT
Very interesting article. Thanks, much.
I like Brian Hench's question.

Trad climber
El Portal, CA
Dec 8, 2008 - 02:20pm PT
Seems like that would be hard for a few reasons...here's a big one I can think of.

CRNs are formed on rock surfaces. If the rock fell off, that would mean a new stage of CRN formation on the wall correlated with rockfalls with different concentrations (the rockfall wouldn't necessarily match up, or only part of the rocks would if exposed to surface rays, making the appropriate dating area impossible to distinguish) - I think.
The Alpine

Big Wall climber
Tampa, FL
Dec 11, 2008 - 03:59pm PT
Check this out:

from here:
surly B

Oakland (what?), CA
Dec 19, 2008 - 03:57am PT
You all realize that this summary of historic documents is really a 'how to' tale, right?

"How to find awesome bouldering is Yosemite: follow the HUGE rockfalls!"

-Greg Stock, Yosemite Association, 2008

Yosemite Valley
Jun 9, 2010 - 05:41pm PT
For anyone interested in a more technical discussion of the El Capitan rock avalanche (plus a preliminary geologic map of the SE face of El Capitan), here is a link to a journal article on the topic:



Social climber
Jun 9, 2010 - 05:44pm PT

Bogus link.

EDIT: Works now!
Clint Cummins

Trad climber
SF Bay area, CA
Jun 9, 2010 - 07:34pm PT
Cool article, Greg - congrats on the publication.

Here is the abstract which gets straight to it, and a key photo/diagram:
El Capitan Rock Avalanche / rock types on El Cap <br/>
from:  "Catastrophi...
El Capitan Rock Avalanche / rock types on El Cap
from: "Catastrophic rock avalanche 3600 years BP from
El Capitan, Yosemite Valley, California",
Greg M. Stock, and Robert A. Uhrhammer,
Earth Surface Processes and Landforms 35, 941951 (2010)
Credit: Greg Stock
White dashed area is computed likely source area for huge rockfall, at around 1600 BC (3.6 ka ago).

ABSTRACT: Large rock slope failures from near-vertical cliffs are an important geomorphic process driving the evolution of
mountainous landscapes, particularly glacially steepened cliffs. The morphology and age of a 219 10^6 m^3 rock avalanche
deposit beneath El Capitan in Yosemite Valley indicates a massive prehistoric failure of a large expanse of the southeast face.
Geologic mapping of the deposit and the cliff face constrains the rock avalanche source to an area near the summit of ~85
10^4 m^2. The rock mass free fell ~650 m, reaching a maximum velocity of 100 m s^-1, impacted the talus slope and spread across the valley floor, extending 670 m from the base of the cliff. Cosmogenic beryllium-10 exposure ages from boulders in the deposit yield a mean age of 36 02 ka. The ~13 kyr time lag between deglaciation and failure suggests that the rock avalanche did not occur as a direct result of glacial debuttressing. The ~36 ka age for the rock avalanche does coincide with estimated late Holocene rupture of the Owens Valley fault and/or White Mountain fault between 33 and 38 ka. The coincidence of ages, combined with the fact that the most recent (AD 1872) Owens Valley fault rupture triggered numerous large rock falls in Yosemite Valley, suggest that a large magnitude earthquake (≥M7.0) centered in the south-eastern Sierra Nevada may have triggered the rock avalanche.
If correct, the extreme hazard posed by rock avalanches in Yosemite Valley remains present and depends on local earthquake
recurrence intervals.
tinker b

the commonwealth
Jun 9, 2010 - 07:48pm PT
i can't get the link to work, but thanks for everything else greg.

Boulder climber
Jun 9, 2010 - 08:57pm PT
are there any park rules that say you can not climb during a major earthquake?

because that would be all we need, more regulations,

Trad climber
Bay Area
Jun 9, 2010 - 11:30pm PT
You're specifically exempted from that regulation.
"Subpart B, Clause 9: Dr. Sprock, owing to his well documented anti-authoritarian nature is exempted from all other sections of this regulation. However, even for Dr. Sprock, the Laws of Nature will not be exempted not excluding the law that being crushed by a 5700 ton boulder will lead to instant death"

Trad climber
Bay Area
Jun 9, 2010 - 11:36pm PT
I had wondered about that big fan of talus spreading out below ElCap. It sure seems a long way for a boulder to travel on its own.
Wonderful research Greg. Chris, thanks for posting.

It will be really interesting to follow this work through the years.
Wade Icey

Trad climber
Jun 5, 2013 - 12:46pm PT
Credit: Wade Icey

Jun 5, 2013 - 01:01pm PT
Big wall field work w/ big rock fall forensics - FANTASTIC!!!

Very cool work!



Trad climber
East Coast US
Jun 5, 2013 - 01:39pm PT
In other words, there are lots of boulder problems that need unearthing.

Trad climber
Jun 5, 2013 - 02:07pm PT

Yosemite Valley
Jun 5, 2013 - 02:14pm PT
Looks like the link I posted above for the more technical paper is no longer valid. This link should work:


For those interested in other Yosemite rockfall publications, they can be accessed from the park's rockfall webpage:



Jun 5, 2013 - 03:08pm PT
Thanks for the new link - fascinating report. Great bump, I had missed this.

Big Wall climber
Reno, Nevada
Jun 5, 2013 - 03:21pm PT
Awesome! Thanks, Greg.

October 11, 2010 - Rock Avalanche:
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