emitted meshes doesn't affect each other - why?

What I want to achieve:
My aim is to create a gravel pile.

There are a “cube”(emitter), a “plane” (as a slide) and another “plane” (as ground) taking place in my scene. The cube just emits little rocks, which are falling onto the slide. From there, they are gliding down the slide and are falling on the ground. Until now, everything works perfect.
So here ist the problem:
The emitted rocks does’t affect each other. They fall into one another so at the end all of them are lined up along the ground. the mesh of the rocks pierces through everything.
how can I achieve, that they fall on top of each other? - that their mesh is “solid”

can someone help?

One way is to use the game engine, allowing lots of objects to fall on top of each other. You can record each objects motion to IPO (blender 2.49) or to the graph editor (blender 2.5x) for further rendering.