DIY - Rock Molds?

[nachoman]

Glaciers are very effective eroding agents, and are pretty non discriminatory at rounding over any rock type. The action of a glacier is likely to obliterate many textures inheirent to certain rock types, as the glacial features dominate the overall landscape appearance rather than the rock features.

You may want to try rubbing a wire brish along your plaster rocks to give that rounded texture. Run the brish in one direction only, paralell to the path of the glacier. This would not only round over the jagged rocks, but also produce stiations indicative of glacial erosion.