ABSTRACTS 413 



United States Geologic Atlas, Folio 2j, Nomi/ii, Maryland- Virginia, 

 i8g6. 



This folio consists of four pages of text signed by N. H. Darton, 

 geologist, a topographic map of the district, a map showing the areal 

 geology, and a map showing the distribution of underground waters 

 and artesian wells. The scale of these maps is 1:125,000. 



The area represented in this folio is about 938 square miles, which 

 lies partly in Virginia and partly in Maryland. In Virginia it com- 

 prises nearly all of Westmoreland county, with parts of Essex, Nor- 

 thumberland, and Richmond, and in Maryland it includes portions of 

 St. Mary, Charles, and Calvert counties. It lies entirely within the 

 Coastal Plain area. The Potomac River extends northwest and south- 

 east across the middle of the area, the Patuxent River crosses its 

 northeastern corner, and the Rappahannock River crosses its south- 

 western corner. To the extreme northeastward it extends to the shore 

 of Chesapeake Bay. These waters are all tidal estuaries. Along the river 

 valleys there are wide, low terraces capped by the Columbia formation, 

 of Pleistocene age. The intervening areas are plateau remnants 

 capped by Lafayette deposits, of supposed Pliocene age. The under- 

 lying formations are the Chesapeake and Pamunkey, the latter extend- 

 ing from the westward only a few miles into the area, along the north 

 side of the Potomac River. 



The Pamunkey formation, of which only the uppermost beds are 

 exposed, consist in greater part of glauconitic marls of Eocene age. 

 It is overlain unconformably by the Chesapeake formation, which is 

 characterized by fine sands, marls, and clays, portions of which consist 

 largely of diatomaceous remains. The formation is very fossiliferous 

 at some localities. Its age is Miocene. The greatest thickness which 

 it presents in the Nomini area is about 270 feet, but it continues to 

 thicken gradually to the eastward. 



The Lafayette formation, which ranges from 25 to 40 feet in 

 thickness, con=.ists of sandy loams of orange, brown and buff tints 

 often variegated, containing irregularly disposed bands and sprink- 

 lings of small quartzite pebbles and coarse sands. The pebbles and 

 larger sand grains are orange tinted, mainly by superficial staining. 

 The plateau surface, capped by this formation and deeply incised and 

 dissected by the larger drainage depressions, inclines gently southeast- 

 ward at an altitude ranging from about 190 feet along the northern 

 and western border of the area to about 90 feet along its eastern 



