Snowslogger wrote:

MoapaPk wrote:Mass Wasting

Bioturbation! (does that cover the brother-in-law with the jack?

yes

So much fun mixed in with correct answers. I had it all worked out but the dang link was down.

First block:
Anyway, not erosion as the correlation of the block to its pedestal is a match and bedded sandstone whose bedding plane has not significantly shifted will not erode or cleave so neatly in the vertical plane unless its undergone metamorphosis. If it had cleaved that way, then there would be a remnant on the left of the block.

So as the plane looks just off the horizontal, leaning down in the direction of travel, then it would be the freeze-thaw with the assistance of gravity. Although an earthquake could be a factor: shakes can be vertical, or horizontal (the more destructive), where the ground would move and the block resist movement by its own inertia (meaning it stayed roughly where it was).

On the second sea pillar, that's erosion in the vertical plane by gradual flaking. Geological formations can be bedded in one plane and split in another. The stack is metamorphic rock (having undergone heat transformation, manifested by the partial hexagonalisation). Still, there is an apparent match between the two halves and shift may have occurred if and when the plane of the whole cliff face was different - due to plate tectonics.

FortMental wrote:

It's not a commonly observed erosive process.... but I've seen it before. Definitely not an earthquake.

If you notice closely in the picture, the fracture plane is not truly horizontal, and the upper block has slid in the downhill direction. I'd agree that erosion started it by washing out a weak thin layer at the fracture plane. I'd also say, though that occasional minor tremmors probably have helped it slide downhill over time. Any constant force (like gravity) would either get it moving and keep it moving or would not get it moving until aided by some other force (like chollo brother-in-law with 10-ton floor jack) and would cease to move it once the other force was discontinued - in short, without some intermittent force, such as occasional tremmors, it would either have slid off entirely or not moved at all. That's my story and I'm sticking to it!

No wonder she wears so much make-up.

Actually, mass-wasting is a catch-all term that includes all the plausible explanations given above.

No horizontal shift, just crack developed from erosion of weaker sections of the rock. The right-side profile of the block and its pedestal seem to match but they were never one continuous face. Imagine if the block was "un-shifted" to the left, it would surely topple because of imbalance; this refutes the theory that it has shifted.

To my eye it looks like it has not shifted but the two sections are coincidentally the same width. I'd need a much closer look before betting money.

the freeze thaw is the most likely explination for the movement of the block. Seasonal ice build up followed by thawing in a small fracture in the rock caused outward stresses on the rock. Thus causing the crack to widen over a long period of time. The shift in the block is due to the angle of the fracture along with the freeze thaw slowly moving the block over time.

If you notice closely in the picture, the fracture plane is not truly horizontal, and the upper block has slid in the downhill direction.

This could be right on, but it could be more a function of the camera being tilted rather than the rock being tilted much. The theory does make a lot of sense though.

Anyway, all the rock bedding in the area is just about as close to horizontal as it gets in nature, though it probably isn't completely plane of course (some have even claimed that the rock has shifted left in historic times, but this doesn't make sense?).

To show how horizontal the bedding is in the area, this rock is just to the right of the other one (you can just pick it out in the lower left of the orginal photo in the thread).



More:



Erosion wise, I guess one way it could have happened (besides frost heaving) is if the left wall (or wall facing the camera) of the tower sheared off, but didn't quite take the black with it but moved it enough to shift.

To my eye it looks like it has not shifted but the two sections are coincidentally the same width. I'd need a much closer look before betting money.

Here's a closer look to zoom in on:

http://www.summitpost.org/images/original/572107.JPG

Anyway, interesting stuff. Thanks for all the replies.

SpiderSavage wrote:To my eye it looks like it has not shifted but the two sections are coincidentally the same width. I'd need a much closer look before betting money.

A closer look does reveal evidence of the lack of shifting (surfaces and details on the upper section that seem to corespond to others directly below them on the lower section), so I guess I won't stick to my story!

A major seismic event occurred along the Moab Fault six million years ago.

In more recent times, exfoliation of the Entrada sandstone could have been the dominant creative force behind what we witness here today.

lcarreau wrote:A major seismic event occurred along the Moab Fault six million years ago.

In more recent times, exfoliation of the Entrada sandstone could have been the dominant creative force behind what we witness here today.

Utah gets an average of 6 to 8 seisms of magnitude 3 of greater, per year. There have been about 16 sizable quakes (> 5.5 magnitude) over the last 150 years. It doesn't take much to send a rock moving under the influence of gravity, especially if you have years to do it.

I don't follow the exfoliation argument.

Really folks, look at the mass-wasting link. It's sort of a "duh", in that the terms are self-referential.

Buz Groshong wrote:

SpiderSavage wrote:To my eye it looks like it has not shifted but the two sections are coincidentally the same width. I'd need a much closer look before betting money.

A closer look does reveal evidence of the lack of shifting (surfaces and details on the upper section that seem to corespond to others directly below them on the lower section), so I guess I won't stick to my story!

Do you guys really think so?

FortMental wrote:Earthquakes can move a lot of rock but don't hold a candle to rain.

A candle in the rain?

http://www.youtube.com/watch?v=GQwKDijLqyg

Day Hiker wrote:Do you guys really think so?

Hey, when shifted back into place, that thing resembles an inverted golf club from Caddyshack!