Proline Decorative Concrete Systems 2LC 20" x 20" London Cobble River Rock Concrete Stamp. Prolines London Cobble Stone will bring the look of an English Garden to your back yard. This pattern offers larger, wider cobblestones that vary in size from 8 Inch x 9.5 Inch to a larger stone of 8 Inch x 8.5 Inch in a running bond pattern. The benefit of decorative concrete versus natural stone is lower cost, higher durability and inherent strength and endurance.