Hmm. Top-down mountains are really tough. The only way I know of to actually get it right is to shade them by hand, which can be tedious and maddening. You could fudge it by doing a beveled path (or fifty). And I'm totally guessing on whether that is available in Illustrator. Even if it is, it would be really simplistic. For an idea of what you're shooting for, check out the mountain section of Torstan's "tips and tricks" tutorial. At least then you'll have steps to attempt to replicate?