Geologic history—the history of the structure of the Earth—is a story. Just like any story there are a number of components that need to come together in order to tell the whole tale. The geologic history of the Chesapeake Bay includes physiographic provinces, tectonics, ice ages, and ocean processes. Each of these storylines helps us understand this complex environment.
About 35.5 million years ago an exploding meteor collided with Earth and formed a massive crater. Because rivers flow along the path of least resistance, the depression created by the crater caused river valleys to converge, setting the stage for the formation of the Chesapeake Bay.
Millions of years later, the prehistoric Susquehanna River carved a 400-foot deep canyon as it flowed from the Appalachian Mountains toward the Atlantic Ocean. Rising sea levels at the end of the last ice age, about 10,000 years ago, flooded the Susquehanna River valley. Sediment has since filled much of the channel, forming the shallow Bay.
But the story of the geologic forces that shaped the Bay watershed began even earlier, with the formation of six distinct types of landforms called "provinces."
The Chesapeake Bay watershed includes six different geologic areas, called physiographic provinces: Continental Shelf, Coastal Plain, Piedmont, Blue Ridge, Valley and Ridge, and Appalachian Plateaus. The appearance of each of these geographically distinct regions is a direct result of the rocks underneath. Young, flat-lying sedimentary rocks create the Coastal Plain. An ancient ocean makes up the Piedmont province. Tectonic forces lifted 1.2-billion-year-old crystalline basement rock upward to build the Blue Ridge and Appalachian Mountains. Wrinkles in sedimentary rock layers comprise the Valley and Ridge province. These same layers form the Appalachian Plateaus.
The modern Chesapeake Bay watershed is framed by mountains on the north and west and the lower elevations on the east and south that formed from the slow collision of the African and North American tectonic plates.
The northern part of the watershed (or drainage basin) is bordered by the Appalachian Mountains of Pennsylvania and New York. The Appalachians also border the western side of the drainage and bend from east-west to the southwest through the states of Pennsylvania, Maryland, Virginia, and West Virginia.
The watershed's eastern and southern boundaries are less dramatic: they do not contain mountains, only high topographic areas connecting ridgelines that separate the Susquehanna River basin from the Delaware River basin in the northeast. The frame of the rest of the eastern boundary of the watershed is along the shallow backbone of the Delmarva (Delaware-Maryland-Virginia) Peninsula east of the Chesapeake Bay. Along the southern side of the Bay, a very subtle topographic rise separates the rivers flowing north to the Chesapeake Bay in Virginia from those flowing south to North Carolina.
Around 250 million years ago, plate movement changed and mountain-building ceased, leaving the Appalachians to the weathering effects of wind, rain, ice, and gravity. Hard, durable rocks persisted under these conditions and became the notable landmarks of the landscape. The erosion-resistant Blue Ridge is the highest part of the Appalachian Mountain range. Today the ridges along the Blue Ridge Parkway in Virginia prominently display these rocks, made of granite and gneiss. In most places, the high topography of the Blue Ridge physiographic province defines the western boundary of the Chesapeake Bay watershed.
Ocean and Coastal Plain
The Atlantic Ocean started to form about 200 million years ago. A rift or gap opened in the supercontinent of Gondwanaland, separating South America and Africa. Sediments eroded from higher elevations into the low-lying areas created as the Atlantic Ocean opened. The sediments draped over the edge of the continent, extending seaward for hundreds of miles to form the Coastal Plain. Glaciation frequently changed the sea level, contributing to the formation of the coastal plain. When the sea level rose, water covered the Coastal Plain, depositing marine sediments to form a type of sand that geologists call the Chesapeake Group.
Thirty-five million years ago a warm, humid tropical rain forest covered the Appalachian Mountains. In an instant, this landscape changed forever when a bolide 1.8–3.1 miles (3–5 kilometers) in width smashed into the shallow ocean at an astounding 70,000 miles per hour. The bolide hurled millions of tons of debris and rocks into the atmosphere, leaving a gaping, smoldering crater in the continental shelf. A tsunami, or tidal wave, resulting from the impact devastated the eastern margin of the North American continent, wiping out much life on land and surrounding marine areas.
At about 80 miles (130 km) in diameter, 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 Peninsula (near present-day Cape Charles, Virginia), across the Bay to Norfolk, Virginia, the roughly circular crater is twice the size of Rhode Island (2,500 sq mi [6,400 sq km]) and nearly as deep as the Grand Canyon (0.8 mi [1.3 km] deep). The depression created by the crater caused river valleys to converge, which led to the eventual location of the Chesapeake Bay millions of years later.
