84 The Lower Cretaceous Deposits op Maryland 



suited in the deposition on the newly wave-ciit surface of a new and later 

 member. Emergence followed, and the waves of the Kecent period are 

 now actively cutting away both the more recently deposited beds and the 

 basal remnant of the older ones, with their beheaded cypress trunks and 

 knees, imbedded in peat. In the basal clays of this Pleistocene swamp 

 deposit, penetrated by the roots of the trees, one finds an occasional, im- 

 perfectly formed nodule of iron carbonate, so characteristic of the Arun- 

 del. When exposed to the air it rapidly changes to a bright vermilion 

 ochre. 



There is little question that some such process as this has figured to 

 a considerable extent in the genesis of certain of the lesser lenses of 

 drab, lignitic, iron-bearing clay occurring at various horizons throughout 

 the Potomac Group; but the large scale — both vertical and horizontal — 

 on which the Arundel formation, or " iron-ore clays " proper, is de- 

 veloped cannot well be explained entirely by this simple theory. Land- 

 ward tilting must be retained as the chief explanation for the Arundel 

 clays until a more satisfactory interpretation can be brought forward. 



The well-marked unconformity occurring at many points between the 

 Arupdel and Patapsco formations, notably in the West Hanover district, 

 indicates emergence and a distinct erosion interval prior to Patapsco 

 deposition, and the marked changes in the floras would seem to indicate 

 that this interval was a long one, during which the Arundel sediments, 

 if originally continiious, were removed by erosion from large areas. 



The highly colored and variegated clays of the Patapsco formation, 

 like the iron-bearing Arundel clays, evidently bear some relation to the 

 great basic eruptive masses, plentifully iron-bearing, which lie to the 

 north and west of them. This phase of the sedimentation is somewhat 

 more prominent in central Maryland, where the rocks of this character 

 are not only well developed, but nearest the eastern margin of the Pied- 

 mont belt. It is also probable that these ferruginous Patapsco clays were 

 also in part redeposited from the more richly iron-bearing clays of the 

 subjacent Arundel. The Patapsco sands were doubtless derived to a 

 considerable extent from those of the Patuxent terrane. 



That the seaward tilting was not continuous or persistent in the same 



