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!