Geograph - photograph every grid square

This extraordinary double-decker bridge was built to carry the main drive to Letterfourie House across the gorge of the Burn of Letterfourie. It is a Category A Listed Building, and on the Buildings at Risk Register. Letterfourie House was built in 1772-1773 by Robert Adam for James and Alexander Gordon, and the bridge is thought to have been built in the late 18th century, also by Robert Adam. However it has been suggested that the lower arch is an earlier bridge, and that the upper tier was added because the approach to the older bridge was too steep for a horse-drawn carriage. In that case, however, why was the lower bridge built wide enough for a horse and cart?