A lot of the final look of a stone bridge, and how satisfying the whole structure is visually, is determined by the pattern of building stones that form the approach to the arch (the skewback) joins up with the individual voussoirs. Care should be taken to find shapes that match. This can be difficult because the tops of the radiating arch stones create acute angles. These triangular shape spaces that are difficult to fill. This can be a problem both visually and structurally.
Too many small stones at the meeting line of the arc and the skewback will look busy and will be more likely to fail over time.
The stones in a bridge have to be laid into the structure much like a dry stone wall. There should be no 'trace' stones or 'shiners' (split stones that are taller and thinner than they are wide and deep). In other words and stones used in the bridge should have their longest lengths oriented on their flat and perpendicular to the plane of the bridge.
Stones that adjoin the arch need to merge nicely, but also be laid long enough into the body of the bridge so as not to be likely to come loose and fall out. Again, if too little attention is given to the joint where the horizontal builder stones and the voussoirs meet the structure will look crude and poorly settled.
In the photo above the stones are being built up over the arch using a combination of level bedded quarried limestone and local granite fieldstone. Coursing helps to keep things looking tidy.
The cope stones over the top also form the outer edge of the pitching of the walking surface which we will discuss later.