About 35 million years ago a large meteor struck near what would eventually become the mouth of the Chesapeake Bay and forever altered the surrounding area. Between 10 and 2 million years ago, multiple ice ages caused a massive glacier to cover much of North America, expanding and contracting with the planet's temperature. Roughly 18,000 years ago, the glaciers began to melt and retreat, creating rivers and streams as water made its way to the coast, while warming climates brought hardwood species such as hickory, maple, and oak into a landscape once dominated by evergreen trees. Approximately 10,000 to 7,000 years ago, continued melting flooded many of the Bay's rivers, including the Susquehanna, Potomac, and James, and rising sea levels submerged the Susquehanna River Valley, creating the Chesapeake Bay. About 2,000 years ago, the Chesapeake Bay assumed its modern-day shape. The Chesapeake Bay is the largest estuary in North America, where rivers and streams meet the open sea to create a diverse ecosystem. Its watershed spans 6 states and all of Washington, D.C., covers 64,000 square miles, includes almost 18 million people, and supports over 3,600 species of plants and animals. Because it has the largest land-to-water ratio of any coastal body of water, actions on land have a major effect on the Bay's health. Maryland enacted the Chesapeake Bay Critical Area Protection Program in 1984 to protect this resource. Critical Area includes any land within 1000 feet of the tidal waters or tidal wetlands of the Chesapeake Bay and is important because it promotes sensitive and limited land development to protect water quality and wildlife habitat. These protected lands are classified as Intensely Developed Area, with 20 or more adjacent acres dominated by residential, commercial, institutional, or industrial uses and very little natural habitat; Limited Development Area, with low to moderate intensity development and plant and animal habitat, with impervious surfaces limited to 15 percent of a site; and Resource Conservation Area, consisting of natural environments or areas used for agriculture, aquaculture, fisheries activities, and forestry.