Some dry stone bridges can be built without having to construct temporary bridges either side to work off of. This bridge in Russell Ontario had very little rise and the creek it went over was dry so our team could build the bridge and transport material across with no trouble.
Bruce's Bridge had a steady stream going under the form while it was being built. The old wooden bridge was used as the new temporary platform while we built the new stone bridge.
At Hubb Creek Bridge in Prince Edward County we laid planks over the big barn beams that were supporting the bridge form in order to walk and carry material across to the other side.
You can see here too that because our 2x4s extended several inches past the form, radiating lines for the alignment of all the voussoirs had to be set up with strings, rather than try to follow the lines drawn on the plywood.