Didn't you get the set of steak knives free when you ordered Bryce?

Oh! you mean chop in the program. Here's one way.

Press the "E" on your terrain to open the terrain editor.

The triangle on the top right of the center window gives a pull down menu. Click on solid and let go to make the mountain solid, otherwise  there will be just a hole in the shell if you chop anything out.

Click on the thing next to it to bring up a second window behind the center window. Click on the sliding bracket next to the bar on the right and drag down. This chops the top off the terrain. The image in the center turns yellow to show what parts you chop. Drag the bottom one up to chop the base. This gets highlighted in red to show what is removed.