The trigger for shifting sands and river channels was the rise and fall of the Atlantic Ocean during various ice ages; water levels were as much as 300-400 feet higher or lower than today.
Over the last 500,000 years, the Delmarva peninsula has grown towards the south, fed by sediments eroded off the Appalachian Mountains in Pennsylvania/New York and carried by ocean currents.
Some of the most useful fossils for dating purposes are very small ones.
For example, microscopic dinoflagellates have been studied and dated in great detail around the world.
Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed.
Correlation can involve matching an undated rock with a dated one at another location.
Many of these organisms have left their remains as fossils in sedimentary rocks.Some fossils, called index fossils, are particularly useful in correlating rocks.For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.Geologists have studied the order in which fossils appeared and disappeared through time and rocks. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.This matching process is called correlation, which has been an important process in constructing geological timescales.Suppose you find a fossil at one place that cannot be dated using absolute methods.That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age.The channel of the Susquehanna River has been pushed south, past the former deltas of the Potomac/Rappahannock/York rivers and down to the channel of the James River.Sediments were deposited during periods of high sea level and extended the ancient Delmarva "spit," while erosion of new Susquehanna River channels occurred during periods of low sea level.The Chesapeake Bay fills the drowned valleys of the Susquehanna and James rivers, plus tributaries that intersected the Susquehanna.The current shape of the bay is less that 10,000 years old - so Native Americans arrived in in Virginia in time to witness the conversion of multiple freshwater river channels into the brackish estuary of the modern Chesapeake Bay.