Bay Geology

The Bay has been changing continuously for thousands of years. All of these changes affect the Bay's ecosystem, for better or worse. Some changes are abrupt, while others take place over such a long period of time that we can only see them by looking back into geologic history.

Geologic History

About 35 million years ago, a rare bolide — a meteor- or comet-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. While this bolide did not actually create the 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. About 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. The 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 length of the Bay's bottom.

Geology of the Watershed

To fully define the 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 and Appalachian provinces. The waters that flow into the Bay have different chemical identities, depending on the geology of where they originate.

Atlantic Coastal Plain

The Atlantic Coastal Plain is a flat, lowland area with a maximum elevation of about 300 feet. It is supported by a bed of crystalline rock, covered with southeasterly dipping wedge-shaped layers of relatively unconsolidated 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 eastward 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 and Fredericksburg, Virginia, were developed 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.

Piedmont Plateau

The Piedmont Plateau ranges from the fall line eastward 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 is low in the 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.

Appalachian Province

The Appalachian Province lies in the western and northern parts of the watershed. These areas are characterized by mountains and valleys, and are rich in coal and natural gas. Sandstone, siltstone, shale and limestone form the bedrock. Most of the water from this province flows to the Bay mainly via the Susquehanna River.

Erosion and Sedimentation

Since its formation, the Bay's shoreline has undergone constant modification by erosion, transport and deposition of sediments. The Bay continues to be shaped by tides and currents, which erode the land and transport sediments to other parts of the Bay.

• Many islands that existed in the Bay during colonial times are now severely eroded or completely submerged. In the early 1600s, Poplar Island, in Talbot County, Maryland, encompassed several hundred acres. By the 1940s, only 200 acres remained, and the island had been cut in two. Today, the area is being restored by the U.S. Army Corps of Engineers using dredged material from Baltimore Harbor.
• Some geologists estimate that the cliffs John Smith explored in 1607 and 1608 along the middle western shore have eroded back 300 feet.
• By the mid-1700s, some navigable rivers were filled in by sediment as more land was cleared for agriculture. Joppatowne, Maryland, was once a sea port, but is now more than two miles from water.