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donini
Trad climber
Ouray, Colorado
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Topic Author's Original Post - Jan 26, 2014 - 09:50am PT
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ST geologists help me out here. The massive rockfall on Elephant Rock was not so long ago and I can remember driving to the base of the Cookie. It seems that every year a number of significant rockfalls occur, who now feels comfortable at the base of the Apron? Whats up...is this just normal geologic process or is something else going on?
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climbski2
Mountain climber
Anchorage AK, Reno NV
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Jan 26, 2014 - 10:09am PT
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I'm no geologist. Have not traveled as much as some but..
The only other place I have been with a similar amount of vertical rock in a comparable area is the Alaska range. Yosemite rockfall does not even come close to it. I would not be surprised if in a day I heard and saw as much rockfall as a person might in a year in Yosemite.
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donini
Trad climber
Ouray, Colorado
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Topic Author's Reply - Jan 26, 2014 - 10:11am PT
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Yes, but there is a lot of chossy rock in the Alaska Range and a lot of ice to lever rock off....not so in Yosemite.
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mooser
Trad climber
seattle
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Jan 26, 2014 - 10:18am PT
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I had the same thought, reading the "Apron Appreciation" thread, donini. I wouldn't spend a heck of a lot of time there, myself. That was the first place I ever climbed in the Valley, and generally one of the last places I feel comfortable at now.
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climbski2
Mountain climber
Anchorage AK, Reno NV
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Jan 26, 2014 - 10:22am PT
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I've always figured that some truly massive rockfall event in Yosemite is inevitable. Something big enough to completely cross the valley even. Gotta be a matter of time, Kinda like big earthquakes or asteroids and such.
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Brokedownclimber
Trad climber
Douglas, WY
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Jan 26, 2014 - 12:10pm PT
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Nobody in their right mind even goes near Lower Brother these days. BITD, that was a Camp 4 locals hangout.
I'm undecided about the Apron, but there are still several climbs there that I have on my "to do" list. Ditto, base of El Cap. Ditto, Church Bowl area.
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mongrel
Trad climber
Truckee, CA
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Jan 26, 2014 - 12:34pm PT
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Calling gstock.. calling gstock. Being the Park geologist, you know he is going to say, rockfall can happen any time blah blah. But the blah blah he has to contribute is always the most interesting imaginable commentary, about the triggers sometimes being ice or water, sometimes heat, big flakes flexing, there's been really interesting stuff about this in other threads (about the Hourglass, I think folks posted up about hexes squashed and the like).
The link above about recent rockfall is a really good paper. And you can download a huge spreadsheet of the details of every recorded rockfall.
If you want to convince yourself never to go to the Valley, do some surfing around and find a USGS map for which the citation is: G.F. Wieczorek, M.M. Morrissey, G. Iovine, and J.W. Godt. 1999. Rock-fall Potential in the Yosemite Valley, California. It's a huge wide map and I think it would blow up the format of the thread if I posted it. But if you can't find it, PM and I'll e-mail it. Basically, the entirety of all the valley slopes that aren't bedrock cliffs is rockfall or slide debris of some era. The issue is, how recent? So for this map, they outlined every deposit they could and colored it by "recent" "historic" or "prehistoric" and by what kind of debris. There's a lot of color! and some of those events were huge huge huge. About which Greg will say, this could happen any time.
But hey, aren't you a big fan of the Black Canyon? How stable is that stuff?
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mongrel
Trad climber
Truckee, CA
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Jan 26, 2014 - 12:39pm PT
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Here's that USGS map. If it ruins the thread I'll delete. Among other cool things you can see that (the former) Mirror Lake was formed by a rock slide that dammed the whole of the canyon. ROCK!
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Byran
climber
San Jose, CA
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Jan 26, 2014 - 12:44pm PT
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Lower Brother solid, it's the far left margin of Middle Brother (Koko Ledge area, just to the left of the Folly) that has all the rockfall. Elephant Rock is also one of the most bomber formations in Yosemite, it's just that obvious right side that has exfoliation issues and there's no reason to be over there. And the GP Apron acts as a natural barrier which diverts rockfall off to either side, so the more central climbs like Harry Daley, Goodrich, and the Grack are probably fairly safe. Common sense seems to keep the crowds away from the most active rockfall locations.
