View from the downstream side
View from the upstream side
These images show that there is a modern, two-lane concrete bridge superstructure superimposed upon the underlying original 1-lane, 2-arch limestone bridge.
Most people know little of the underpinnings of society--what makes things work--and I suspect this bridge falls into the same category. I doubt whether even half the people who use it every day even know that they are driving on an antique (but fully functional) structure.
This is what the circa-1850 bridge would have looked like originally. This photo is of the nearby Social Island Road bridge, from a way cool site called Bridgehunter: