Modeling a river in Archicad Part 3/3 (Rocky areas and Bedrock - Finishing the MAP)

At the end of the trilogy, of the river modeling videos in Archicad, is this video that shows how to model the rocky areas and the bedrock. Here you will understand the use of boundaries, (there are not only contours and ridges). Also Earth is actually a rock. And soil and sand are the results of erosion. So the only continuous Mesh sould be the rock. It comes to the surface where it is to steep to keep the soil or where the action of water or wind is to strong mountaintops, riverbed. The sand is generally