The ice sheets started melting faster than they were growing about 18,000 years ago, after the peak of the Wisconsin glaciation. Initially, as the Atlantic Ocean water levels began to rise, the main change was the height of sloping hills/cliffs at the edge of the ocean (today's Continental Slope). Virginia's eastern coastline did not move inland until about 12-15,000 years ago, when the ocean began to flood over the Coastal Plain. Roughly 9-10,000 years ago, the rising Atlantic Ocean submerged the mouths of the James and Susquehanna rivers, and may have reached as far north as the Rappahannock River.1 Sea level rise tapered off about 6,000 years ago,2 and the bay took on its present shape about 3,000 years ago.3
The creation of the bay may have occurred not in a steady process but in stages. Changing chemistry in sediment cores suggest one rapid expansion of the bay occuring roughly 8,000 years ago. The bay could have expanded in a quick surge because sea level had increased quickly worldwide, perhaps as continental ice sheets released a pulse of melted water and increased volume of the oceans.
Another possibility: the Susquehanna River valley flooded slowly within its channel until sea water finally rose onto the floodplain, and submergence of the Susquehanna River floodplain created a sudden one-time increase in the size of the bay.4
When the ice sheet reached into Pennsylvania, the Susquehanna River carried glacial debris to its mouth and formed a delta at the edge of the Atlantic Ocean. Other rocks and sand transported by the ice sheet were deposited along the Atlantic coast further north. Terminal moraines of the glaciers still remain today as Cape Cod, Nantucket Island, Martha's Vineyard, and Long Island.
Sand from the northern coast has been pushed south by longshore currents, building up beaches/barrier islands along the East Coast shoreline south to Cape Hatteras in North Carolina. Sediment barriers moved by currents have blocked the flow of the Susquehanna River directly into the Atlantic Ocean, forcing the river several times to etch a new channel to the south.
The sand that blocked the Susquehanna River (forming the Delmarva Peninsula today) may have been transported all the way from Canada during the last Ice Age, scraped up by the ice sheet before being deposited on the shorelines of New Jersey-Maine. Some of the material etched out of the bedrock to form basins, such as those now filled by the Great Lakes, could remain as sand grains on Virginia's Atlantic Coast shoreline or on the bottom of the Chesapeake Bay today.
Over the last 18,000 years, the mouth of the Susquehanna River has been pushed further south, as ocean currents and storms dumped mounds of sand in front of the mouth of the Susquehanna River. The river adjusted its path several times, carving new channels to empty into the Atlantic Ocean as the Eastern Shore was deposited by longshore currents and piled up in various storm events.
Old channnels of the Susquehanna River can be identified underneath the current Eastern Shore, and on the edge of the continental shelf where "fossil" canyons such as the Washington Canyon show the former route of the river. Today, the channel of the Susquehanna River has been pushed past the former deltas of the Potomac/Rappahannock/York rivers, down to the channel of the James River. The Potomac/Rappahannock/York rivers flow into the old Susquehanna channel, which exits to the Atlantic Ocean between Cape Charles and Cape Henry in a channel that parallels the exit of the James River.
The two sets of tunnels for the Chesapeake Bay Bridge-Tunnel mark the Thimble Shoal Channel (on the south for the James River) and the Chesapeake Channel (on the north for the Susquehanna River). Ships sailing to Baltimore use the Chesapeake Channel and follow the path of the drowned Susquehanna River north, while ships headed to Norfolk use the Thimble Shoal Channel and follow the drowned James River west.
The sea level of the Atlantic Ocean has been rising for roughly 18,000 years. When the ice sheets stretched to their greatest extent, the Atlantic was a much smaller ocean because so much water was still trapped in the ice. As the ice melted, fresh water surged into the oceans in various meltwater pulses, and flooded the valley of the Susquehanna River (and other tributaries to the Chesapeake Bay).
Today, the Atlantic Ocean is roughly 300-350 feet (100-120 meters) higher than when glaciers reached into Pennsylvania, and sea level has been rising slowly at the rate of 6 inches/century.5 If sea level rise predictions of the Intergovernmental Panel on Climate Change6 come true, however, the Chesapeake Bay could expand significantly - or disappear and become a salt water gulf comparable to the Gulf of Maine, if the Eastern Shore is completely covered by a rising Atlantic Ocean.
Sea levels have risen in the past, depositing sediments into various irregular "embayments" along what is now the eastern edge of the United States. The Coastal Plain, east of the Fall Line, consists of sediments lying on top of the metamorphosed Piedmont rock formed when the Iapetus Ocean closed. Underneath the Coastal Plain, there is a "basement layer" of Piedmont rock that dips towards the east. The sediment layer gets thicker as you drive east on I-64, from Richmond to Virginia Beach. As described by one geologist:7
 depth to bedrock from the surface, Coastal Plain between Fall Line and Atlantic Ocean Source: US Geological Survey, Professional Paper 1404-C: Hydrogeologic Framework of the Virginia Coastal Plain (Figure 8) |
 embayments along Atlantic Coast Source: US Geological Survey, Circular 1264: Geology of the National Capital Region—Field Trip Guidebook |
