Frank,
That's the section of my layout that has been called Pinnacle Peaks State Park. It first appeared in OGR Run 258 (Aug/Sept 2012). That article covered benchwork, the basic hardshell scenery, and some details. It's overall size is about 5'x9'.
Another article, OGR Run 261 (Jan 2013), covered the methods used to produce the rockwork, which included a combination of plaster castings, carved pink Styrofoam, and chunks of white Styrofoam beadboard.
Both issues are available in the OGR Digital Library that can be accessed through premium forum membership. Back issues of the printed magazines may also be available if you give the OGR office a call.
While it was built to match the surrounding areas of the layout, the track is not connected to the mainline so it could be called an add-on module.
In fact, I had thought about putting it on casters, but was glad I didn't bother when I had finished emptying my third 60 lb bag of Structolite on the project!
It is fun to operate by itself for a change of pace on evenings when I don't fire up the big layout.
I've always enjoyed smaller layout projects. You get lots of satisfaction and an operating layout in a relatively short period of time. 
Jim
