Gray's Reef is a submerged hard bottom (carbonate-cemented sandstone) area that, as compared to surrounding areas, contains extensive but scattered rock outcroppings. The rocky ledges can be as tall as six feet but lie under 60 to 70 feet of ocean water. The rocky ledges are complex - they have nooks, crannies and bumps, and plenty of places for invertebrates (animals without backbones) to latch on to and for fish to hide in. The rocky places provide a firm base for a variety of invertebrates that live their lives permanently attached to the rock. These animals include bryozoans (moss fauna), ascidians or tunicates (sea squirts), sponges, barnacles, and hard-tubed worms. The complex structure provides shelter for mobile invertebrates such as worms, shrimps and crabs. Many fished shelter in the reef or hover in the water column above.
Algae (marine plants) and invertebrates grow on the exposed rock surfaces: dominant invertebrates include sponges, barnacles, fan corals and other soft corals, hard coral, sea stars, crabs, lobsters, snails, and shrimps. The reef attracts numerous species of fish that live on or near the substrate (benthic) or that swim in the water above (pelagic). These include black sea bass, snappers, groupers, and mackerels. Since Gray's Reef lies in a transition area between temperate and tropical waters, fish population composition changes seasonally. Loggerhead sea turtles, a threatened species, that use Gray's Reef year-round for foraging and resting and the reef is near the only known winter calving ground for the highly endangered North Atlantic right whale.
Gray's Reef is a consolidation of marine and terrestrial sediments (sand, shell, and mud) which was laid down as loose aggregate between six and two million years ago. Some of these sediments were probably brought down by coastal rivers draining into the Atlantic and others were brought in by currents from other areas. These sediments accumulated until a dramatic change began to take place on Earth during the Pleistocene Epoch, between 1.8 million and 10,000 years ago. It was during this time that the area which is now Gray's Reef was, at times, exposed land and the shoreline was as much as 80 miles east of its present location. As a result of this exposure, the sediments there became solidified into porous carbonate-cemented sandstone sandstone rock. As the glacial ice melted, the water flowed back towards the sea, filling the ocean basins back to their earlier levels.
Archeological research at Gray's Reef has found fossil bivalves and gastropods and mastodon bones located in this area that indicate that the reef was once a shallow coastal environment and an exposed land form.