The form used for supporting a dry stone bridge while it is being built can be held in place in a number of ways. Most often temporary, fairly thick, beams need to be used for the form to rest on.These beams or heavy planks need to be positioned in such a way that they can be dropped down and the form lowered and slid out sideways after the bridge arch is built and has become self supporting. In the photo above the beams that support the form are resting on big stones bedded in the creek. Wedges will be slid between the top stones and the beams to bring the form to the correct height. These wedges will be removed later when the dry stone arch is completed.
The form for Crown Bridge was supported by a crib of cedars. The posts in the crop running parallel to the direction of the bridge were knocked out after the bridge was completed so that the form could be lowered and taken away.
Springdale bridge in Port Hope was supported by 4 by 4s laid on 12 inch blocks. Later after the bridge work is done, the blocks will be smashed and the form will drop down.
On a couple of bridges I have used four pairs of these block wedges under the supporting beams.
Plastic is sometimes used between the wedges so that the wood doesn't bind when the time comes to spread them to lower the beams supporting the form.