Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting (13-16 March 2010)

Paper No. 11
Presentation Time: 5:15 PM


WARD, Lauck, Virginia Museum of Natural History, 21 Starling Avenue, Martinsville, VA 24112,

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 Chesapeake Bay, being a series of drowned rivers, has little strength in its currents to move sand-sized particles along the bottom, but silt and clay are supplied in plenty from the Piedmont. The benthic organisms of river, bay, or sea require a substrate of sand, silt, and clay mixed. Any predominance of one or the other makes the diversity drop. An even mixture of the textures causes diversity to maximize. The sand in the Chesapeake Bay is supplied from the erosion of the natural cliffs. This erosion not only creates the Bay, but is necessary for its ecological health. This natural mixing of the cliff sand, silt, and clay created maximal bottom constitutions for benthic organisms such as shrimp, worms, oyster, crabs, and others. These conditions are now being threatened by the rip rapping of the shorelines to “protect” homes. The result is that only silt and clay will line the bottom causing conditions too soft and soupy for the benthic exploitation. The proper setback of building would make this ecologically harmful practice unnecessary.