Because of its tremendous strength under compressive forces, stone can bear an almost unlimited load when arranged in the form of an arch. The ancient Romans used stone arches for large bridges and aqueducts, and the tradition of building bridges in stone continued in medieval and Renaissance Europe.
Bridges varied considerably in their masonry techniques. At its most basic, a stone-arch bridge could be built using flat or wedge-shaped fieldstone laid up in the shape of an arch. Most bridge builders, however, used cut stone at least for the arch ring, since shaping the ring stones resulted in a more perfect arch. The sides or spandrels of the bridge were less critical; their function was simply to retain the fill for the roadway. Spandrels were often of rubble (randomly shaped stone) even when the ring stones were cut. In more ornate stone-arch bridges, the spandrel masonry was ashlar (rectangular-shaped cut-stone blocks).
Stone bridges cost far more than comparably sized wooden bridges. Towns built them only on their most important roads. In the second half of the 19th century, many stone arches were built just downstream from mill dams. Bridges in these locations were in danger of being washed out by flood waters bursting through the dams. Because they were so strong and durable, stone arches also were favored by railroads.
Most Connecticut stone arches used locally available material: brownstone in the Connecticut River valley, gray granite in eastern Connecticut. The elaborate urban stone arches built around 1900 sometimes made use of granite brought from other New England quarries. Brick arches, though far less common, were built on the same principle as stone.
Stone bridges could be built with locally available skills. Most Connecticut communities had both carpenters, who were needed for the arched-shaped wooden forms or centerings upon which the stones were laid, and masons, who laid the stone.