The Chesapeake Bay has its origins with the Susquehanna River, one of the oldest rivers on earth—even older than the mountains through which it passes. The flow of the Susquehanna may have begun as early as 300 million years ago. Traces of the ancient riverbed can be detected today beneath the waters of the Bay.
Several geologic events transformed the lower part of the Susquehanna into the Bay, which is a formation known as a ria. A ria is a drowned river valley that typically takes the form of an estuary such as the Chesapeake Bay. To create the ria, either the land sinks or the water level rises, or both. In the case of the Bay, the waters rose to flood the valley beginning approximately 10,000 years ago, at the end of the last Ice Age. Because the water rose but the land did not sink, the result is a relatively shallow estuary.
The advance and retreat of glaciers during succeeding ice ages helped carve the landscape around the Bay. As a result, water, ice, and other geologic forces produced the typical Tidewater landscape: flat, fertile land with broad, navigable rivers fed by numerous smaller watercourses. But because the Bay was once a valley, the land is not uniformly flat; bluffs and eroding cliffs are prominent features in many locations, especially along tributaries.
Probably the single most dramatic geological event to affect the evolution and shape of the Chesapeake Bay occurred about 35 million years ago. A meteor-like object from outer space crashed to earth in the shallow ocean waters off of what was then the eastern seaboard. The impact of this bolide is an important piece of the Bay’s geologic puzzle. The impact did not create the Bay, but it influenced the Bay’s eventual location and created the Chesapeake Bay crater.
The crater is not visible anywhere on land; it is completely submerged beneath the mouth of the Chesapeake Bay and 35 million years’ worth of sediment.
Did you know?
The Chesapeake Bay impact crater is the largest known crater in the United States. Located beneath the mouth of the Chesapeake Bay running from underneath the tip of the Delmarva (Delaware-Maryland-Virginia) Peninsula across the Bay to Norfolk, VA, the roughly circular crater is twice the size of Rhode Island and nearly as deep as the Grand Canyon.
Geologic forces such as these formed the landscape that Captain John Smith encountered. The English adventurers first explored the southern end of the Bay, where the land is low and marshy. As they traveled northward and westward, they discovered that the land became higher. In some places they found exposed cliffs, eroded by actions of sea level rise, waves, and tide.
As they continued up the tributaries, the rivers narrowed and the uplands between the rivers became more extensive. Along all the rivers that were long enough, the explorers eventually reached a fall line characterized by water falls and rapids. The outcroppings of bedrock along these fall lines prevented further exploration except on foot.
The Bay’s shorelines are constantly changing. Currents and tides erode and smooth peninsulas and headlands and transport materials to other parts of the Bay, such as channels. Rivers carry sediments and deposit them at the mouths of tributaries and along the margins, forming broad, flat deposits of mud and silt.
By the mid 1700s, as more and more land was cleared for agriculture, sedimentation filled in some navigable rivers. Joppatown, MD, for example, once a seaport, is now more than two miles from water. The forces of erosion and sedimentation continue to reshape the details of the Bay.
Sea-level rise also changes the landscape. Many of the islands that existed in the Bay at the time of Captain John Smith’s travels are now submerged. Poplar Island in Talbot County, MD, for example, encompassed several hundred acres in the early 1600s. Over the centuries, rising sea level has inundated Poplar Island, and today only a chain of small islands remains.
Learn more about the Chesapeake's formation