Don't just perch the rocks on the surface of the soil - make sure there is a good percentage beneath the surface, too, and that the soil-filled gaps between them drain downwards at a backwards angle - i.e. under the rock - that way, the soil is less likely to be washed out in heavy rain. Also, don't spread the rocks too evenly - it won't look natural. Looking forward to some pictures of the work in progress, Charnwood.