by J. V. Reistrup
photographs by John Bildahl

Have you ever passed a channel marker and wondered what's holding it in place? Ever motor under a bridge and wonder what those massive concrete piers look like underwater? Ever pass a shoal lighthouse and wonder how they put it there? . . . Well, we have. So we've looked into the matter, nosed around a bit and buttonholed a few experts. And we devote the following pages to what we found out. Herewith, we unravel at least a few mysteries of the deep. Or of the shallows, as the case may be.

From Screwpile to Caissons

The first lighthouses on the Chesapeake were built as they had been for millennia, on land and out of mundane materials like stones and brick and mortar. But on the shallow Bay, onshore lighthouses proved of little use in warning skippers away from the shoals that jut out dangerously beneath the surface, often miles from the visible shore. So the U.S. Lighthouse Board had to figure out a way to build some lighthouses on the shoals themselves-and to make them safe enough for keepers and their families to live in.

What they came up with early on was what looked like a cottage on long iron stilts-the stilts being essentially giant self-tapping screws that could be augured deep into the Bay bottom by crews manning capstans. The last of those screwpile lighthouses still in service, Thomas Point Light [pictured at left], sits at the western edge of the Bay shipping channel off the South River near Annapolis, more than a mile from its namesake point of land.

Most of us know very well what it looks like above the surface; it's a rather charming six-sided white clapboard cottage perched on eight legs. Below the surface, it looks much like you'd expect: those long spidery legs reach down some 20 feet, where their threaded tips were bored through the muddy bottom into firmer footing by crews manning capstans. If the Bay were suddenly drained of water, Thomas Point Light would look more like an ornate water tower than a cottage on stilts.

There were of course many more screwpile lights on the Bay in the 19th century, though all but Thomas Point Light eventually succumbed to the elements or, in later years, were moved to safety on land. Ice was always the screwpile's worst enemy; more than one of the lighthouses was carried away by an ice floe, its legs sheared off as if made of straw. The Thomas Point Light, built in 1875, survives because it is surrounded by protective riprap and has an ice-breaker installed upstream-a concrete barrier with a pointed end that slices and deflects the ice floes before they reach the light. Although the light no longer has a crew (it's solar-powered and fully automatic) the old structure was recently restored and, for the first time in its history, opened to visitors-who can get to it by boat on tours arranged by the Annapolis Maritime Museum. If you want to get a close look at the submarine end of a screwpile structure, though, the other three surviving lighthouses are better suited, because they're on land. There's Seven Foot Knoll Lighthouse in Baltimore's Inner Harbor, Drum Point Lighthouse at Calvert Marine Museum in Solomons, Md., and Hooper Strait Lighthouse at the Chesapeake Bay Maritime Museum in St. Michaels, Md.

Looking for something more durable than the screwpile design, lighthouse engineers turned increasingly to caisson construction in the latter half of the 19th century. Indeed, of the dozen or so surviving shoal lighthouses on the Bay and its tributaries, all but Thomas Point are on caissons. Basically, although there were variations, one end of a fat hollow cylinder (initially cast iron, later steel) was sunk into the floor of the Bay, with the other end sticking above the surface. The water was pumped out, and the cylinder sank deeper as the pump drew sand and other material from the bottom. After workers climbed down inside to clean out the muck, the caisson was filled with concrete and rock, surrounded by stone riprap . . . and topped with a lighthouse a la the Baltimore Light [pictured at right] or squat tower, the Craighill Channel range light. In each case, what you'd see below the surface would be a massively thick metal-clad cylinder planted firmly in the bottom mud. If your x-ray vision were particularly sharp you'd see the concrete, sand and rock inside and maybe even some former workspace.

Although the method was more costly than screwpile construction, caissons held up better-though they haven't all been impervious to ice damage. The Sharps Island Lighthouse off Tilghman Island on the Eastern Shore was built in 1882 to replace a screwpile light that ice had carried five miles downstream, with the keepers aboard. (They got off okay when it ran aground). The caisson replacement still survives, though ice floes in the winter of 1976-1977 left it with a 15-degree tilt.

Channel Markers
Aids to navigation, such as the channel marker pictured here, are anchored to massive concrete blocks on the floor of the Bay with lengths of chain about three times the depth of the water-so they drift a bit. The size of the chains and blocks is proportionate to the size of the buoy. With the largest markers, those for the shipping channels, the combined weight of buoy, chain and sinker can be as much as 15 tons. The Coast Guard regularly maintains navigation aids with boom-equipped tenders. In areas prone to large ice floes, like the upper Bay, they substitute smaller and less vulnerable buoys in the winter [see "Buoy, Oh Buoy," January 2005]. Less frequently, the blocks and chains are hauled out too, so damaged links can be cut out and replaced.

