142 W. J. McGee — Formations of Middle Atlantic Slope. 



the Potomac formation, when freely interpreted, give the out- 

 lines of an intelligible and harmonious picture of the Atlantic 

 slope during and for some time antecedent to the Potomac pe- 

 riod : Before the initiation of Potomac deposition, but subse- 

 quent to the accumulation of the Triassic and Rhsetic deposits 

 and to the displacement and diking by which they are affected, 

 there was an eon of degradation during which a grand moun- 

 tain system was obliterated and its base reduced to a plain which, 

 as its topography tells us, was slightly inclined seaward and 

 little elevated above tide — the Piedmont zone alike of the later 

 Mesozoic and the present ; and over this plain meandered the 

 prototypes of the Delaware, the Susquehanna, the Potomac, 

 the James, and the Roanoke, within a few miles at most of 

 their present courses and but a few hundred feet above their 

 present channels, flowing slack and in shallow valleys because 

 at base level. There followed a slight elevation of the land, 

 when the rivers attacked their beds and excavated valleys as deep 

 as those to-day intersecting the Piedmont plain ; but whether 

 or not there was concomitant tilting of the land, the phenomena 

 thus far fail to indicate. Then came the movement by which 

 the deposition of the Potomac formation was initiated — the 

 deeply ravined base level plain was at the same time submerged 

 and tilted ocean ward ; its waterways became deep but short 

 estuaries ; deep oceanic waters extended quite to the inter- 

 mediate shores; the declivity and transporting power of the 

 rivers was increased ; and the accumulation of coarse delta 

 and littoral deposits progressed rapidly. With continued 

 deposition the sea gradually shoaled, the declivity of the land 

 decreased, the materials became finer and finer; there was 

 probably temporary emergence of the land about the middle 

 of the Potomac period, followed by renewed submergence 

 without seaward tilting during which the clays of the upper 

 member were laid down ; and the period was finally closed by 

 an emergence represented by the unconformity between the 

 upper Potomac and the glauconitic deposits of the Maryland 

 Cretaceous. 



There is a great hiatus in the geologic history of the 

 Atlantic slope : The history is fairly legible up to the ter- 

 mination of the Paleozoic deposition, and it is even more 

 clearly legible from mid-Cretaceous time to the present ; but 

 the hiatus includes the most interesting period in the evolution 

 of the eastern portion of the continent. The transfer of sea 

 and land; the elevation and corrugation of the Appalachians, and 

 the profound displacement and metamorphism of the Piedmont 

 rocks ; the degradation of thousands of feet if not miles of 

 strata and the transportation of the materials whither no man 



