In the Gulf of Maine, the coastal zones of Maine and Nova Scotia have the greatest density of marine organisms, the highest productivity, and the largest number of people dependent on marine resources for their livelihood. Most lobsters, sea urchins, clams, scallops, shrimp, and mussels are harvested from the coastal zone, which extends three miles from land. This zone also has the most diverse habitats of the Gulf of Maine. The habitat diversity — a patchwork of rock, mud, sand, and gravel deposits scattered across the seafloor — is a consequence of sea-level changes that accompanied the Ice Age. As the glaciers melted away, they left a mixture of materials underwater. These materials were washed by waves and currents as the sea fell about 10,000 years ago, and washed again as it rose to the present shoreline. This diversity of bottom habitats contributes to the variety of species found in the Gulf of Maine.