128 THE GEOLOGY OF PRINCE GEORGE S COUNTY 



original character. Consequently it is scarcely possible to explain 

 the conditions under which they were originally deposited. Yet it 

 may be said that a large portion of the area which the Piedmont 

 metamorphic rocks now occupy was under water at one time, or per- 

 haps many times, and received in some places deposits of sand and 

 mud carried in by streams, while in other places beds of limestone 

 were formed. It is not known how long this sedimentation continued 

 or hoAv many breaks took place between successive periods of depo- 

 sition. It has been thought by most recent workers in the Piedmont 

 region that the rocks there include not only representatives of the 

 Archean, to which most of the earlier geologists referred them, but 

 of the Cambrian and Ordovician as well. These old rocks have been 

 broken through in many different places by sheets and dikes of 

 igneous material. Thus the Piedmont metamorphics comprise repre- 

 sentatives of both igneous and sedimentary rocks. The structure of 

 these rocks when first formed was undoubtedly much more simple 

 than at present, but they have been repeatedly subjected to various 

 processes of metamorphism by which the beds have been folded and 

 crumpled and the original mineral composition has been greatly 

 changed. 



There is no e^ddence to show a submergence of this area during 

 the latter part of the Paleozoic era nor during the Triassic period. 

 It probably remained as a land mass during most of this time, fur- 

 nishing terrigenous materials to the Paleozoic sea to the west and to 

 the Atlantic Ocean far to the east. It is of course possible that it 

 may have been depressed beneath the ocean waters and covered with 

 sediments many times, but, if so, later erosion has removed such de- 

 posits from the crystalline surface. 



SEDIMENTARY RECORD OF THE LOWER cnETA( E()^^. 



The earliest of the kno^vn unconsolidated deposits lying upon the 

 floor of crystalline rocks belong to the Patuxent formation of the 

 Potomac group. It indicates a submergence of the Coastal Plain in 

 this region of sufficient extent to cover the whole area with shallow 

 water. The cross-bedded sands and gravels furnish evidence of shift- 



