Oblique aerial view of Passmore Creek and
marsh, Jamestown Island, James City County
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he Coastal Plain extends from the Fall Zone eastward to the Atlantic
Ocean. Through the Fall Zone, the larger streams cascade off
the resistant igneous and metamorphic rocks of the Piedmont to
sea level. Large tidal rivers, the Potomac, Rappahannock, York, and James, flow
southeastward across the Coastal Plain to the Chesapeake Bay. The Bay, in turn, empties
into the Atlantic Ocean.
The topography of the Coastal Plain is a terraced landscape that stair-steps down to the coast
and to the major rivers. The risers (scarps) are former
shorelines and the treads are emergent bay and river bottoms. The higher, older plains in
the western part of the Coastal Plain are more dissected by stream erosion than the lower,
younger terrace treads. This landscape was formed over the last few million years as sea
level rose and fell in response to the repeated melting and growth of large continental
glaciers and as the Coastal Plain slowly uplifted. During the glacial maxima, much of the
continental shelf was emergent and the Susquehanna flowed through the Chesapeake lowland
and across the exposed shelf to the sea 80 km or more to the east. The Chesapeake Bay was
created about 5000 to 6000 years ago when the lower course of the Susquehanna River
through the Chesapeake lowland was flooded as meltwater from the large Pleistocene
continental glaciers raised sea level. Continuing sea level rise and shoreline erosion
caused the bay to expand its aerial extent.
The Virginia Coastal Plain is underlain by a thick wedge of sediments
that increases in thickness from a featheredge near the Fall Zone
to more than 4,000 meters under the continental shelf. These sediments rest on an eroded
surface of Precambrian to early Mesozoic rock  . Two-thirds of this
wedge is comprised of late Jurassic and Cretaceous clay, sand, and gravel; they were
stripped from the Appalachian mountains, carried eastward by rivers and deposited in
deltas in the newly formed Atlantic Ocean basin. A sequence of thin, fossiliferous marine
sands of Tertiary age overlie the older strata. They were deposited in warm, shallow seas
during repeated marine transgressions across the Coastal Plain.
This pattern of deposition was interrupted about 35 million years ago by a large meteorite
that plummeted into a shallow sea, and created a crater more than 90 km in diameter, termed the
Chesapeake Bay Impact Structure. It
was subsequently buried under about 1.2 km of younger sediment.
Latest Tertiary and
Quaternary sand, silt, and clay, which cover much of the Coastal Plain, were deposited
during interglacial highstands of the sea under conditions similar to those that exist in
the modern Chesapeake Bay and its tidal tributaries.
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(3/19/99) |
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