LOCATION OF POTOMAC BEDS. 41 



It is not necessary to give desi.'riptioiis of the Potomac formation as ex- 

 posed in the central and western portions of the area Tlie account given 

 of the strata at the several plant localities will give an idea of its variable 

 character. It may be stated generally that along the western margin there 

 is a larger proportion of argillaceous matter tlian in the eastern outcrops. 

 The clayey matter sliows also a greater tendency to assume various colors, 

 due to oxide of iron, and mucli of it is mottled. These features are seen 

 more especially in the upper portions of the formation. 



At Fredericksburg the western margin is found about two miles west 

 of the town. It bends around so as to strike the Rappahannock near the 

 mouth of the Massaponax River. Here the Eocene lies above it, and this 

 is the southern termination of exposures in the Fredericksburg area. 



From the mouth of the Massaponax to the vicinity of Hanover Junc- 

 tion no Potomac has been seen. Hanover Junction is the point of cross- 

 ing of the Atlantic Coast Line, and the Chesapeake and Ohio Railroads. It 

 is situated in the triangle formed by the North Anna and the South Anna 

 Rivers, near their junction. The material exposed in this interval is mainly 

 a tliick mass of sands and sandy clays, yery variable in composition and 

 color. The name Appomattox is proposed for it, from its great develop- 

 ment in typical form in the vicinity of the Appomattox River. It is 

 younger than the Miocene, for it rests on the eroded surface of this, but 

 being witliout fossils, its exact age can not be fixed. It is probable that no 

 exposures of tlie Potomac can be found between the points mentioned 

 above, because the erosive power of the streams in the belt where out- 

 crops may be looked for is not sufficient to remove the thick covering of 

 superjacent material. The only streams of any size are the up[)er waters 

 of the Mattapou)'. None of these cut tln-ough the Eocene. 



In the vicinity of Hanover Junction the Potomac shows in several 

 places a few feet of strata. It lias evidently lost much l)y erosion. In the 

 cuts on the Atlantic Coast Line Railroad, between the Junction and North 

 Anna River, a tiiin remnant may bo seen of the usual character, resting on 

 the eroded surface of the older Mesozoic (Rluetic), ami capped by Qua- 

 ternary and Appomattox. On the North Anna River, a short distance 

 above its junction with the South Anna, the Potomac sand may be seen 



