North Arch and South Arch are clearly” twins” and thus have a common genesis. The ridge on which they occur is a major drainage divide, and the rock that forms the lintel of the arches is the most erosion-resistant stratum in the region. Headward erosion on both sides of the drainage divide has shaped the arches. Eons in the future, continued erosion should remove enough support from the arches to cause their collapse.
In most dimensions, South Arch is the larger of the two. At one point, the deck is 103 feet(31.4 meters) high with a clearance of about70 feet (21.3 meters). North Arch, in contrast, has a height of about 62 feet (18.9 meters)with a 51 foot (15.5 meter) clearance. Each arch is a major topographic feature. On the north, South Arch blends into a rockshelter and a widened bedding plane. Thus, measuring the breadth of the span involves subjective judgment. The span is clearly greater than 135 feet (41.1 meters). North Arch has a more easily defined span, about 93 feet(28.3 meters). Each bridge has a near-perfect arch shape.
In the eastern United States very few bridges are as high or as broad as the South Arch. If both components of the Twin Arches are regarded as parts of a single landscape feature, then very few natural bridges in the world equal the Twin Arches in size.