Castlewood Canyon Rock-fall (Cliff-fall)

I was running around at Castlewood today and noticed that the cliff, climbers left, of Morning Sun Wall had calved off and tumbled down the hill. The section right before the draw.

Missing cliff face.

Cliff debris.

Broken Castlewood cliff.

Large rock.

The rock that bowled down all the trees was about the size of a box truck. This would have been neat to see ( from a safe distance.)

What wouldn't be neat to see would be the sketchy semi-detached section of cliff to climbers right of Morning Sun Wall come down. There's trail under that one.

Garrick- that happened this Summer. We've been wondering what happened, as that was an explosive rockfall, but the cliff there was maybe only 20' high. Everywhere else in the canyon, the blocks just seem to sort of slough off and slide a little downhill. But this thing went with a boom.

I'm curious; what is an explosive rockfall? Thanks, Jane

AKA: Crublewood Canyon

Chosslewood Canyon? Can't believe this happened!

Saw this the other day (1st time in castlewood in quite some time) and was curious as to what happened as it looked massive from across the canyon. Crazy how it appears to have been a major wall explosion and not just a few pieces peeling off.

Here is a photo from the Buoux Block area near the Fall's Wall on 11/24/2012:

Rockfall at Castlewood Canyon. Just past the Morning Sun wall. Apparently happened sometime in Summer 2012. More info here: mountainproject.com/v/castl…

and a photo for a larger perspective (Fall's parking lot on the RH bottom side):

Crumblewood!

^^^^See what I mean, Jane? That was a relatively short section of cliff, but the debris field is huge and goes right up to the base. There was no real vertical footage for the falling block to explode like that, it was already on the ground. Something happened here that caused -for lack of better terms- an explosion. Chosswood does not have any debris/talus fields like the one that was made here. Plenty of boulders, some huge, that have calved off the rimrock and tumbled downhill; but mostly intact.
I would love to hear from a geologist-type on what might have happened here, but MP.com seems to be getting more and more devoid of interesting people.

Thanks, Mike. I'm not sure what you mean by this sentence "There was no real vertical footage for the falling block to explode like that, it was already on the ground."

Let me know if anyone has any theories as to why that huge fall happened.

Jane

Jane- excuse any incoherencies from me, I am recovering from a serious heart event last week and and not really all together 100%.
The block that 'exploded' was simply the front of that section of cliff. It did not fall vertically through space. Think of real crags, you know 100's of feet of face that blocks can whip down from; then crash and explode on the deck. This thing was on the deck already, and just seems to have exploded anyway. if you look at Andy's picture, to the right you see what typically happens in that canyon: blocks just sloughing off the rim rock and sort of rolling some down the hill.

I'm a geo engineer type.
Would have to verify it in the field, but an initial guess is that it is a fault zone, leading to extra pore water pressure (the burst) and weathering (why the grain size appears more clayey than the surrounding arkose conglom/coarse SS).
If it happened in the summer, there was probably a large buildup of water (pressure) in the slope during the monsoons localizing in the fault zone (basically a bunch of cracks that allow water to flow easier).
In addition, the bend in the road and topography seems to lend credence to a fault zone.

while it may look explosive, from what it looks, nothing special needs to happen to make that happen. When rocks fail and fall, they can crush and break into smaller pieces like that pretty easily. Water may have been a contributing factor, but a large build up of pressure behind a fault that is at the surface (and next to a cliff face) seems very unlikely.
If you look at the pictures, there are other boulders and debris from past failures that go right down to the slope to the same distance.

Mikey Lane, is this the same area we crossed a couple months ago? I hope you are doing well.

Yes it is T. Maybe you can vouch for how bizarre this event was, not typical erosion or rockfall for this place at all.
My prognosis is a bit sketchy, btw. Been told to have my affairs in order, just in case.

Look at the block size of the failures. Yes, they can break into smaller pieces, but the sizes of the failures are significantly smaller. Granted "explosive" is a bit misleading. In addition, look at the nature of the cliff face - no sharp quartz and feldspars - they have been weathered to smaller grain sizes. Also note the tree root growing along the discontinuity. When a stream removes material, discontinuities/joints tend to form parallel to the stream as the removed material no longer exerts a horizontal pressure (note the fracture orientations still visible on the wall). Water can build along the parallel fractures. A significant inflow of water can occur, as well as a buildup of pressure, especially with clay filling up the fractures. The pressure buildup doesn't have to be super large - just enough to send the factor of safety above one and allow for failure. Also, look at how the failure corresponds to the bend in the river.

