All bridges rest on foundations that transfer loads from the bridge into the ground.
The type of bridge suitable for a particular location depends on length of span, terrain, available materials, and navigation needs. The arch shape channels all the weight from above to the foundations below. The earliest arch bridges were built of stone. Stone arches are built over a temporary wooden form called a centering. Once the last piece of stone, called the keystone, has been set in place the arch proper is finished.