MAKYLAND GEOLOGICAL SURVEY 175 



which has not been determined. That the elevation did extend far to 

 the east beyond the present borders of the Piedmont Plateau seems 

 however, to be an hypothesis which almost amounts to a certainty. 



A vast amount of erosion followed the uplift of the Patuxent forma- 

 tion before the region was again lowered beneath the sea. The 

 formation which succeeds the Patuxent is known as the Patapsco. 

 In regions further to the south, a formation known as the Arundel 

 has been found interpolated between the Patuxent and Patapsco for- 

 mations; but in Cecil county its presence is doubtful. The 

 Patapsco formation is composed of the same sort of materials as are 

 found in the Patuxent formation, with the exception of arkose. Not 

 only is there; a similarity in materials but there is also a great likeness 

 in the way in which the different sorts of ingredients are assembled. 

 There is the same rapid change, both horizontally and vertically, as 

 was found to exist in the Patuxent formation, and, consequently, like 

 inferences must be drawn regarding its mode of formation. The 

 Patapsco therefore indicates a repetition of the conditions which 

 existed during Patuxent time. They were off-shore deposits made in 

 shallow water by shifting currents of varying intensity. There is, 

 however, a striking change in the character of the plant remains. In 

 the Patuxent, few dicotyledons have as yet been discovered; while in 

 the Patapsco, a large number of dicotyledons have been found. The 

 meaning of this difference in the fossil flora has led to much discus- 

 sion; it seems probable, however, as suggested by Professor Clark and 

 Mr. Bibbins, 1 that the stratigraphic break between the Patuxent and 

 Patapsco formations indicates a lapse of time of long duration, during 

 which the flora became greatly modified. The evidence, to be sure, 

 is incomplete, but the few and primitive dicotyledons in the Patux- 

 ent formation suggest the possibility of the Jurassic age of that form- 

 ation, while the presence of Lower Cretaceous dicotyledons in the 

 Patapsco, places the age of that formation beyond question. Further 

 south in certain other counties of Maryland, remains of animals which 

 Professor O. C. Marsh referred to the Jurassic, are confined to the 



ir The Stratigraphy of the Potomac Group in Maryland. Jour, of Geol., vol. v, pp. 

 479-506, Chicago, 1897. 



