C EN O ZOIC HISTORY. 57 1 



Coastal plain, the Lafayette formation lies on a very smooth sur- 

 face, but there are low depressions along the lines of the present 

 valleys of the larger streams south of, but not including, the 

 Potomac. In the Piedmont region, there is a system of very 

 low flat divides coincident with those of the present drainage 

 systems. There are a number of "monadnocks" or unreduced 

 areas of hard rocks, which rise more or less abruptly to various 

 heights above the plain. These are shown on figure 5. Parrs 

 Ridge, the large, unreduced area west of Baltimore, rises gradu- 

 ally to only a moderate elevation, but its slopes are nearly every- 

 where clearly demarked from the peneplain. The monadnocks 

 are portions of the old Cretaceous peneplain which was the slope 

 on which the Tertiary peneplain was excavated. Probably the 

 tops of some of the higher monadnocks stood above the Creta- 

 ceous plain. 



During the development of the Tertiary peneplain, there 

 were deposited the Chesapeake, Pamunkey, and possibly portions 

 of earlier deposits, and the long time intervals by which these 

 formations are separated represent intervals of uplift and in- 

 creased planation. It has not been possible, as yet, to differen- 

 tiate the topographic products of these epochs in the Piedmont 

 region, and probably the local features to which the earlier con- 

 ditions gave rise were effaced in succeeding epochs. A certain 

 amount of base-levelling progressed in the Piedmont region dur- 

 ing the deposition of Lafayette formation, but the relative 

 amount is not known. Probably it was not great for the forma- 

 tion represents but a small time compared with preceding depo- 

 sitions and uplifts. 



TERTIARY DEPOSITS. 



There are three formations of Tertiary age in the Coastal plain 

 region, Pamunkey (Eocene), Chesapeake (Miocene), and Lafay- 

 ette (Pliocene?). The Pamunkey formation consists of glau- 

 conitic sands and marls, which attain a thickness of about 180 

 feet east of Washington. It represents but a small proportion 

 of Eocene time, and according to a recent comparison by G. D. 



