The metal Observation Tower at Big Stone Beach was one of five constructed in Delaware between World Wars I and II to reinforce nearby Fort Saulsbury. These towers formed an integral part of the fort’s fire control system.

The arch is composed of solid grey limestone. It is 215 feet high (55 feet higher than Niagara Falls) 40 feet thick, 100 feet wide, and spans 90 feet between the massive walls. The span contains 450,000 cubic feet of rock. If man had scales to weigh it, the mass would balance about 72,000,000 pounds or 36,000 tons. The rocks that compose the bridge are early Ordovician, about 500 million years old. The internal forms of these rocks, that fold and break in the layers, were imposed on them during the Appalachian Mountain building process toward the end of the Paleozoic Era, more than 200 million years ago. At its highest point, the bridge is approximately 1160 feet above sea level.