Reading a keystone
The keystone is the last stone set and the first one anyone looks at. Builders knew this, which is why it is so often the one that lies.
On most small bridges the keystone is not structurally special. A segmental arch distributes load along the whole ring; remove the centre voussoir and the arch will usually stand, sulkily, for years. What the keystone carries instead is information: a date, initials, a mason's mark, sometimes a coat of arms belonging to whoever paid.
Three things to check
| Feature | What it suggests |
|---|---|
| Projecting keystone, tooled face | Built or refaced after about 1760; earlier work rarely bothers |
| Date carved in relief rather than incised | Later repair — relief carving on soft stone does not survive two centuries of frost |
| Keystone of a different stone | Replacement, or a deliberate contrast; look for the same stone in the parapet copings |
The honest approach is to read the keystone last. Start at the springing, where repairs are rare and expensive, and work upward. By the time you reach the centre you will already know roughly when the bridge was built, and the keystone will either agree or tell you something about the people who later owned it.
The bridge in the sketch on the front page has a keystone dated 1811 over a ring that is plainly older. Nobody I asked knew why. That is normal.