In this situation, excavate the proposed garden area about 12 inches deep. Then lay down four inches of rubble, clinker or coarse stone. Then mix the topsoil with the required quantity of sand before replacing it. To avoid the possibility of nearby tree roots or shrubs encroaching on the rock garden, bury a metal strip about 12 inches wide between the offending roots and the rock garden. Because sandstone is porous, it should be your first choice.
Limestone is less satisfactory because it is less porous, but well-weathered limestone is striking.
Such stone is readily seen along the face of the Niagara Escarpment. Use granite rock to simulate a glacial till, or an area in nature where glaciers deposited rock and debris. In your garden, group rounded weathered rocks of various sizes and shapes and surround them with smaller broken, cracked stones. This type of garden calls for some special attention to get the alpine plants to grow in the midst of the rocks. You can also use lightweight rocks known as tufa and pumice for smaller rock gardens.