DIY - Rock Molds?

[nachoman]

Harold (hminky) had a good tutorial using ceiling tiles on his website, and that is probably the one you are thinking of. Unfortunately it seems to be offline (!).

All rock types can show layering. I've seen granite weather into a layered appearance because of a paralell fracture pattern and successive lava flows or volcanic ash deposits that make a distinct layered pattern. And I have seen sedimentary rocks that make relatively massive homogeneous cliffs, especially limestone and sandstone. The layers come about when the sedimentary beds have a variation in texture from coarse to fine or a variation in composition. Often sandstone has coarse and fine layers, or has mudstone intermixed, and as the different layers weather differently, they produce the layered appearance. Geologists recognize rock units as ledge formers or slope formers, with weak layers tending to form slopes and durable layers forming cliffs. Metamorphic rocks can also show a strong layered appearance, but in those cases the rocks are almost always folded or tilted.

The height of your individual "layers" really depends on the rock units you are modeling. In the Grand Canyon for instance, many of the rock units are hundreds of feet thick, and are relatively homogeneous from a distance. That means your scale "layer" needs to be hundreds of feet thick. Other rocks may show distinctive layers on a scale of only a few feet. That ceiling tile method looks good for rocks that have narrow layers, but I would try hand carving foam for rocks that have thicker layers.

For non-layered rocks, plaster castings in rubber molds is perhaps the best compromise for speed and cost. The drawbacks are that they are often too jagged and generic looking. That's what I used on one half of my layout. While they look adequate, they also look rather generic. I just didn't want to spend the time carving a more accurate representation out of foam blocks. The rock molds (and crumpled aluminum foil) often look best in places where the hillside has been blasted away for a road or railroad cut. Of course, how you arrange and paint your rock castings can also go a long way into improving their appearance.