Buried beneath the sea, the Gulf of Maine's underwater landscape rivals that of any portion of the Eastern Seaboard. Valleys plunge to depths of 1500 feet (460m). Small mountains rise abruptly, thrusting upward from 800 feet (250m) to fewer than 100 feet (30m) below the surface of the sea. On Georges Bank, long-submerged tree trunks are occasionally drawn up in fishing nets; water depths can be just 13 feet (4m) on the peak of Georges. These hidden landscapes were forged through a combination of physical processes that shaped and then erased the Gulf's physical features many times over the millennia.

The Gulf of Maine's history begins 430 million years ago when volcanic mountains formed in the region, depositing solidified molten masses of granite. Then, approximately 50 million years later, another mountain chain rose, called the Appalachians. This process of orogeny, as mountain building is called, once again violently reworked the region. All this activity occurred when the continents were bound together as one, called Pangea. Pangea began to break apart approximately 190 million years ago. As the continental plates tore the super-continent apart, rift valleys formed. The Bay of Fundy is one such rift valley which, as it formed, poured additional igneous rock onto the granite base of the Gulf.

The Gulf had a brief spell of quiet, in geologic terms. The region stabilized as a shallowly submerged, smooth, seaward-sloping shelf, much like today's continental shelf off the mid-Atlantic. Then, 15 million years ago, the Gulf was exposed as dry land, either due to a drop in sea level or a rise in the height of the sea bottom. As dry land, the Gulf was subject to normal erosion from water and wind.

Keep in mind that not all rocks are the same. Each type of rock resists erosion differently. Granite tends to resist erosion the longest, forming headlands and highlands, while metamorphic rocks are more easily eroded and disappear, forming bays and lowlands. Thousands of years of wind and rain can create dramatic topographical relief.

The large topographic features of the Gulf of Maine — such as its banks, basins, and the Northeast Channel — emerged as a result of erosion. The glaciers then reworked the landscape in their turn. The slow progress of the mile-thick ice sheets effectively scoured away softer rocks and sediments lying in the Gulf of Maine. Approximately 13,000 years ago the glaciers reached their southern-most extent and began to retreat. As they melted, the glaciers released great volumes of meltwater which cascaded across the ice-free plain as rivers and streams. The Northeast Channel was the principal water gap for the glacial meltwater system. The power of the water once again reworked the Gulf's malleable landscape.

With the retreat of the glaciers, the sea began to rise. Cape Cod took form as a recessional moraine, marking the point where ice paused as it receded. Eventually sea level rose high enough to broach the great banks left by the glaciers and flood the Gulf. Four thousand years ago, the Gulf was a calm embayment tenuously connected to the open Atlantic. As sea level continued to rise and the Gulf of Maine grew ever deeper, the tides grew stronger. Still effectively barred from the Atlantic by Georges and Browns Banks, and with ever strengthening tides, the modern Gulf of Maine was born.

The ancient Northeast Channel continues to separate Georges and Browns Banks, and provides the major inlet of ocean water in to the Gulf of Maine. Through this deep-water conduit dense, high salinity, nutrient-rich North Atlantic water courses into the Gulf, then settles into the deep basins.

Best known of the Gulf's banks, Georges Bank reaches depths of no more than 200 feet (60m) over much of its extent. Two high points, Georges Shoals and neighboring Cultivator Shoals, lie just 13 feet (4m) beneath the surface.

Shallow features include Stellwagen Bank off the eastern coast of Massachusetts, nearby Jeffreys Ledge off the coast of southern Maine, and Cashes Ledge in the central Gulf. Gulf. At 1236 feet (377m), Georges Basin, off Georges Bank and near the Northeast Channel, is the deepest of the 21 basins found in the Gulf. Other basins deeper than 600 feet (190m) include Wilkinson Basin and Jordan Basin.