Global Sea Level Change
The geography of the world changes over the course of millions of years due to the motion of Earth's tectonic plates. Continents are pushed across the surface of the Earth; they join; they split apart. On shorter time scales continents can be joined or separated by changes in sea level. The true boundary between continental lithosphere and ocean basin is the edge of the continental shelf. Currently, many continents have broad continental shelves--areas of submerged continental lithosphere. This means that relatively small changes in sea level can have a major effect on the geography of a continent.
Let's examine world geography at the height of the last
ice age, 18,000 years ago. At that time the Northern Hemisphere was
substantially glaciated. In eastern North America, ice caps that originated in
northern Canada extended as far south as Long Island and central Pennsylvania.
The ultimate source of all this ice was water evaporated from the ocean,
transferred by snowfall, and stored on the continents as large ice caps. The
amount of water required to make this much ice was sufficient to lower sea level
by 125 meters (410 feet).
Visit Cornell's get Geophysical Lab for many answers.