Ghosts under the Chesapeake -- Prehistory of Delmarva, Part 3:

Bob Jones posted in Worc. Co., MD, History, Bob Jones, Coordinator

Bob Jones 9:50am Nov 6 Ghosts under the Chesapeake -- Prehistory of Delmarva, Part 3: The colossal impact-crater buried under the southern tip of Delmarva is not the only ghost lurking in the murky depths of the Chesapeake Bay. There are at least three ancient "skeletons" that had much to do with the shape of our land. In 1948, when Bay Bridge engineers in search of firm footing drilled boreholes across the Bay, they hit coarse river gravel 120 feet down, uncovering the first evidence that an ancient river valley was buried beneath the Bay. The Delmarva Peninsula used to be a much larger land mass. In fact, it was not a peninsula at all. It was high and dry. In those Olden Days, eons ago, if you had wanted to go catch some crabs at Public Landing or enjoy a meal at Greenback's Crusty Crab you would have driven 75 miles -- that's how far it was to the coast line. On the other hand, if you had wanted to drive to Crisfield, you would have driven all the way to Smith Island before arriving at water's edge -- and not salt water, but fresh . . . and not on the bay but on a river . . . the Susquehanna River, the great river at the head of the Bay that pours in nearly 50 percent of the river water entering our estuary today. In those distant times, it was not an estuary, but a river like any river flowing towards the ocean, which it did not reach until some 70 miles east of Norfolk. But at that time -- millions of years ago -- we were in a great ice age. Sea levels drop due to the removal of large volumes of water above sea level in the icecaps. These sheets of ice could be a mile or two thick. The sea level dropped 400 feet or even more, exposing the continental shelves. The weight of the ice sheets was so great that they deformed the Earth's crust and mantle. When an ice sheet melted, torrents of water would flow down river valleys to the ocean, and it was in this way that the ancient Susquehanna River got flooded and its bed got covered with the course river gravel discovered there in 1948. In those distant remote times, this ancient river did not empty into the sea at Norfolk -- it emptied at Exmore. So, Delmarva was then much shorter. As the geologic ages unfolded, glaciation alternated with warming periods, which cause sea levels to retreat, and then advance. Whenever, the sea level rose, it would bring in sand to deposit in the Exmore region, and also flood the ancient Susquehanna River valley. Later, when sea level fell, the river bed would trap gravel and sediment and get filled up. Each time this cycle reoccurred, a new Susquehanna channel would be gorged out, and each time further to the west. Why did the shift always head southward? There is an important south-flowing longshore current. Sand came sweeping down the ocean side of the Delmarva Peninsula, carried by a south-flowing longshore current. At the mouth of the Bay, the longshore current meets the tidal currents entering the Chesapeake. This confrontation causes the sand to stop, and it then builds up. The work of Robert Mixon, a researcher with the U.S. Geological Survey, helped put together the pieces of this Delmarva puzzle. He explains the phenomenon this way: Though the peninsula began as a short spit of land, it was a spit susceptible to great growth spurts — and great growth pauses. It grew with rising sea levels, and it paused with falling sea levels. When warm eras brought rising seas, ocean currents would pile up sand and sediment — extending the southern end of the spit. As the river at the end of the spit became an estuary, it filled up — and the Delmarva spit simply extended itself right across the flat channel. The forces of nature buried the Exmore Channel first, then it buried the Eastville Channel (150,000 years old). The Cape Charles Channel is buried under Fisherman’s Island. Each growth spurt ended in an ice age: falling sea levels drained the ocean away, terminating sand deliveries, and exposing the Delmarva as a long, low hill along the empty, dry plains of the continental shelf. In our present era, the ocean has risen so high that the "hill" we're on has the illusion of being a peninsula. Looking at the three maps, we can see how the sand deposited in the Exmore Region became an obstacle causing the mouth of the river to move southward. The three ancient channels thus shifted, emptying first into the Atlantic at Exmore, later at Eastville, and finally at Cape Charles (18,000 years old). Those towns thus sit astride the mouths of old river beds. I drew my material from various web sites, the most helpful being this one: http://www.chesapeakequarterly.net/V10N1/main/