Yay for debris! I'm actually pretty happy with the heap of rocks too. I'm a big fan of scattering bits of crap around a map to make it look well used. This is looking nice.

I'm not sure what type of overall look you're going for, but a little bit of shadow around the rocks might make them pop a bit more, or some texture/height to the rocks themselves. Even just a subtle emboss and drop shadow might do it.

cheers,
Meshon