Firstly, that's a really cool map so far! And welcome to the forums
So, plateaus… Well, you've already drawn the hard part To close it off, a slightly jagged line will be all that's needed, maybe with a few small peaks here and there to match the look of the front. But just imagine that you are standing on the plateau, or hovering a little bit above it; all you'll be able to see is the edge. I can do a paintover on your map if you want, to demonstrate, though Psylence already showed the basic idea! (A masterpiece by the way, Psylence! )