366 W. B. Clark — Potomac River Section 



particularly characterizes the Eocene deposits of Delaware and 

 the eastern shore of Maryland as well as of Anne Arundel 

 County on the western side of the Chesapeake. A very coarse 

 phase of the consolidated sand-rock is seen at Mt. Misery on 

 the Severn river. 



In the less completely weathered portions of the formation 

 the change is shown in the mottled yellow and brown character 

 of the more superficial beds, the glauconitic grains still show- 

 ing their green color when crushed. Thin iron crusts at times 

 appear under these conditions. "When the glauconite is 

 largely or more rarely entirely absent, the deposits consist of 

 black or grey sands or clays, the latter at times micaceous and 

 in a few instances carbonaceous. 



A pebble bed has been found at some localities at the base of 

 the formation. 



Strike, Dip and Thickness. — The strike of the Eocene 

 deposits in Delaware and Maryland is approximately N.E. to 

 S.W. while in Virginia the prevailing trend is more nearly 

 N". to S. This change in the direction of strike is attained in 

 the area between the Patuxent and Rappahannock rivers, 

 chiefly in the Potomac basin. 



The dip of the strata differs in the various portions of the 

 area, as shown by section-lines and well-borings, from 10 to 20 

 feet to the mile, but along the Potomac river section, where 

 detailed measurements were made by the writer, it is on an 

 average about 12J feet to the mile. 



The results obtained from a study of the various section-lines 

 and well-borings show that the average thickness of the deposits 

 is somewhere in the vicinity of 200 feet, although estimates 

 based upon the Potomac river section as well as upon well 

 borings in the area to the east of Fredericksburg show that it 

 reaches quite 300 feet in that portion of the middle Atlantic 

 slope. 



Potomac River Section. 



The most extensive section of the Eocene in the middle 

 Atlantic slope is found in the valley of the Potomac river, a 

 nearly complete sequence of the several members of the forma- 

 tion being exhibited in the bluffs between Aquia Creek, Stafford 

 County, Virginia and Pope's Creek, Charles County, Maryland. 



Detailed stratigraphy. — The accompanying columnar section 

 shows the relative thickness and character of the deposits in 

 this area. The several members of the formation are numbered 

 in ascending order. The full thickness is about 300 feet. 



