Drywall mud doesn't have much strength unless it's backed by something solid, will crack if applied too thickly, and is soft and easily damaged. I used it to smooth the surface of a couple of roads, but I'm forever touching-up the colour where the tip of the vacuum cleaner hose touches it during the annual (well, maybe not that often) layout cleaning.
For landforms, I use Durabond 90 patching plaster, applied over aluminum window screen. It can be mixed as thin or thick as desired and it will still harden in approximately the 90 minutes stated in the product name (it comes in various setting times, as indicated by the number following the name). It can also be applied as thickly or thinly as you wish, and it won't crack. Once fully-hardened it is extremely strong, and it you wish to make changes to it, a hammer or sawz-all is required. I colour mine with flat interior latex house paint, thinned one-to-one with water. This is easier to apply than unthinned paint, dries quickly, and covers well enough that you can apply ground cover over it with no further preparation. It also won't re-soften like drywall mud when you're adding water-based scenery.
Here's a partially-finished area, with the bare-but-coloured Durabond visible, along with some sceniced portions. The area to the right is in a corner, and I supported myself on the hill at left (minus the trees) while reaching in to apply ground cover and trees.
This river is also Durabond, applied over a 3/8" plywood riverbed, with a depth of no more than 1/8". After colouring with full-strength latex paint, I gave it three coats of high gloss water-based urethane, and it's extremely durable, requiring only an occasional dusting. Many times, cameras are placed directly atop the "water" for photo taking, with no signs of scratches, chips or cracking.
You can get Durabond at most home improvement stores, in 5lb. boxes or 33lb. bags.
Wayne