Ice sheets that covered the poles and Canada some 2 million years ago periodically pushed their way into the present-day United States. The ice was a major cause of changing sea levels: sea level rose as ice melted and fell as the water froze and more ice formed. Water from the melting ice sheets fed the ancestral Susquehanna River, giving it the power to move massive amounts of sediment as it cut its way to the coast. As glaciers and sea level adjusted, so did the rivers. During glacial times (lower sea level), rivers had to work their way across the exposed continental shelf to dump their sediment loads into the ocean.
These rivers travel across an important geologic feature, the fall line, which marks the transition from Coastal Plain to Piedmont. Geologically it represents the change from the softer sedimentary rock and unconsolidated sand and gravels of the Coastal Plain to the harder metamorphic rocks of the Piedmont core of the ancient Appalachians. Rapids and waterfalls visually mark the fall line, where rivers flow off the topographically high rocks of the Piedmont and onto the flat, softer sediments of the Coastal Plain. These fall lines were important to later settlement patterns.
Five substantial rivers feed the western half of the Bay: the Patuxent, Potomac, Rappahannock, York, and James. The Potomac is the longest and has the largest watershed. The enormous watersheds of these rivers produce huge flows of freshwater, which cause the western half of the Bay to be less salty, more silty, cluttered with woody and other vegetative debris, and generally more active than rivers on the eastern side. See a larger image of the rivers to the Bay.
The eastern rivers of the Chesapeake Bay are quite different from those in the west. The largest of these rivers draining into the Bay are the Pocomoke, Wicomico, Nanticoke, Choptank, and Chester. These eastern rivers are not particularly long, do not collect freshwater from large drainage areas, and are tidal for most of their length. Because they contribute only a fraction of the water produced by the western rivers, the eastern half of the Bay is saltier, slower moving, more given to marshes, and more productive of saltwater plants such as submerged aquatic vegetation.
At the end of the last glacial stage, some 15,000 years ago, a great meltdown began and sea level rose relatively rapidly. During this period, the lower Susquehanna River basin started to fill up with seawater. Rising sea level moved up into the river, "drowning" it. By 10,000 years ago, the main channel of the ancient Susquehanna River valley was flooded and became a narrow estuary. Between 6,000 and 7,000 years ago, the rate of submergence began to slow, and the Chesapeake Bay took on its characteristic drowned-river-valley shoreline pattern. Sea level at that time stood approximately 30 feet (9 m) lower than the present level.
Enclosing the Bay
During periods of high sea levels, the Delmarva Peninsula, the Chesapeake's "Eastern Shore," lengthened into a major barrier spit. Progressive phases of high sea levels/renewal of Delmarva Peninsula development and low sea levels/cessation of landform development continued throughout the last 2 million years.
The significant phase of forming the peninsula occurred in the early or middle Pleistocene period (1.8 million to 11,500 years ago) with the development of the Accomack spit.
The creation of the Accomack spit protected the western shoreline of the Bay from the ocean's power for the first time, which was important to the formation of the Bay. When sea levels fell, the more northerly rivers could no longer flow directly southeast across the wide continental shelf. The emerging rivers were diverted southward around the tip of the lengthened peninsula. The diverted river systems progressively joined the next river to the south and excavated a new channel. The pattern of lengthening the Delmarva Peninsula during sea-level highs and diverting all the western rivers into the dominant Susquehanna River continued until the modern Chesapeake Bay formed.
The Bay Today
Erosion, transportation, and deposition of sediments are constantly changing the Bay's shorelines. Currents and tides erode and smooth peninsulas and headlands and deposit materials in other parts of the Bay. Rivers transport sediments and deposit them at the mouths of tributaries and along margins of the Bay, forming broad, flat deposits of mud and silt.
These natural causes of sedimentation are often accelerated by human activity. By the mid-1700s, as farmers cleared more and more land for agriculture, sedimentation filled some of the navigable rivers used for travel. For example, Joppatowne, Maryland, once a seaport, is now more than 2 miles (3 km) from water. The clearing of forests and other impacts of population growth continue to alter the Bay's landforms and increase sedimentation.
Sea-level rise has also changed the landscape. Many of the islands that existed in the Bay during colonial times are now submerged. For example, in the early 1600s, Poplar Island in Talbot County, Maryland, encompassed several hundred acres. Today, a chain of small islands is all that remains of the original Poplar Island.