Rising high above the floor of Lexington Canyon, this imposing natural arch was created by the forces of weather working slowly over a span of centuries. Lexington Arch is unusual in one important respect: it is carved from limestone. Most of the natural arches of the western United States are composed of sandstone. The fact that Lexington Arch is made of limestone leads to speculation that it was once a passage in a cave system. Flowstone, a smooth glossy deposit that forms in caves has been found at the base of the opening, lending support to this theory.
It is even possible that Lexington Arch is actually a natural bridge. The distinction: an arch is formed by the forces of weathering, such as ice, wind, and chemical breakdown of the rock. A natural bridge, by contrast, is formed by the flowing waters of a stream. It is possible that long ago when Lexington Canyon was less deep, the waters of Lexington Creek flowed through a cave in the wall of the canyon, in the process enlarging the tunnel that later became Lexington Arch. If this happened then the Arch is truly a natural bridge.
Whatever the case may be, the forces of weather continue to sculpt the Arch. The limestone is particularly vulnerable to the dissolving action of rainwater. As time goes on the rain, ice, heat, and cold chisel the Arch into a unique natural form that will continue to change with the passage of centuries.