Pisa Stone SIMPLE CONSTRUCTION AND LONG LIFE: Retaining walls built with Pisa Stone, lean at the proper angle to assure stability. This slope is formed automatically by the patented off-set interlock. A pad of compacted gravel in a shallow trench is all that is required for a Pisa Stone wall up to 4 feet in height. No other foundation is required. Blocks are simply laid dry, without mortar, one on another. A coping stone, also interlocked, provides a finished top. Pisa Stone blocks are supplied with splitting grooves. Easy breakage with a hammer and chisel produces a fresh cut finish. Special blocks provide a joint arrangement for inside and outside corners. These are supplied in 12" depth and interlock with Pisa Stone.