crushed

[theeagle]

was it crushed rock or just round rock.
crushed rock will compact and give a fairly solid surface.
putting cement into it will create lumps that will popup when other parts sink down.
you could get some rock waste(also known as decomposed granite,etc.) and mix that in to create a road mulch consistancy that will firm up the rock at that thickness.

Remember the past or all is lost.