Watson and Powell — Age of Virginia Piedmont Slates. 37 



for a distance of 40 miles, extending in a southwest direction 

 from Lorton, Prince William County, where it passes beneath 

 the Coastal Plain formations, to Shady Grove, Spottsylvania 

 County, five miles south of the Orange and Fredericksburg 

 railroad. It lies partly in the three following counties : Prince 

 William, Stafford, and Spottsylvania. The slates have not 

 been traced continuously on the surface between the extreme 

 limits given above, because of their concealment beneath the 

 Coastal Plain sediments in the northern half of the belt, but 

 they have been recognized in every section measured along the 

 streams which cross from the crystalline rocks of the Piedmont 

 onto the Coastal Plain sediments (map, fig. 2). The principal 

 streams have carved narrow canyon-like gorges in the crystalline 

 rocks of the northeastern Virginia Piedmont region, and they 

 usually afford excellent exposures along their courses. North 

 of Accakeek Creek the slates are concealed on the interstream 

 areas by the Coastal Plain cover (map, fig. 2). 



The slate belt varies from less than one mile to about 2 miles 

 in width, with a probable average of about one mile. It is a 

 part of the crystalline rocks of the Piedmont region, and north- 

 eastward from Accakeek Creek, Stafford County, it is the east- 

 ernmost representative of the Piedmont crystallines. This 

 part of the belt is bordered on the west by gneisses and gran- 

 ites, and on the east it is concealed beneath the cover of Coastal 

 Plain sediments. Southwestward from Accakeek Creek the 

 gneisses and granites are the limiting rocks on the east and 

 west sides of the belt. 



Structurally the slates show considerable variation in places 

 both in dip and strike. With but few exceptions the dip is 

 to the northwest, usually from 70° to vertical (see fig. 4) ; and 

 the strike ranges from 1ST. 20° E. to N. 60° E. The strike and 

 dip are given below for the sections measured. They are : 



Name of Section. 





Strike. 





Dip. 



Eemarks, 



Occoquan Creek 



N. 



40° - 45° 



E. 



70° N.W. to vertical. 





Marumsco Creek 



N. 



35° -42° 



E. 



Nearly vertical. 



Variable, 



Powells Creek 



N. 



20° E. 





50°-70° N.W. to vertical. 



Quantico Creek 



N, 



30° -35° 



E. 



15° N.W. to vertical. 





A qui a Creek 



N. 



25° -40° 



E. 



70° N.W. to 'vertical. 





Austin Run 



N. 



30° -45° 



E. 



80° N.W. to vertical. 



Variable. 



Accakeek Creek 



N. 



38° -00° 



E. 



8o° N.W. to vertical. 





Potomac Creek 



N. 



50° E. 





35°-59° N.W. 





Wilderness, one 













mile west 



N. 



20° E. 





Nearly vertical. 





Brock Station 



N. 



38° E. 





Vertical. 





Shady Grove 



N. 



25° E. 





Vertical. 





