Recently a demonstration module was completed for the Hurstbridge Model Railway exhibition. The module shows various stages of construction using foam cut, glued and shaped. Next a layer of goop (see below) was added followed by painting to represent bare clay.
Goop was also used to start the creek running under the bridge on the shelf layout. Cardboard strips were hot glued in place. Next a layer of goop without the Vermiculite was used.
This recipe came from Mark Laidlay and I have used it successfully.