UP South Street Bridge (Woodstock)

Located in Woodstock, this unique stone arch bridge carries the former Chicago & North Western Railway over South Street. It is unknown if a previous bridge existed at this location. In 1867, a 16-foot skew stone arch span would be constructed to cross South Street. In 1897, this bridge would be extended south for a second track. Currently, the bridge consists of a 16-foot helicoidal skew stone arch, set onto stone abutments. The width of the bridge is 32 feet. The bridge utilizes wing walls, which extend from the structure at different angles. Wing walls for the northern track utilize a curved design, which was occasionally used on mid-19th Century stone arch bridges. Stone for the north track was quarried at an unknown location, while stone for the south track appears to have been quarried at Duck Creek, Wisconsin. It is believed that an unknown contractor constructed both halves of the bridge.

Helicoidal stone arches are a rare example of a skewed stone arch. On a typical stone arch bridge, courses of stone are laid parallel to the abutments, giving the structure a rectangular shape. However, skew arches are required to be constructed in a parallelogram shape, requiring precise stone cutting and placement. The helicoidal design uses a spiral pattern, which allowed for stones to be cut in a consistent pattern. Stones for the voussoir stones would be cut with curved surfaces, which formed parallel spiral courses. This design was invented by British architect Peter Nicholson in 1828. While requiring a greater level of mathematical calculations in designs, this design allowed for a simpler construction and provided a stronger structure than a ribbed arch.

The bridge across South Street is one of only a handful of examples of a helicoidal stone arch in the United States, and one of only two known examples in Illinois. In addition to the rare design, the bridge was widened simply by adding a parallel structure, and no stones cross between the two halves. This method of modifying and widening the structure is a testament to the durability and strength of this design. Stone arch bridges were popular with the C&NW, as the railroad had access to plenty of quality stone. This design provided a durable and easy to construct design. Since the 1897 widening, the bridge has remained largely unchanged. No significant repairs have been made to the structure, other than the addition of handrails to the bridge. The future of this bridge is uncertain, as it is believed that it may be replaced in the future. Overall, the bridge appears to be in fair to good condition, with only minor deterioration to the stone. The author has ranked the bridge as being highly significant, due to the age and unusual design.