The Iron Bridge over the River Severn in England is the first major bridge built entirely of cast iron, and it marks the beginning of a material revolution in architectural history. Until then, large spans were crossed in stone or timber; here, for the first time, the new industrial metal of the Industrial Revolution became the load-bearing system itself.

The span of about 30 metres is crossed by five parallel cast-iron arch ribs. What is striking is that metal joining techniques — riveting, bolting, welding — had not yet matured: the parts were assembled with joints borrowed from carpentry — mortise-and-tenon, wedges, and tongue-and-groove. In other words the first iron bridge was built with the logic of a timber bridge, but in iron. This moment of transition shows that the new material had not yet found its own language but had taken over the old one.

Cast iron is strong in compression, so the arch form (which generates compression under load) suited the material. The work, overseen by the Coalbrookdale ironmaster Abraham Darby III, proved that iron could serve as a bridge and then a building frame; it opened the way for the iron-and-glass stations, greenhouses, and towers of the 19th century. The Iron Bridge thus marks the threshold where architecture passes from the age of stone and timber into the age of metal.