MARYLAND GEOI-OGICAL SURVEY 127 



sand, and gravel make up the formation and these materials are de- 

 posited in deltas, flood plains, beaches, bogs, dmies, bars, spits, and 

 wave-bnilt terraces. 



Interpretation of the Geoeogical Record. 



Almost all the formations which occur within Prince George's 

 County have a much more extensive development in the regions be- 

 yond its borders. If study were confined to the area of this county 

 alone many of the conclusions drawn from such investigations might 

 be unsatisfactory and erroneous. The geologic history of the county, 

 which is here outlined, has been based on work done not only in this 

 area but also throughout the I^orth Atlantic Coastal Plain from 

 Raritan Bay to Potomac River and in certain localities in Virginia 

 and the Carolinas. 



A study of the geologic history of the county shows that it has 

 been long and complicated. This is indicated by the many different 

 kinds of strata represented and by the relations which they bear to 

 one another. There are deposits that were formed in fresh or brack- 

 ish water; others that show evidence of their deposition in marine 

 waters, some in water of shallow depth, others in deep water; while 

 breaks in the conformity of the different strata indicate that from 

 the time of the formation of the earliest beds down to the present 

 day the region has undergone many elevations and subsidences. 



sedimentary record of the crystalline rocks. 



In this section rocks older than the Cretaceous are present only 

 in the Piedmont Plateau. It is exceedingly difficult to interpret the 

 past history of the Piedmont region for the reason that the whole area 

 has been subjected to many great changes which have essentially 

 modified the original materials; yet the studies of Williams, Keith, 

 Mathews, Bascom, and others have revealed many facts concerning 

 the original condition of the rocks noAv occupying this region. 

 ISTearly all the rocks of the Piedmont are metamorphic in character. 

 Many of these rocks were originally sedimentary deposits, but in the 

 processes of metamorphism have now lost nearly all traces of their 



