Set a 12-foot two-by-six plank on edge, spanning the gulley and resting on piers one and three. Place another across the gulley and resting on piers two and four. Placement of the planks is temporary so that their length indicates the positions for the third and fourth piers. Locate piers three and four in approximate position on the other side of the gulley, but be prepared to adjust them several times in the next few steps.