This shows the basic concepts that I use. Various layers of foam board to build up a very rough surface contour. This is then smoothed down with hot knives, surform tools and whatever else it takes to get the surface close to the final shape. Roads, water drainage, cliffs, should all be roughed in at this point. This should be about 1/4 inch lower then the final contours. Sculpamold is then used to bring the surface to almost where you need it.
Ground foam and dirt will finish it off.

It’s hard to see, but there is a road going up this hill side. Later when more material is added it will stand out.

A sprinkling of dirt helps define the shape

This shows the 2 inch base better with a river channel cut into it. Then space for a dam and mill to be built later.