After a big cold storm, for the first couple days of sun I avoid the large steep formations and go climbing at the Cookie or somewhere small and solid. Really though the likelihood of being clobbered by spontaneous rockfall in Yosemite is extremely slim, most rockfall accidents are self-inflicted or caused by a party above. Compared to other climbing destinations of its size and caliber, the Valley probably has the least objective hazards to be found on earth.
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Spider Savage
Mountain climber
The shaggy fringe of Los Angeles
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Jan 26, 2014 - 02:53pm PT
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I am always very cautious. If you are standing on fresh talus, you are in an active zone. Although some of that could be from winter ice falls. Inside the great Royal Arch seems to get hammered pretty hard by that type of work. El Cap base too.
I'd rather not climb on The Apron, chased off by falling stones in the late 1980's.
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McHale's Navy
Trad climber
From Panorama City, CA
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Jan 26, 2014 - 03:27pm PT
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The massive rockfall on Elephant Rock was not so long ago
Jim, you mean the one in the early 70's? I was rappelling on the other side of the valley straight across from it, heard a crack like sharp thunder, turned around, and a huge ceiling was sailing through space. It hit the river and washed a wall of water over the hwy and closed the road.
There had been a huge slide earlier maybe that year that created the huge ceiling we were admiring just that morning. It's the one that came off for us. Must have been the size of a large house or so.
I was going up to Sickle Ledge once in the late 60s or early 70s and it had to be the the next morning after that big Magic Mushroom area collapse. The Nose up to Sickle was covered in dust, and every time the wind blew, it was in our eyes. We had no idea what was going on - nobody knew it had happened yet. Later that day we hiked around toward the Salathe and the forest and everything was obliterated. Soccer ball sized rocks came down for many months after that.
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donini
Trad climber
Ouray, Colorado
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Topic Author's Reply - Jan 26, 2014 - 10:42pm PT
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You got it Dan.
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Peter Haan
Trad climber
Santa Cruz, CA
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Jan 26, 2014 - 11:09pm PT
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Someone laminate up a copy of that Stzzo map asap for Jim. J-Do thought he was on solid ground all this time. You know, like on those things he used to do down under. I mean solid like a rock.
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bigbird
climber
WA
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Jan 26, 2014 - 11:11pm PT
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Always active rock fall= Troll Wall....
Its like the dark ugly cousin of the big wall world, that no one talks about....
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gstock
climber
Yosemite Valley
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Jan 27, 2014 - 02:27pm PT
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Well, it’s funny you should ask, as several of us geologist/climber geeks (Brian Collins, Lauren Austin, Roger Putnam, and I) just spent two days last week thinking hard about this issue.
To the opening question about whether Yosemite Valley is inordinately active, I think the answer is “no”. Exfoliation is an ongoing process in Yosemite and seems to be relatively steady over geologic timescales. We have a decent amount of data now to support that conclusion but it takes a bit of work to get there.
The database of historical rockfalls in Yosemite (http://pubs.usgs.gov/ds/746/ ) spans 155 years, from 1857 to 2011. If you simply plot the number of rockfalls per year, the obvious conclusion would be that rockfalls are increasing, with something like 5-6 times more rockfalls now than in the 1970’s for example. However, this conclusion would be flawed for the simple reason that the database depends almost entirely on reported rockfalls, and the rate of reporting has increased significantly through time. The earliest reports were either from local newspapers or the journal writings of people like John Muir and Joseph LeConte, but nowadays reports come in a variety forms (including postings on SuperTopo!) and from a much larger cadre of people. The plot of rockfalls by year below shows this bias in the “steps” that are associated with certain milestones the park’s level of rockfall documentation, e.g. the founding of the National Park Service in 1916, the involvement of the U.S. Geological Survey in rockfall studies beginning in 1980, and the hiring of Yosemite’s first park geologist (me) in 2006. I am assuming that the cliffs are not responding with a vengeance to these milestones! Bottom line – we cannot safely interpret any change in the number of rockfalls through time from the database.
