How did the slabs move since 1983?

Careful study of these quicktimes showing the apparent motion of the slabs between 1983-2005 appear to make sense based on erosion of the ground underneath the slabs.

However some questions can be raised regarding the apparent movement across the face of the supporting cliff wall.

The images below illustrate two things:

1) The photograph and models show protrusions of the sandstone cliff face that support the slabs. The protrusion supporting the center slab shows possible erosion to all sides except where contacting the slab. While the motion of the slabs appears to be tangent with the cliff wall, the angle of the protrusions are at an angle that would oppose that motion. Additionally, the protrusion have a small surface area that would support the slab. If the slab moved substantially from these locations, would the apparent erosion to the protrusions take place in only 27 years?

2) The 2005 model compares with the photograph as a reference to how the slabs contact the protrusions/cliff face.
Examination of the "1983-morphed" model provided by Western Mapping appears to place the stone slabs with a gap between them and the cliff wall, suggesting the slabs were further away from the wall in 1983?

click on images for pages with larger views.



Relative to these images and the image below, some measurements are taken:
1) the approximate alignment from north of the cliff face supporting the slabs (blue) = 21 degrees

2) the angle from north of the face of the protrusion supporting the east slab (green) = 39 degrees

3) the distance of travel for the selected point (red) between the 1983-morphed model and the 2005 model =

The point moved north 0.005 meters (5 mm / 0.197 inch)
The point moved downward 0.048 meters (4.8 cm / 1.89 inches)
The point moved east 0.025 meters (2.5 cm / 0.98 inch)

The linear distance travelled is a total of 0.054 meters (5.4 cm / 2.125 inches)
The angle from north travelled is 11.36 degrees





I don't know how much sandstone can erode in 27 years, but the ground can change a lot. How different is the ground around the east slab? 1983 (?) on left and 2005 on right. Did anyone notice that rock under the east slab then? (update: Anna says they did and that it did not appear to bear substantial weight of the slab.)