The Geologic History of the Bay

Picture of the Earth in the clouds The Chesapeake Bay we see today began about 10,000 years ago, when the glaciers that had advanced as far south as present-day New York City finally began to recede. They receded because the world began to get warmer.

The change of the temperature was enough to affect the entire face of the globe. During the cold winters of the planet, wich we call an Ice Age, much of the planetís moisture is caught up in ice. The level of the oceans falls.


For ninety percent of the recent geologic past, the sea has remained in a well-defined basin, with a sharp drop off at its edge. But when the glaciers melt, the sea rises. The overflowing ocean covers the edge of that basin, flooding the long flat shoreline under a shallow sheet of water. That shoreline, now covered with seawater, we call the continental shelf.

If you go to sail out of the Chesapeake Bay, you would find the depths fairly shallow - about 40 feet for 20-50 miles out to sea. This area is the old shoreline, the flat plain that once bordered the coast where land animals like bison, deer and panthers roamed for centuries. Only about a hundred and twenty miles out of the Bay, you would be beyond the continental shelf and over the ancient sea bottom.

Photo of the Bison
The Chesapeake, for much of its history, was not a bay at all. It was a river, the river we now call the Susquehanna, a name given it by the native Americans. This river starts in New York State and run through Pennsylvania. Once it continued straight to the sea, emptying into the ocean basin.

Those 10,000 years since the glaciers receded created a very special place in the Chesapeake Bay. The Bay is an estuary, where salt water from the sea mixes with fresh water from the rivers. This natural mixing bowl, fed with nutrients off the land, makes for a fertile feeding ground and nursery ground for fish and shellfish, the forest made home for an animals.