62 Bulletin 1, Biological Society of Washington, 1918. 



and the rivers cascade from narrow rock-bound gorges into 

 broad tidal (p. 230) estuaries. Now, during the Columbia 

 period, ocean water overwhelmed nearly all of the lowlands 

 between the present coast and the fall line and washed the 

 upland margin where now stand Fredericksburg, Washing- 

 ton, Baltimore, and Philadelphia ; and north of Philadelphia 

 it swept still further inland, submerging the uplands to 300 

 and even 450 feet above present tide level. Then the Poto- 

 mac embouched into open ocean at the "Three Sisters"; at 

 the culmination of the submergence, shoal ocean-waters 

 rolled over the highest land between Washington and the 

 coast, and the Anacostia, the Patuxent, and Chesapeake Bay 

 were not ; while during even the inferior stages of water the 

 lower Potomac was an estuary many times broader and 

 deeper than today." 36 



The greater part of the Piedmont Plateau remained a 

 land surface while the Coastal Plain was submerged several 

 times. Not only was all land life on the Coastal Plain 

 entirely extirpated during these submergences, but even 

 during the emergences ecologic conditions for long periods 

 and over great areas were radically different from those on 

 the Plateau. 



The tide flats, the salt marshes, the low-lying flood-plains 

 of the rivers which long characterized the landscapes after 

 each emergence were not suited to the needs of most of the 

 upland species. On the contrary, the land was occupied 

 and held by species already adapted to the conditions. As 

 the Coastal Plain was elevated after the last depression, 

 upland species gradually advanced upon it, but to accom- 

 plish this they needed powers of migration and the ability 

 to take and hold a place in the face of probably somewhat 

 adverse conditions. Not all species were able to do this, 

 and some, on account of lack of adaptiveness, or small 

 ability to extend their ranges, required a very long time to 

 reach, or may not yet have reached, places in the coastal 

 Plain ecologically suited to them. Conversely, certain other 

 species found only in the Coastal Plain the conditions neces- 



s«The American Anthropologist, Vol. II, 1879, pp. 230-231. 



