72 THE niYSlOGEAPHY OF PRINCE GEOKGE S COUNTY 



In the Piedmont Plateau the topography is mainly due to these 

 facts and almost invariably the broader valleys have been developed 

 in limestone regions while the ridges are composed of less easily 

 eroded rocks. In the Coastal Plain where the strata are almost 

 entirely composed of unconsolidated sediments there is in general 

 little difference in the rate at which erosion proceeds. However, it 

 can be seen in several places that the Potomac strata are generally 

 easily eroded because of the large amount of sand present. 



As a result of the difference in the ease of erosion of the Potomac 

 strata a strike valley occupied by Anacostia River and its tributaries 

 has been produced in this region. The valley is carved out of 

 Potomac sands and clays and is confined between the crystalline rocks 

 of the Piedmont Plateau on the west and the younger formations of 

 the Upper Cretaceous and Tertiary on the east. Contrary to what 

 might be expected the eastern wall of this valley is even steeper than 

 the western. This is especially noticeable in the southeastern portion 

 of the District of Columbia. 



The most prominent feature in connection with this difference in 

 ease of erosion has been its effect upon the direction of the streams in 

 the area in which the Potomac strata fonn the surface formations. 

 A glance at the map will show that the escarpment between the Lower 

 and Upper Cretaceous rocks has served as an efficient divide between 

 two sets of secondai';N' streams. In only one instance in this county 

 does a secondary stream flowing mainly through the area of the later 

 formations drain any Potomac area. The Western Branch is the 

 sole exception as it has cut through the escarpment just west of Wood- 

 moor. The larger volume of water carried by the Western Branch 

 has aided this stream in pushing its head backward into the Potomac 

 area. 



It is also probable that the ease with which the Potomac material 

 is eroded is responsible for the change in direction of the Potomac 

 River where it emerges from the crystalline rocks. This stream 

 traverses the Piedmont Plateau in a general southeasterly course 

 regardless of the character of the country rock, but as soon as it 

 strikes the Potomac strata it changes its course suddenly and flows 

 southward in a direction almost parallel to the strike. This direc- 



