[Dennis]

The "Create Convex Surface/Object" command will ignore surfaces that go "inside" what would be considered a shell of your selected vertices. It doesn't sound like that's the command you want, since you'll have many ins and outs in your ditch.

You could try selecting pieces of the ditch and use the "Create 2D Mesh". you might want to do this for one side, the bottom, then the other side, then merge the meshes together. Depending on how irregular the vertices in the ditch are, however, this could be troublesome.

I came up with an alternate method you might want to consider. It does require a free 3rd party plugin for AC3D called "Noise" that works on Windows or Mac. If you're on a Mac, we can try to get this available to you if you're interested in this alternate method.

The results shown here are with very little care for customization. The contours of the bumps seem to hint more at a path eroded by a waterway, but you can tweak the noise parameters and the main structure of the ditch to get very different results, probably very close to what you're looking for.

The method is explained in the attached images.