The bridge's two towers were built by floating two caissons, giant upside-down boxes made of southern yellow pine, in the span of the East River, and then beginning to build the stone towers on top of them until they sank to the bottom of the river. Compressed air was pumped into the caissons, and workers entered the space to dig the sediment, until the caissons sank to the bedrock. The whole weight of the bridge still sits upon a 15-foot thickness of southern yellow pine wood under the sediment.