The terrain editor has a nice tool called the "Bulldoze 2 points". It will cut a down sloped trough between 2 points if one is higher than the other. It will even raise a ramp if you pull one point from up on a 90 degree cliff, then out and down to the lowest point like the ocean. Makes it very nice to raw cut canyons and gully's, and setup raw raised ridges for an extensive runoff area from a mountain.
This is my first cut with the bulldozer tool set to 3miles diameter. I'll make a pass over all three 5,000ft land bases under the 25,000ft barrier wall to raise buttresses and create runoff canyons. Then I will have to stop and eyeball all of it for a minimum of secondary cuts off each primary cut. I'll be using brush with a smoothing tool set to a very low feet per second to slowly act like erosion to blend these cuts down into something that looks like canyon terrain running off of a high mountain into the ocean. Once you get the hang of it, each primary cut is simply setting your elevation level about 10,000ft, use the W key to push to the barrier wall, then the S key to pull you away to the ocean 1 1\4 sector away from the base of the wall. It's just finger painting with a 3mile diameter finger tip. Kind of like building a sand castle with damp sand.
If you make booboo's, incorporate them as a feature into the landscape or, use the elevation tools to put the area back you booboo'd and start over. Worst case, immediately close the editor and do not save. But, if you have not been saving all along or set the auto save to something faster than 15 minutes. You loose all your work like I did yesterday when I made the terrain editor CTD. It was fortunate for me, I really didn't like the block of work I was doing just before the CTD.
First screen shot is half the distance from the wall to the ocean. Second is at the ocean and you can see the buttress and trough the bulldoze tool creates 3miles wide. This is cut No.1 on the first crescent shaped land footing and I have three of these crescents to cut before I then look at doing secondary cutting. By the way that smooth round over at the top edge of the 5000ft cliff is courtesy of running a global blur against the 16bit grayscale file before converting it to raw format for import into the terrain editor. I found if you don't, the edges of cliffs and the water just off the edge of land ends up with vertical spikes of land all over the place. And it flattens random vertical spikes of terrain on land bodies.
One of the real problems using the terrain editor is visual scale. That 25,000ft wall, the 3mile wide trough and the 5,000ft cliff do not look like they can be that large in those screen shots which are 1:1. You have to constantly give yourself reality checks about scale. A 100ft cliff in the editor looks like the width of a hair from the alt I'm working in the screen shots. The mistake is trying to make things look right from 10,000, 20,000 or 50,000 feet above the project land base. Then you run a build and login offline to find you have built a port with a 10,000ft half circle cliff wall around it. But, it looked right in the editor because the port object is on a 1x1mile tile.