Calstone 129000092 18" x 12" x 8" Allan Block Stone 12 Degrees Gray - The Allan Block Collection can create more useable space or multi-level patios. With walls built as tall as 50 feet, Allan Block has set the standard since 1989. When using the Allan Block collection, you can claim more space by turning sloping yards into beautiful gardens and planted terraces. With different shapes and setbacks available, you have many design options. Upgrade your backyard space and extend your home outdoors adding beauty and functionality.