The foam before gluing or rough shaping.
After sleeping on it, the best method was more obvious to me, and involved gluing every two inches of foam for the 6 inch cliffs and all three of the panels for the three inch cliffs.
Spraying adhesive on the foam.
I used 3M Styrofoam adhesive to glue up the foam panels. It sprays in a sort of stringy figure-eight pattern that is easy to control, and has basically no mist-like overspray that you get with most spray adhesives. If offers a long last bond, and is easy to cut through with knife, saw, and hotwire.
3M Styrofoam Spray Adhesive.
The springy "figure-8" pattern of the adhesive
sprayed onto the foam
Adhesive was sprayed on both surfaces to be glued together, a couple of minutes was allowed to pass, then the two pieces were matched up along an edge and pressed together.
I had previously collected about 40 photos of cliffs, offering different textures and shapes, and settled on about half a dozen as guides for the cliff shaping process.
I made two cliff side templates, one 3 inches high, and the other 6 inches, out of thin styrene sheet, which will make it easy to give the modules matching edges. The 3" template is identical to the upper 3 inches of the 6 inch template.
Three inch and six inch tall side templates.
Starting with the first 6"x12"x6" section, I carefully aligned the three two inch thick subsections, and drew the cliff profile on the edges. Note that the first 6 inch measurement is actually the depth of the module, and is a minimum dimension. Some panels were cut such that they were deeper than 6 inches to allow more variation in the finished cliff modules, such as the one depicted below.
The three 2 inch thick panels lined up with a foam backing panel,
and the template at the side. Note that the three lower foam pieces
in this module are approximately 9 inches deep, rather than 6"
A top profile was then drawn onto the uppermost section, and cut out with a hotwire,
The top profile drawn onto the the upper cliff section.
The foam panel was then cut along this profile line with the hotwire
The hotwired top panel was then place onto the middle 2 inch panel, the edge of the top panel was then traced onto the second panel. Next, the top profile edge was drawn onto the second panel and cut, creating a stepped effect when the panels are stacked. The same process was then performed on the bottom panel, yielding the first straight cliff section ready for the stone detailing.
This process was repeated for each cliff section until all were roughed out on the hotwire. The 3 inch tall cliff sections only required the top profile to be drawn on, and then hotwired once to get the rough shape.
A view of all of the cliff sections after cutting with the hotwire.
Example of how the outside corner, inside corner, and straight
section mate together. Note the stepped appearance of the panels
making up the 12 inch long straight cliff module.