Go to a world viewer application (Google maps will work) and look at the tepuis in Venezuela. A tepui (or tabletop mountain) is a high plateau that's usually above forests and have steep edges. Take a look at an image search for "tepui" and you'll get a lot of ideas.

One way to show height is with something called ambient occlusion. Tall things block part of the sky (ambient) light around them, which results in what amounts to a dark glow near the base of the item. Another way is to add a drop shadow to show that the directional light is being blocked. Finally, consider a darker outline right at the edge to show the dark pit below the floating chunk. Mixing these techniques might give you a result like that below:
Azil-floater.jpg
The looks a little taller than the adjacent mountains, but you can probably fade out that effect a bit as you work with it.