Build a Bridge
If you could see what lies beneath the bridges across the Bay and its tributaries, you would find they rest upon a couple of different types of footings, dictated in part by the channels they cross. Both spans of the Chesapeake Bay Bridge, for example, sit on similar reinforced-concrete pillars-though the pillars look somewhat different because of developments in construction techniques in the years between 1952 and 1973, when they were built. In the center of the picture at left is a tall reinforced-concrete pylon that anchors the cables holding up the suspension bridge beyond it. To build these, workers would drive flat sheet-steel piles as a kind of fence all around the area, linked at their edges into something called a cofferdam. Then the water was pumped out, allowing crews to work as if they were on land, putting in reinforcing bars and concrete. When they were finished, the cofferdam was removed.

The deepest footings look like the pair in the left foreground, with their steel casings sticking up out of the water kind of like upended binoculars. Tom Jenkins, a structural engineer who worked on that second span of the Chesapeake Bay Bridge as well as the Francis Scott Key Bridge across the Patapsco River in Baltimore, the Naval Academy and South River bridges in Annapolis, and the replacement Woodrow Wilson Bridge now being built across the Potomac, says there is a common factor among all those bridges: mud. Each crosses a fairly deep channel, and where there's a deep channel there's deep mud-as much as 100 feet of it, as is the case in the Bay's main channel. And that, Jenkins says, was the greatest challenge for the builders of the Bay Bridge spans.

"The infill of soft material [mud] is 100 feet deep, and that's below 100 feet of water. So you have to penetrate first 100 feet of water and then 100 feet of mud, and that's a daunting challenge," he says. The piers for both spans of the Chesapeake Bay Bridge are "Potomac-type" piers, so called because the construction technique was first used to build the U.S. Route 301 bridge over the Potomac River near Colonial Beach, Va., in the late 1930s. After dredging out about 15 feet of mud, explains Jenkins, crews would drive in small steel posts and lower a massive prefabricated platform-a drilling jig, essentially-onto them. Then, using holes in the jig to guide them, they'd drive H-beam steel pilings deep into the Bay floor. "It's like threading a needle to get the piles in, and of course you can't see anything at the bottom of the Bay," Jenkins says, adding that hard-hat divers would have to guide them, mostly by feel. (Computers made the job easier for the second span.) The next step was to lower steel shells around sets of H-beams and fill them with what is known as tremie concrete-designed for underwater use. After each layer of concrete hardened, the tube would be raised and another layer would be poured. Voila-a 100-plus foot column of concrete, all fabricated underwater.

The Chesapeake Bay Bridge-Tunnel [pictured at bottom] is of course a different creature altogether-unique because its 23 miles (most of those over or under water) include so many different construction methods and structures: tunnels, trestles, manmade islands and earth-filled causeways. The American Society of Civil Engineers named it one of the seven wonders of the modern world after it opened in 1964 because of its unusual engineering features, usefulness to mankind and size. It has the added attraction of spookiness, when the highway upon which you're driving dives into a hole in the water.

And that, of course, the tunnel part of it, is what makes it unlike anything else in the Bay. If you could see through the water at the tunnel portions, below the Thimble Shoals Channel leading to Hampton Roads and the Chesapeake Channel heading north . . . well, you wouldn't see much. That's because the tunnel tubes, made of steel and reinforced concrete and prefabricated in 300-foot sections, were laid into trenches and then covered with tons and tons of concrete and then about 10 feet of backfill. This isn't a matter of tidiness; it's because the tunnels are filled with air and, as Jenkins wryly puts it, "With tunnels, you want to be careful they don't float."

The bridge portions of the project were built on cylinder piles like those of earlier bridges, but the whole structure required extra precautions because of the threat of hurricanes, northeasters and general Atlantic moodiness.

Signs of the Harvest
For recreational boaters, perhaps the greatest mysteries of the deep come in the form of commercial fishing gear-or, rather, the things that identify them at the surface: stakes, poles, floats, flags, orange balls, a two-liter bottle of Dr. Pepper belonging to someone named "Skeeter," etc. Each indicates a different kind of fishing operation-or not, depending on how much respect said fishing operation has for the local regulations. Obviously we can't speak with any authority about the dizzying variety of renegade markers you may encounter in your travels, but we can speak in general terms about what lies beneath the most common and presumably legal fishing gear markers.

