Just plotted out corner of backyard where I want to put a free form waterfall feature; approximately three feet across and four feet high made of locally "found" rocks of various lineage: river rock, shale for flat, etc. My question is . . . what is the best "cement" to secure the rocks? I will have several 12" x 12" cement squares as a pseudo sub-structure to achieve the approximate form and height of waterfall, then wanted to cover entire thing with rocks. Any insight would be greatly appreciated