However, a bright spot in the database is that rockfall volumes appear to be less affected by this bias. Plotted below are the largest rockfall volumes by year, from 1857 to 2011 (note the log scale in the y-axis). Ignoring the years when there are no data, you can see that there isn't an obvious trend of volume through time; i.e., there are not more large rockfalls now than there were a century ago. The annual variability in this plot may actually be similar to the real annual variability of large rockfalls. Without a doubt the small rockfall events have been – and still are – under-reported in the database, but because these small events make up such a tiny fraction of the total cumulative volume this doesn’t seem matter much.
So rockfall volumes appear to have been pretty consistent over the past 155 years, but as I’ve argued here before that isn’t a very long time from the geologic perspective. Can we do better? As it turns out, Yosemite Valley is a great place to derive long-term rates of rockfall activity because we know (1) the Valley was glaciated during the last ice age, (2) this glacier probably removed any previous talus, and (3) the Valley was free of ice by about 15,000 years ago, resetting the Valley floor. The talus that flanks virtually every major cliff in the Valley has accumulated in the past 15,000 years. Here is an example of what the Glacier Point Apron talus looks like in filtered lidar data:
We can measure the volume of the talus piles and divide by 15,000 years to come up with a long-term rate of rockfall activity (volume/year). When we compare these long-term rates with those derived from the historical database, they are basically in the same ball park, suggesting that rockfall activity has not appreciably changed through time.
This “steadiness” in rockfall rates through time makes sense to me because Yosemite is an exfoliating landscape. When a slab of rock falls away it reveals a new cliff surface, which progressively weathers and fractures and eventually fails again. Climate plays a role here, but ultimately the pace of exfoliation may be dictated by the properties of granite, which shouldn't change with time.
So those are my thoughts on whether the Valley is inordinately active now as opposed to back in the day. Next up – rockfall seasonality and triggering…
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gstock
climber
Yosemite Valley
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Jan 27, 2014 - 02:59pm PT
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One of the neat things about the Yosemite rockfall database is that it includes a lot of information about antecedent environmental conditions and probable triggers. We’re still analyzing this information in detail, but here are a few take-aways:
First, there isn’t really a pronounced “rockfall season” in Yosemite Valley. The plot below shows rockfalls by month from 1857 to 2011, and although there is some seasonality (there are more rockfalls in the spring and fewer in the summer), the difference isn’t huge. The fact that rockfalls occur pretty regularly at all times of the year suggest that there isn’t one dominant environmental trigger for rockfalls in Yosemite, as there might be in other settings.
We can also look at rockfalls by trigger. The first thing to note here is that more than half of all documented rockfalls either have “unknown” or “unrecognized” triggers. In the former case this means that there wasn’t enough information provided to evaluate triggering, and in the latter case it means that although the event was closely observed the usual triggers (precipitation, freeze-thaw, earthquakes, etc.) could be ruled out and another trigger wasn’t obvious. This is a particularly interesting subset of rockfalls, typified by those that occur on clear sunny days, and one we are actively investigating. If we ignore the unknown and unrecognized rockfalls, we’re left with the plot on the right, which shows the percentage of known and recognized rockfall triggers. Precipitation dominates here, which is good for climbers because we’re typically not out climbing during and immediately after rainstorms. The more subtle triggers that operate during good climbing weather probably pose a greater hazard – unfortunately those are the least understood of the triggering mechanims.
The NPS rockfall webpage is a good source of general information and includes links to just about everything that's ever been published about rockfalls in Yosemite: http://www.nps.gov/yose/naturescience/rockfal.htm
Greg Stock
Yosemite Park Geologist
(209) 379-1420
greg_stock@nps.gov
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stevep
Boulder climber
Salt Lake, UT
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Jan 27, 2014 - 03:14pm PT
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Very interesting Greg, though your rockfall by trigger graph appears to be missing the "Trundle by Climber" category.
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