Maryland Geological Survey 83 



formation, and to a less extent in the I'atapsco. Where the chloritic 

 schists of the Piedmont are developed near its eastern margin, as in the 

 northern Virginia area, the Patuxent clay lenses tend to be greenish 



in color. 



The close of the Patuxent epoch was probably marked by a slight 

 elevation of its dej)osits and a trcncliing of the surface by streamis; or 

 perhaps the process was merely one of differential warping. This was 

 followed by a subsidence or tilting, which was emphasized to the land- 

 ward by the occupation of the ancient valleys by swamp deposits. The 

 tough clays of the Arundel formation, charged with lignitic accumula- 

 tions, in which tree trunks are at times found erect with their roots in- 

 tact, find their most satisfactory explanation on this basis. It was in 

 these ancient swamps and estuary marshes that the iron, derived to a 

 considerable extent from the adjacent areas of basic eruptives, was de- 

 posited, first, no doubt, as bog ore, which by contact with the excess of 

 carbonaceous materials was later altered to the carbonate and redeposited 

 in its present nodular form. It was in these swamps that the remains 

 of dinosauria became entombed. On this hypothesis the lenses of Arun- 

 del clays re|)resent crudely the ancient drainage lines of the eroded sur- 

 face of the Patuxent terrane. The widening of the areas seaward may 

 possibly be interpreted on the basis of lagoon deposits into which the 

 Arundel estuaries merged. 



The Pleistocene " buried-f orest " deposits of the Chesapeake shores 

 may furnish some clue to the origin of the Arundel iron-ore clays, as 

 well as similar beds in the Patuxent and Patapsco formations. The 

 Pleistocene deposits of this character appear to have originated by the 

 impounding of the estuaries by sand spits — a process which may be ob- 

 served at many points within the Chesapeake and elsewhere at the 

 present day. The closed estuary then speedily silted up and was con- 

 verted into a peaty cypress swamp in which bog iron ore was deposited. 

 Meanwhile the bay shore adjoining the mouth of the swampy estuary 

 was gradually receding by virtue of wave action until the swamp mate- 

 rials themselves were invaded and more or less cut away. This process 

 was followed, or perchance attended, by gradual subsidence, which re- 



