GEOLOC.Y OF THE POTOMAC BEDS. 51 



nioiul, wliere a well du<^' tliroiiiili the Kocone to the top of the Potomac 

 disclosed a layer of larj^'e cobbles, many of them composed of Potsdam 

 (juartzite, and some ten inches in diameter. The same thing- may be seen 

 at Deep Bottom and near Hanover Junction. Indeed, so common is this 

 featnre, that Professor Rog-ers, in his earliest pnblications on the geology 

 of Virginia, called attention to the trenched and eroded character of the 

 lower Potomac snrface npon which tlie Eocene rested, and to the accnniu- 

 lation of large stones in places on this snrface. 



These stones are to be fonnd also generally where the Quaternary 

 rests on the Potomac, and then they attain their maximnm in abundance 

 and size, deserving sometimes to be called bowlders, since some of the 

 Potsdam stones are two feet in diameter, while the Azoic masses are occa- 

 sionalK' ninch larger. 



The stones within the Potomac beds are generally small, being- two to 

 three inches and under in diameter, but sometimes they attain the diam- 

 eter of five to six inches and more. They occur sometimes aggregated 

 into irregular beds and pockets, then attaining their largest size. The 

 smaller pebbles are often very abundant, scattered through the general 

 mass of the rock, and showing no particular relation to the inclosing mate- 

 rial. This ma's' be sand, coarse or fine, or it may be clay. This shows 

 that there was little sorting action in the waters. 



As stated before, Potsdam material is not seen in the northern areas 

 except in the vicinity of Washington, for in the areas north of Fredericks- 

 burg the inclosed stones are usually quartz or Azoic. In the Peters- 

 burg- area, as before stated, these are accompanied by an abundance of 

 Potsdam cobbles. No Potsdam stones were seen in the Nottoway area. 

 The Nottoway River does not cross the Blue Ridge and touch the Pots- 

 dam terrane,' as do the James and the Potomac. The Potsdam quartzite 

 is found in place on the western flanks of the Blue Ridge, but not east 

 of it. 



^ye find these stones in the top of the lower Potomac where the 

 Variegated Clays rest on the sand, but sufficient exposures of this contact 

 have not been seen to enable one to say what is generally the case at such 

 'I- junction. It is not an uncommon thing to find plant-impressions in the 



