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Lighthouses: 1 hen and Now	/)	^	a	\/	!//)■>	/	Page	5	of	7
tight and it had a slow rate of deterioration. A number of cast iron towers were lined with brick for additional stability and insulation. By the close of the 19th century, cast iron towers proliferated throughout the United States. The tallest cast iron tower was the Cape Henry light built in 1881; it is 165 feet and is still standing.
Crib foundation construction was used extensively on the Great Lakes. Wooden cribs were constructed ashore, towed to the site, and filled with stone. Once the crib had settled to the bottom, it was capped with concrete or other masonry. Frequently it was necessary to level the structure by adding weight to one side or another. From an engineering viewpoint, the two most significant crib lighthouses in the Great Lakes are Spectacle Reef and Standard Rocks. The first was completed in 1874 and the second 8 years later. Spectacle Reef is 10 miles from the nearest land and Standard Rocks is 23. Each required a number of years to complete.
Cofferdam construction was used where it was desired to build the foundation on a dry site and where it was not necessary to penetrate the seabed to any great depth. This method could only be used in very shallow water. The wooden walls of the cofferdam were constructed ashore, taken to the site, assembled into a box in the water, bolted together and sealed, and the water pumped. Workmen then entered this open-topped structure and prepared the foundation for the lighthouse.
Three lighthouses were constructed upon man-made foundations built upon underwater ledges, stone by stone without the use of a crib. They were Stratford Shoal, New York; Race Rock, New York; and New London Ledge, Connecticut. These sites were exposed to strong currents which these foundations had to absorb. These lighthouses are characterized by a typical light tower, which might be found at any shore site, placed on top of a massive stone foundation designed to absorb the fury of the waves. However, the tower itself was not designed to sustain the force of waves crashing against it, as is the case for the wave-swept structure. Each of these foundations was very expensive to build. This fact, coupled with the development of the submarine site lighthouse, probably accounts for the fact that only three of this type were built.
Cast iron revolutionized the construction of lighthouses in Northern bays and sounds because it significantly reduced the cost of building a lighthouse foundation in the water to a fraction of what it had been. The screw-pile structure had revolutionized lighthouses in the bays and sounds of southern waters; however, this technology was not applicable in northern waters due to its vulnerability to swift currents and ice. The traditional method of preparing a foundation on an underwater site in northern waters was by laying and interlocking a bed of large stones weighing from three to five tons each. But this method was time consuming and expensive. The 10,000 ton foundation at Race Rock, New York, took five years to prepare and cost a quarter of a million of 1875 dollars. And the lighthouse still needed to be built. By comparison, a hollow cast-iron shell could be sunk to the seabed in water up to 30 feet and filled with sand, rock, or concrete. Cast iron was selected because of its ability to resist corrosion in salt water. A lighthouse, typically of cast iron, was then placed on top of the caisson although other materials were also used for the tower. By the 1870s these caisson lighthouses had proliferated through the northern waters of the nation. Approximately 50 caisson lighthouses were built.
Most cast-iron caisson bases were simply lowered to the seabed and filled with concrete. However, sites where the seabed was uneven, unusually soft, or exposed to strong currents and waves, required special preparation. For these lights, known as submarine site
http://www.uscg.mil/hq/g-cp/history/h_lighthouses.html
5/22/2005