Building a dry stone bridge -Part 9

Pre-shaped outer voussoir stones ( essentially, corner stones ) and inner voussoirs (any good parallel-planed, flat faced 'builder stones') are fit carefully together  laying them in courses over the wooden form. Each row of stones is supported by 2 x4s suspended over the ribs of the form. The 2 x4s are placed loose on the form as the courses are added making it easier to stand inside the form until the last few courses are set in place. These 2x4s will be easy to remove when the form is lowered slightly, making the centering much lighter to carry out from under the bridge.