General:
* Rock fragments
* Stone fragments
* Boulders
* Cobbles
* Gravel
* Rubble
* Scree (especially for loose, unstable rock)
* Talus (similar to scree but often used for a larger accumulation)
More specific:
* Shattered rock (emphasizes the broken nature)
* Debris flow (for a moving mass of rock and other material)
* Landslide debris (specifies the origin)
* Glacial till (debris deposited by glaciers)
* Outcrop fragments (emphasizes the source)
Figurative:
* Ruins (for a more poetic or metaphorical sense)
* Remains (suggesting something destroyed or broken)
The best synonym for "rocky debris" will depend on the specific situation and the desired level of detail.