The Chesapeake Bay has been changing continuously for thousands of years. All geologic changes affect the Bay ecosystem, for better or worse. Some changes are sudden, whereas others take place over such a long period of time that we can only see them by looking back into geologic history.
About 35 million years ago, a rare bolide – a comet- or asteroid-like object from space – hit the area that is now the lower tip of the Delmarva Peninsula, near Cape Charles, Virginia. The bolide created what geologists call the “Exmore Crater,” which they believe was as large as Rhode Island and as deep as the Grand Canyon.
Although this bolide did not create the Chesapeake Bay, it helped determine that a bay would eventually be located there.
During the last Ice Age, mile-thick glaciers stretched as far south as Pennsylvania, and the Atlantic coastline was about 180 miles farther east than it is today.
Approximately 18,000 years ago, the glaciers began to melt, carving streams and rivers that flowed toward the coast. Sea level continued to rise, eventually submerging the area now known as the Susquehanna River Valley. This drowned river valley became the Chesapeake Bay.
The Chesapeake Bay assumed its present shape about 3,000 years ago. Remnants of the ancient Susquehanna River still exist today as a few troughs that form a deep channel along much of the Bay's bottom.
To fully define the Chesapeake Bay ecosystem, we must go far beyond its shores. Although the Bay itself lies entirely within the Atlantic Coastal Plain, its watershed includes parts of the Piedmont Plateau and Appalachian Province.
The waters that flow into the Bay have different chemical identities depending on the geology of where they originate.
The Atlantic Coastal Plain is a flat, lowland area with a maximum elevation of about 300 feet.
The coastal plain is supported by a bed of crystalline rock covered with southeasterly dipping wedge-shaped layers of sand, clay and gravel. Water passing through this loosely compacted mixture dissolves many of the minerals. The most soluble elements are iron, calcium and magnesium.
The coastal plain extends westward from the continental shelf to a fall line that ranges from 15 to 90 miles west of the Bay. Waterfalls and rapids clearly mark this line, which is close to Interstate 95.
Cities like Baltimore, Maryland; Washington, D.C.; and Richmond, Virginia, were built along the fall line to take advantage of potential water power generated by the falls. These cities became important commerce areas, as colonial ships could not sail past the fall line and had to stop to transfer their cargo to canals or overland shipping.
The Piedmont Plateau ranges from the fall line westward to the Appalachian Mountains. This area is divided by Parrs Ridge, which stretches across Carroll, Howard and Montgomery counties in Maryland and adjacent counties in Pennsylvania.
Several types of dense crystalline rock, including slates, schists, marble and granite, compose the eastern side of the ridge. This variety creates a very diverse topography. Water from the eastern side of the Piedmont is low in calcium and magnesium salts.
The western side of the Piedmont consists of sandstones, shales and siltstones layered over by limestone. This limestone bedrock contributes calcium and magnesium, making the water “hard.” Waters from the western side of the ridge flow into the Potomac River.
The Appalachian Province lies in the western and northern parts of the watershed. These areas are characterized by mountains and valleys, and they are rich in coal and natural gas. Sandstone, siltstone, shale and limestone form the bedrock.
Most of the water from the Appalachian Province flows into the Bay from the Susquehanna River.
Since its formation, the Chesapeake Bay's shoreline has constantly been shaped by tides and currents that erode the land and move sediments to other parts of the Bay.
There are many examples of erosion and sedimentation throughout the Bay.