Natural History of District of Columbia — McAtec 59 



Indefinite and often ill-defined, the meeting place of these 

 provinces — that higher bench at the foot of the Allegheny 

 Mountain system, known as the Piedmont Plateau, and the 

 lower land or Coastal Plain composed of materials 85 car- 

 ried down from the plateau and westward areas and marine 

 sediments deposited during its several submergences below 

 sea level — is better called a zone, a zone of interdigitation. 



The irregular character of the boundary between the sur- 

 fical deposits of the two provinces is due to two causes: 

 (1) the rocks of the Piedmont are exposed eastward beyond 

 the line of their higher outcrops by the erosion of stream 

 valleys; and (2) westward of this line Coastal Plain forma- 

 tions mantle the divides between such streams, sometimes 

 to a distance of several miles. Hilltops far back in the 

 Piedmont province bear caps of Coastal Plain deposits, an 

 indication of the former extent of these sediments and of 

 the vast amount of erosion necessary to have carried off the 

 remainder of them. The reverse condition — that is, islands 

 of Piedmont rocks in the Coastal Plain, does not exist for 

 the reason that the rock surface dips steeply and is now 

 exposed as far as the base-levelling action of the streams 

 makes possible. 



I'he Piedmont Plateau originally was a shore line against 

 which the Coastal Plain sediments were deposited and in its 

 superficial characters, the boundary zone between these prov- 

 inces still retains many of the characters of a strand. The 

 upper border of this zone is undulating, suggesting the 

 tongues of sand left on the beach by the last tide, the exposed 

 rocky stream beds resemble the gullies cut here and there 

 in the sand by the receding water, while the Coastal Plain 

 debris capping the hills, is like the drift cast up by storm 

 tides, patches of which hither and yon have not yet been 

 washed back to the sea that upheaved them. 



^The sedimentary deposits which make up the Coastal Plain are chief- 

 ly clays, gravels and sands, which, with the exception of some sandstones, 

 are unconsolidated. The estuarine streams of this region have broad open 

 valleys with muddy vegetation-covered banks. The streams of the Pied- 

 mont Province, on the contrary, flow in narrow gorges cut into and usual- 

 ly across the structure of the metamorphic and igneous rocks. These 

 rocks include granite gneiss, gneiss, schist, soapstone, serpentine, diorite 

 and gabbro. 



