GEOLOGY OF THE CHESAPEAKE BAY AREA--THE CLASSIC STANDARD FOR THE ATLANTIC COASTAL PLAIN

The area where the Chesapeake Bay lies today is the site of Paleo-Atlantic transgressions and regressions since the time of the continental break-up in the Late Triassic. Beneath the Bay a series of deltaic sands and fossiliferous marine silty sands records a rich history of climate change; sea level fluctuation, and structural movement.

Today the Atlantic Continental edge is tilted by tectonic forces to the Northeast. The result is that the Coastal Plain north of Massachusetts is tilted below sea level. The shelf is progressively emergent south of that area so that the Chesapeake Bay area is submerged partially allowing tidal waters to impinge to the Fall Line or Piedmont.

The Salisbury Embayment, the site of the present Chesapeake Bay, has a rich history of Cretaceous and Tertiary sediments. Glacial streams and rivers have cut through these sediments to expose these older beds in the resulting valleys. Subsequent flooding during interglacial periods allowed these rivers to overflow their channels and then be widened by predominate northeast and northwest winds. The result is erosion of the river banks and the construction of wide, flat flood plains. When sea level dropped, broad terrace plains were exposed, with high escarpments to mark where the cliffs were at the edge of the river.

The exposures along the Chesapeake Bay and its associated rivers, the Patuxent, Potomac, Rappahannock, York, and James, have created almost continuous sections that extend for miles. Within these beds are countless fossils of almost every type from land plants to marine invertebrates. This extensive record has served as a “Type area” to study the stratigraphic succession and the facies within the beds south of the Maryland-Virginia area. The embayment of the rivers in North Carolina, South Carolina, and Georgia continuously decrease. The result is smaller exposures that are farther apart. For this reason much of the stratigraphy south of the Bay area is interpreted on data obtained from the Chesapeake Bay exposures.