The snorkel transect beyond the relative safety of the docks is best done with at least two people and a floating diving flag or better yet a boat. Fast moving skiffs pose a definite menace to the snorkeler. The transect from the reef into the fore reef requires calm weather, a boat and knowledgable guide to go through the reef and approach it from the front side. If you wish to examine anything deeper than the very shallow fore reef, scuba gear and a dive master are required. Under no circumstances should one attempt to swim out through one of the passes in the reef to the fore reef without the presence of someone knowledgable about the waters. Currents in these passes can exceed the ability of even a powerful swimmer.

Click here for information showing the main points of the Outer Reef Lagoon, San Pedro and South, and the main cuts through the reef east of the island.

The beach sand (if there is no sea wall) consists of rounded grains of calcite from the skeletons of all the organisms that live between the beach and the reef. Geologists refer to it as coralgal sand because most of the grains are coral and algal fragments. If there is a sea wall, then the sand has been trucked in from excavations in the interior of the island and consists of varying mixtures of beach, shelf and lagoonal sands with all sorts of skeletal fragments.

Further seaward in about 8 to 10 feet of water the mud and sand gives way to Pleistocene limestone with a very thin sand cover and soft corals (gorgonians, sea whips and sea fans) are able to attach to the solid bottom and survive. In this more open environment starfish, sea slugs and gastropods are more common. In the vertically solutioned fractures or joints, fish and eels find habitats, and on the rocky bottom the tile fish will build its nest of stones. A lot of the lime mud and sand that is generated and transported into this environment filters down through these cracks into the cave system in the limestone. The environment remains about the same in the deeper part of the lagoon which also has a mostly rocky bottom.

In the back reef, carbonate sand again becomes thick and broken coral fragments become more common. Soft corals live in abundance and patches of Thalassia and algae again appear in larger numbers. in the immediate back reef there is a profusion of broken and dead coral fragments and scattered colonies of living coral. As this back reef material builds up it can also be colonized by living coral and be incorporated into the reef crest. The reef crest itself is dominantly coral and binding or encrusting algae.

The immediate fore reef (from 2 to about 15 feet deep) is sometimes referred to as the barren zone. This is the part of the reef that takes the constant pounding of the sea and consists mainly of large coral fragments. Small patches of encrusting coral and algae survive to bind the debris into a wave resistant mass. The reef slowly grows out over this shallow environment. The immediate fore reef is the site of numerous ship wrecks, as over the centuries many sailing ships were driven onto the reef by storms. This process continues today, but internal combustion engines and accurate weather predictions make it much less common.

In the deeper part of the shallow forereef marine life is again able to grow in profusion and the hard and soft corals dominate although algae is also very common. This portion of the reef is dominated by what is called spur and groove topography. The spurs are ridges that rise above the intervening grooves which are filled with white carbonate sand and scattered larger coral fragments. The spurs consist of hard corals partially colonized by soft corals and algae. The origin of the spur and groove topography has been hotly debated. Many feel that they are erosional ridges and gullies formed when this environment was dry land during the last ice age. Others argue that the movement of sand down the gullies prevents the growth of corals and other reef organisms which then preferentially build the spurs up above the level of the grooves. Like so many other controversies, the truth probably lies in the middle. The grooves may have originated as erosional gullies and then been magnified and preserved by the preferential coral buildups between them. The grooves tend to get deeper with increased water depth and at depths of 100 to 130 feet deep the grooves can be 30 or 40 feet deep.

As water depth increases, the amount of light that can reach bottom diminishes. As this occurs, coral growth is slowed and the massive and branching forms are replaced with bladed and dish shaped types that have more surface area to capture the available light. Below about 130 feet, the forereef drops away steeply with much less active coral growth as light penetration decreases and virtually ceases around 300 feet.