Crab Pots
Anyone who's been out on the Bay for more than five minutes has seen a crab-pot float. Indeed, many of us have had the dubious pleasure of getting a crab-pot float and line tangled in our propeller. When they're not tangled in propellers, the float of course is marking the location of a crab pot-a wire trap, that is-which has been baited and sunk to the bottom of the Bay to catch crabs. During the crabbing season (April to mid-December), commercial crabbers check and rebait each of their pots once a day. The law requires that the crabber put his or her permit number on each float.

Pound Nets
Pound nets are under those fence posts you may see stalking across the Bay at right angles to the shore. They are always stationary, being complicated structures that take a lot of effort to build. Nets are typically submerged in 12 to 20 feet of water. Viewing them from the side, as the fish see them, you would see a maze of nets. From above, however, you would see a kind of arrow-shaped pattern.

One long straight net corresponding to the shaft of the arrow leads away from the shoreline and intercepts fish as they migrate. This "leader" or "hedging" points the fish into a heart-shaped net, which in turn leads into the crib, or pound, where they wind up. In Maryland, sites must be registered with the state, which provides maps of them (, search for pound net sites). The sites aren't always in use, though, so there are more of them on the charts than in the water as watermen rotate their nets among the poles they have staked out. Most pound nets are set and fished between April and November. Owners check their nets daily, using dip nets to collect the fish they want and throwing the rest back.

At night these structures are darn tough to see, so keep a sharp lookout if a chart indicates one is nearby. Despite their near invisibility to boaters, both Maryland and Virginia regulations for night markings are few.

In Maryland, stakes at each end of a pound net must be marked,  regardless of whether the nets are in the water, with reflective tape at least six feet above normal high water. The stake at the head of the net must also have a light that works between sunset and sunrise and is visible from at least a mile away on a clear night.

In most Virginia waters, marking requirements for pound nets are minimal. The owner's license must be attached to a pole so it is visible four feet above normal high water, but otherwise there are no requirements for lights or color markings-unless owners want to discourage fishermen from dropping a hook in their pounds, in which case they can enforce a no-fishing zone by posting 12-inch-square blaze-orange flags around them.

In the Potomac, the prescribed bands of reflective tape are red for the Maryland side and green for the Virginia side, corresponding to the colors used for aids to navigation.

Sea turtles posed a particular problem for a while with pound nets in the lower Virginia waters of the Bay, where some swim in while migrating up the Atlantic coast in springtime from their breeding grounds farther south [see "A Summer Guest," April 2007]. Scores of the air-breathing reptiles were trapped by the nets and drowned. After a two-year test, however, the National Marine Fisheries Service found the animals could escape if the nets themselves occupied only the lower third of the water column, so it issued a federal regulation to that effect last year for the affected waters-which include the Bay south and west of New Point Comfort, down to the Chesapeake Bay Bridge-Tunnel, plus the James and York rivers up to the first bridge in each tributary. Instead of mesh in the top two-thirds of the water column, there must be stiff vertical lines at least two feet apart so turtles can swim through. The rule applies from May 6 to July 15, but watermen can leave them in longer if they choose-they just cannot have deeper pound nets during that time.

Gill Nets
Unlike a pound net, which appears as a cluster of stakes, a staked gill net requires only two. Imagine a badminton net, except it's underwater and it's, say, 600 feet long. That's what staked gill nets look like. They're set at the edge of a channel, perpendicular to it, and snag a fish by their gills. Fish that are too small swim through the net's mesh, and fish that are too big don't try to. But fish of just the right size for a given net will get only their heads through-and then find themselves stuck. They can't go forward because their bodies are too large, and they can't back up because their gills get caught.

You'll see staked gill nets all around the Bay-though they're legal only in Virginia waters, as is the case with so-called anchored nets, which operate on the same principle but are held stationary with anchors or weights. Maryland allows only drift gill nets-which are like the anchored nets, except . . . well, they're not anchored but are suspended in the water on sinkers and floats. In both states-and on the Potomac River, which has its own fisheries commission-drift nets must always be attended. Virginia requires orange flags on gill net buoys and stakes, as does Maryland for drift net buoys. The Potomac River Fisheries Commission requires square red and triangular green flags depending on which side of the river they're on, matching the navigation aids.

U.S. Lighthouse Society, Chesapeake Chapter,
Lighthouse Friends,
U.S. Coast Guard,, click on lighthouses
National Park Service,,

Chesapeake Bay Bridge,
Chesapeake Bay Bridge-Tunnel,
Woodrow Wilson Bridge,

Fishing and Crabbing
Maryland Department of Natural Resources,
Virginia Marine Resources Commission,

Chesapeake Bay Bridge-Tunnel: