Here's how I did it. (This used to be so easy in Bryce! :-) This is a little complicated so bear with me.. Insert a terrain in your scene. In the terrain editor add a layer and generate a terrain. I used the Advanced>Fractal Generator. Choose Edit>Convert to other modeler and convert it into a vertex model. In the vertex model room it will be very large. I would leave it that way for the moment. Select Construct>Add thickness and only make it a little thicker. If the terrain is very large the default 2in should do. DON'T DO ANYTHING YET. After the thickness is added the top of the object you've created will be selected. Using the camera view and the move tool, hold down the alt key and move the top surface upwards a good amount, 3-4 times the height of the terrain. OK now we have a solid shape we can use as a boolean tool. One end gives us one surface and the other end gives us a mating surface. I made a cylinder, duplicated it and the tool and arranged them as in this picture for the boolean operation