makyi>a:\d geological suka'ey 101 



Areal Distrihidlon. — The Aquia is exposed throughout a broad 

 belt 10 miles or more in width, extending from the vicinity of Col- 

 lington southwestward as far as Western Branch of Patiixent Kiver. 

 Beyond the latter point the formation is buried beneath later deposits 

 and outcrops only in a thin band near the headwaters of the creeks 

 along the western margin of the county and along the Potomac Kiver 

 at Fort Washing-ton. The Aquia formation dips to the southeast 

 and is supposed to underlie the younger Eocene and Miocene beds 

 throughout the southern portion of the area. In its wider distribu- 

 tion it extends from Virginia nortlieastward across Maryland to 

 Delaware. 



Character of Materials. — This formation consists usually of loose 

 sand in which there is a considerable admixture of glauconite, the 

 latter in places making up the body of the formation. Where the 

 material is fresh it ranges in color from a light blue to a very dark 

 green, but in regions where it has been exposed to weathering for a 

 considerable time it has assumed a reddish-brown to light-gray color. 

 The beds are in most places unconsolidated, although locally some 

 have become very firmly indurated by oxide of iron. vSmall, well- 

 rounded pebbles coated with iron oxide occur in a few places near 

 the base of the formation. These gravels are typically exposed about 

 a mile northwest of Westphalia and in numerous places about Col- 

 lington. About half a mile southwest of Collington this pebble layer, 

 which is about 2 feet in thickness, has been cemented by ferruginous 

 material into a hard, compact rock that has been used for building 

 I)urposes. Where the Aquia deposits have been exposed to the action 

 of the atmosphere, as on the tops of divides, the iron present in the 

 glauconite has been segregated to form l)ands of iron sandstone. 

 These are very numerous and in places attain a thickness of 1 

 to 2 feet. ]S"ear Upper Marlboro there are a few ledges of indurated 

 marl from which numerous species of fossils have been obtained. 



