MARYLAND GEOLOGICAL SUEVEY 87 



conifers and cycads. During the excavating for the new reservoir 

 in Washington a siliciiied trunk 50 feet in length and several feet in 

 diameter was found in beds belonging to the Patuxent formation. 

 Another similar trunk about 4 feet in diameter was found in the 

 vicinity of the Maryland Agricultural College. 



Strike, Dip and Tliiclcness. — The strike of the Patuxent forma- 

 tion in this county is almost due north and south along the Poto- 

 mac River, but at Washington it changes to a northeast-southwest 

 direction. 



The dip of the Patuxent, as well as of the overlying beds of the 

 Potomac group in Maryland, ranges in direction from east-southeast 

 in its more southerly exposures to south-southeast farther north. 

 The normal dip of the basal beds of the formation reaches about 60 

 feet to the mile. In the vicinity of the '"Fall line," which is toward 

 the landward margin of the Patuxent outcrop, the dip of the basal 

 beds is considerably greater than this. Southeast of Washington it 

 ranges from 50 to 75 feet, but near the '•Fall line" it amounts to 

 about 90 feet to the mile. 



The observed thickness ranges from a few feet to 340 feet, increas- 

 ing toward the east. On the basis of well data the estimated maxi- 

 mum thickness is about 500 feet. 



Stratigraphic Relations. — The Patuxent formation overlies the 

 granite-gneiss, of Archean age, and is overlain unconformably by the 

 Arundel formation. In many places where the Arundel has been 

 removed by erosion the Patuxent is overlain unconformably by clays, 

 sands, and gravels belonging to the Columbia group. 



The Arundel Formation. 



The Arundel formation received its name from Anne Arundel 

 County, where the deposits of this age are typically de^^eloped and 

 well exposed. 



Areal Distribution. — The outcrops of the Arundel formation 

 within Prince George's County are confined entirety to its north- 

 western portion between Washington and Laurel, but it is believed to 

 underlie the greater portion of the count}^ south and east of Anacostia 



