MAJiVl.AM) OKOLOORAh SLUVKY 47 



ever, ]iy wcillicring, lose tlieir characteristic green color, and ])y the 

 depusilioii ui' a greater or less amount of hydrous iron oxide ]>ec(»nie 

 firm red or brown sandstones or incoherent red sands. At times, notably 

 in Southern ^Maryl.iiid and \'ii;uiiiia, the strata become highly argilla- 

 ceous, the glauconitic elements largely or quite disa|>|»t'ariii;/. Infre- 

 quently coarse sands and even gravels are found, the latter cliietiy toward 

 the base of the formation and near the ancient slmre line, especially 

 toward the northeast in central and castci-n ^laryland. 



Very commonly the shells of organisms are so numerous as to form 

 the chief constituent of certain beds. Notwithstanding these facts, the 

 deposits are remarkably homogeneous, although recent investigations 

 have shown the possibility of dividing the deposits into two well-marked 

 formations on both lithologic and faunal grounds. The lower or Aquia 

 formation is mueli more highly arenaceous than the upper or Nanje- 

 moy formation which, particularly in its loM^er part, is generally highly 

 argillaceous. The Aquia formation is also much more calcareous than 

 the jSTanjemoy formation, indurated layers frequently appearing in the 

 former. 



NEOCENE. 



The jSTeocene deposits occupy the region to the southeast of and over- 

 lie the Eocene. The lower beds comprise the Chesapeake group, so 

 named from the superb sections found exposed on the shores of Chesa- 

 peake Bay, and recognized to consist of three well-defined fonnations; 

 the upper beds comprise the Lafayette formation. The Xeocene de- 

 posits lie unconformably upon those of the Eocene and overlap them 

 along their western border, where they ultimately come to rest upon 

 the Cretaceous toward the northeast. They are in turn unconformably 

 overlain by the Pleistocene deposits. 



The Chesapeake group consists of sands, clays, marls, and diatomace- 

 ous beds. The latter, composed almost exclusively of the tests of dia- 

 toms, are chiefly confined to the lower portion of the basal formation, 

 where they afford striking, light-colored bluffs along many of the larger 

 stream channels. The nearly pure diatomaceous earth reaches a thick- 

 ness of 30 or 40 feet, although the remains of diatoms are found scat- 

 tered in o-reater or less amounts throughout much of the overlying strata. 



