If there are no ledges or rocks, excavation is required to get started. It's not just for drainage. If the rock garden is laid out only at ground level, then at this point there will be a break, which will greatly affect the moisture supply. If you want to get the best idea about rock garden then navigate to this website.
The foundation of the rock garden structure must be completely buried in the ground. The base is mostly made of stone and the statues in the courtyard are left for later.
Here small fractions, flat stones, soft stones or slate, any rock fragments not suitable for use above ground, can be used for "filling" purposes.
Old brick or stone scraps will answer; Slag (not soft ash) performs admirably. The largest rock foundation, the top of which appears above the ground, must be deep below the surface. Incidentally, excavation provides soil for use above ground.
Under normal conditions, when soil drainage is good, 12 or 15 inches is sufficient for this initial excavation; often less on slopes or on land. If the ground is hard, it must be well crushed with a pickaxe.
Many authors recommend digging three or four feet. It's much easier to recommend than to stick with – you're more likely to meet the recommender. Such depth is only necessary if drainage is abnormally poor.