Also, I'm not saying that this is what happened. Just that it is possible and more than likely in my experience.

Ben Beard wrote:while it may look explosive, from what it looks, nothing special needs to happen to make that happen. When rocks fail and fall, they can crush and break into smaller pieces like that pretty easily. Water may have been a contributing factor, but a large build up of pressure behind a fault that is at the surface (and next to a cliff face) seems very unlikely. If you look at the pictures, there are other boulders and debris from past failures that go right down to the slope to the same distance.

This is really interesting. I have to get out there to see it! There are NO faults in the area (see map excerpt below; the Castlewood Conglomerate is light purple marked Tcr). There are many open fractures parallel to and behind the cliff face and orthogonal sets connecting those to the cliff face (big enough to crawl inside) so I doubt water could build up to cause the instability (and it was an extraordinarily dry year). The Castlewood conglomerate is a broad alluvial fan that incorporates clasts (cobbles down to silt and clay size) from the mountains to the west, including Idaho Springs metamorphics, Pikes Peak Batholith (K feldspar granites), Silver Plume (Al-An Granites), even Pennsylvanian Fountain formation red arkosic cobbles. The Caslewood alluvial system incised into the Dawson Sandstone and Wall Creek Volcanic Tuff, adding lots of clay and volcanic cobbles to the mix. The Morning Sun area has always felt a little hollow and unstable, and the unweathered rock in the photos looks very clay rich. The roots and maybe expansive clay may have had something to do with it, but it has been a dry year. Very cool (as long as I wasnt climbing it at the time)

Castlewood geologic map.

Consider your map scale and level of detail. You sound like a geologist - would you want to put your name on a baseline report stating there are no faults due to that map interpretation? I'd want to take a look at it in the field. Edit - though I should state that it doesn't have to be a fault, just some sort of discontinuity. Good point.

I do like the depositional subenvironment suggestion too though. Could be a weaker deposit, which leads to the drainage location in the first place.

Also, yes, I'll still maintain that water can build up behind the fractures enough to cause failure. We've got plenty of case studies for it.

Greg Twombly wrote:There are NO faults in the area

I say that having verified the geology in the map, walked the outcrop on the canyon rim and climbed there many times. I didnt see evidence of offset on any of the fractures in any part of the canyon but one. There is some offset in the Castlewood Conglomerate, but not in the underlying Dawson, at Canyon Point, but that seems to be gravitational offset of the Castlewood when the Dawson is undercut. That may be what happened here, since the slope below the cliff was steep and the Dawson is not well lithified. Water may have played a role, but the Castlewood is very high permeability (Darcy level rather that milliDarcy) and I havent seen water in the open joints when crawling around there (except ice in the winter). The nearby Cave Wall (to the north) has a chimney entrance to the Cave. The Chimney entrance is at the intersection of face parallel and face purpendicular joint sets, again too open to allow water to build up.

Mike Lane wrote:...My prognosis is a bit sketchy, btw. Been told to have my affairs in order, just in case.

Are you at risk of exploding like that cliff face?

Ryan Marsters wrote:Consider your map scale and level of detail. You sound like a geologist - would you want to put your name on a baseline report stating there are no faults due to that map interpretation? I'd want to take a look at it in the field. Edit - though I should state that it doesn't have to be a fault, just some sort of discontinuity. Good point. I do like the depositional subenvironment suggestion too though. Could be a weaker deposit, which leads to the drainage location in the first place. Also, yes, I'll still maintain that water can build up behind the fractures enough to cause failure. We've got plenty of case studies for it.

It is highly plausible that some water/weathering and pore pressure could have contributed to the failure. "Explosive" levels, less likely.
There is no question that the rocks failed. Trying to prove some "explosive" force for the failure, good luck. Anyone check the USGS seismic records?

Rednecks from Franktown with a couple cases of M-80's?

Mike, sorry to hear about your health problems. Here's hoping for a good outcome, good luck man.

"Explosive" wasn't my word - I said it was misleading. Ran the thread past a few other geo engineering grad students at Mines and the consensus seems to be increased pore water pressure and water infiltration due to a higher concentration of discontinuities (faults/fractures/joints - unknown without field verification). Doubt seismic had anything to do with it; probably just a period of heavy precipitation.

Ben Beard wrote: It is highly plausible that some water/weathering and pore pressure could have contributed to the failure. "Explosive" levels, less likely. There is no question that the rocks failed. Trying to prove some "explosive" force for the failure, good luck. Anyone check the USGS seismic records?