CE NO ZOIC HISTORY. 575 



ocean, covering all of the eastern shore of Maryland and Virginia, 

 and extending up the tidal estuaries to and into the Piedmont 

 region. They pass below tide level along the coast line and extend 

 far out the submarine slope. To the westward, they gradually 

 rise to from 60 to 100 feet in the depressions near the western 

 margin of the Coastal Plain province. They extend up the Pied- 

 mont gorges for some distance, but are, of course, there greatly 

 narrowed. Along the north side of the Potomac gorge above 

 Washington, there is a narrow discontinuous shelf which grad- 

 ually rises to 145 feet at Great Falls, where it becomes the floor 

 of the valley. The inner gorge below the Falls has been cut 

 through the later Columbia terrace. The later Columbia terrace 

 extends up all the small valleys along the Coastal Plain, but is 

 often considerably degraded in them. 



PLEISTOCENE DEPOSITS. 



The Pleistocene deposits consist of gravels, loams, and sands. 

 In the typical development, there is a basal member containing 

 gravels and bowlders, which merges upward into loams. On the 

 earlier terraces, there is a formation of this character, which for 

 the present may be designated "earlier Columbia." On the later 

 terraces, there is a similar deposit which has long been known 

 as the Columbia formation, and this for the present shall be 

 differentiated as "later Columbia." 



In the region to the west and north of Washington, where the 

 earlier terrace is highly elevated, the earlier Columbia is at high 

 altitudes, and the later Columbia deposits lie on the low terraces 

 in the deeper portion of the depressions, but to the east and 

 south, the later Columbia lies in regular succession on the 

 earlier Columbia deposits. The evidences of the separateness 

 of these two formations westward are the bare scarp of erosion 

 intervening between the terraces, and, in a measure, the differ- 

 ence in degree of inclination. In some districts there are wide 

 areas of bare slopes between the upper and lower terrace levels. 

 In portions of the region eastward, a series of cross-bedded sands 

 has been found to intervene between the earlier and later 



