Over the past several million years, seas rose and fell many times, sometimes making Florida very small or covering it entirely, because what is now Florida is only the top of the larger Florida Plateau, much of which lies under as much as 300 feet of water and forms the continental shelf. Approximately 100,000 years ago, no land existed where the Keys are today, and coral polyps built underwater reefs in clear water during the Sangamon interglacial period, when ancient glaciers melted and water levels were 25 to 30 feet higher than today; these reefs included the present reef about five miles offshore and an inner reef that would become the Florida Keys. By about 17,000 years ago, during the Wisconsin glacial period, colder climate and the accumulation of seawater in the Polar Regions lowered sea levels by up to 300 feet, enlarging the continents and shrinking the oceans, so that the Keys were only the tops of hills, Florida was more than twice its present width, there was no Florida Bay, the Keys were part of the mainland, and the channels now crossed by bridges were rivers, streams, and creeks flowing to a distant sea. Shortly after this, the first humans came to south Florida and settled on beaches and streams near the ocean, leaving paleo and archaic Indian sites that are difficult to find because they now lie under many feet of water and may be part of the reef. Around 10,000 years ago, as the last Ice Age ended, sea levels began to rise again; about 4,000 years later the outer reef was submerged and coral proliferated, the old inner reef became the Florida Keys, and 2,000 years after that water began to cover lower areas west of the Keys, creating the Everglades and Florida Bay. As sea levels continued rising, Florida Bay reached its current depth, Hawk Channel deepened, more of the Keys became submerged and assumed their present configuration, and the outer reefs continued to expand, with some formations barely below the surface. The Upper Keys are made of Key Largo Limestone, a white-to-tan, moderately hard limestone composed mainly of coral skeletons along with invertebrate shells, marine plant and algal debris, and lime-sand; in times of rising sea levels, dead reefs provided foundations for new coral growth, and through successive phases this limestone accumulated to 75 to 200 feet thick in places. Key Largo Limestone underlies the oolitic limestone that forms the surface of Miami Beach and the Keys from Big Pine through Dry Tortugas; unlike coral-based limestone, oolitic stone formed from small calcareous egg-shaped ooids that settled in layers, then were cemented together when falling sea levels left them dry and rain dissolved upper layers and washed lime downward. At Windley Key Fossil Reef Geological State Park, layers of limestone and fossils show how the Keys formed over thousands of years, and the site was later quarried for limestone used in building the Railway until 1968.