Rocks in the area are limestones and dolomites from the Ordovician Period, which implies deposition about 470 million years ago. The carbonates of the Natural Bridge occur within the Chepultepec Formation, making the buttress of the bridge, and the Beekmantown Formation, deposited conformably over the Chepultepec and comprising the span and the upper part of the bridge. Strata at Natural Bridge are close to horizontal, in contrast with dipping and folded beds as close as away along Highway 11, which points to the fact that structurally the rocks making the Natural Bridge are the bottom part of a syncline.