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I think the real trick with terrains in Carrara is that you need lots and lots and lots of polygons. I made the included image by turning Rendered Mesh Density to 4096x4096 and turning off smooth normals. In Edit, I set the image size to 1024x1024, generated great peaks and applied the Erode 2 filter. You could probably achieve better results than this by messing around with erosion and other filters.