60 Bulletin 1, Biological Society of Washington, 1918. 



For expression, in a single line, of the boundary of the 

 Piedmont Plateau and the Coastal Plain in the region of 

 the District of Columbia, the line of the present easternmost 

 outcrops of the metamorphic rocks (shown as a heavy line 

 on map, which see, p. 61) will do as well as any. It does 

 not coincide with a line connecting the principal waterfalls, 

 for which the line is named, since these generally are some 

 distance back from the actual edge of the outcrops. 



The boundary between Coastal Plain and Piedmont floras 

 and faunas agrees well with the outcrop line north of the 

 Potomac. Eoughly the Potomac itself then becomes the 

 boundary as far down as the great bend north of Fredericks- 

 burg, where the river cuts its way across the Coastal Plain. 

 However, the Coastal Plain is recognizable as a tangible type 

 on the Virginia side comprising the flood plain of the river 

 and its tributaries. The hills have a prevailing Piedmont 

 biota. Nevertheless, one must be prepared to find some 

 Coastal Plain species accompanying their native geologic 

 formations which cap the hills near the river. Conspicuous 

 patches of these formations lying farther back are shown on 

 the map by heavy broken lines. 

 The Fall Line as a Faunal and Floral Line. 



Be it ever so indefinite and ill-defined, the fall-line must 

 be recognized as having some significance as a faunal and 

 floral line because it lies between regions that have had very 

 distinct geological histories. A succinct sketch of this his- 

 tory may be quoted from W. J. McGee, who says : 



"Today there is a lowland plain rising gently from the 

 Atlantic and stretching mountainward to the elbow of the 

 Delaware at Trenton and Philadelphia, to the embouchure 

 of the Susquehanna into Chesapeake Bay, to the great elbow 

 of the Potomac at Washington and Alexandria, to Fred- 

 ericksburg, and to the bend of the James at Richmond. This 

 lowland plain widens from a point at Sandy Hook to 150 

 miles at Cape Hatteras. Still further westward there lies 

 an upland plain, 10 to 100 miles wide, overlooking the low- 

 lands and overlooked by the mountains. Along the common 

 boundary of the lowlands and the uplands the land surface 

 drops from a mean altitude of 300 feet to less than 100 feet, 



