At the bottom of the terrain editor, you have a slider that goes from zero to 100, if you move those sliders, the one on the left up, the one on the right down, you will change the clipping planes positions, so you can put them almost next to one another to limit the carving. To be able to see the planes, you have to activate the "Show clipping planes" button, it's the 7th from the left at the top of the editor. Now that your planes are in place, you can use a 2D brush (I would recommend you create your brush yourself in Photoshop or the like, a square black brush would be best, and there are no such brushes already available in Vue). Before sculpting, you need to activate the "constrain to clipping plane"  box. Then use the "Raise" brush, but check the "Invert" box before starting sculpting.