Brock Performance Turf Underlayment Brock is a patented high performance drainage and impact layer that replaces the stone bases commonly used under synthetic turf. Brock installs faster than stone requires a simpler drainage system is less expensive in most cases and makes the design and installation process easier for the architect and builder. Brock is a consistent predictable material that takes the guess work out of specifying stone. Brock improves safety by lowering the G-max only at high impact levels (i.e. during falls) without sacrificing speed and performance during regular play. So you get the best of both worlds faster more natural feeling play and softer landings.