Definition: Generate a correct and concise answer for the question based on the words in the context.
Input: Context : Culham Bridge was built of stone between 1416 and 1422 to replace an ancient ford called Culham Hythe by a religious body known as The Brotherhood of Christ (later Christ's Hospital) who subsequently looked after its upkeep.
Question : What is the surface of Culham Bridge made of?
Output:
stone