You mean that's not finished already?
It already looks like a ledge spiraling into darkness
Hmm... well... the only way to improve it would be to get really nit-picky. Like...
It does look like there's a bit more shadow on the second ring of the actual ledge, in the top left corner. There's no overhang in that spot, and the darkest bits further down are on the right side and on the bottom right.
The uneven shading makes it look flatter than it normally would. If you don't want to mess with your cavern, I'd just try some layers of transparency with black to one size (play with the opacity a bit, something like:
You're also adding most of your depth by darkening the path as you go down. But the Ground at the top is darker than your first level of the "path"... so... it kind of throws it off a bit. I'd reverse those shades, if possible... lighten the ground at the top if not. (Red circles below)
Your rocks are really even in some places... the shape of the rock would change a lot as you get further down. (Green areas ADD to your perception of depth, purple areas DETRACT from it.)
Of course, this is all: 1) Personal opinion, and 2) REALLY nitpicky, because you asked how to do something you'd already done really well
It looks really great as is.